Florigen, the flowering hormone

Many plants are ‘short-day’ plants, meaning that the plant starts to bloom once the days shorten. This is why horticulturists cut the amount of light from eighteen hours a day to twelve when they want to start the flowering phase. It’s actually a shame from the plant’s perspective: six fewer hours of light a day is six fewer hours of photosynthesis, and thus less energy for your plants in the form of sugars. There is also a substance that can get your plants blooming without your having to cut down on their light.

D. Kroeze, CANNA Research

What really happens when the days get shorter or when you cut back to 12 hours a day? When the light goes down to 12 hours or less, the leaves start to manufacture a substance that triggers flowering, which gets transported to all over the plant. This substance is called florigen or flowering hormone.

The term ‘short-day plant’ isn’t completely ac- curate. It’s not the fact that the days are getting shorter that makes the plant decide to flower, but that the nights are getting longer. Although the difference may seem trivial, it does explain why a night-time visit to your growing space will delay flowering for your plants. When you turn on the light, the plant’s night is over; it has now become too short to stimulate flowering. The plant has to start over counting the hours of darkness from zero.

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Other short-day plants include maize, chrysanthemum and chicory. There are also long-day and day-neutral plants.

Examples of long-day plants are spinach, let- tuce and barley. One day-neutral plant is tobacco.

Julius and the discovery of florigen

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In 1865 a German scientist named Julius von Sachs discovered that when he transferred sap from a flowering plant to a non-flowering plant, the non-flowering plant started to flower as well. This even happened when the two plants were from different species. Unfortunately, no matter what he tried, he never succeeded in isolating the substance responsible for flowering.

Many after him also have tried in vain to isolate florigen, which made it into something of a mystery. It got to a point where the question was not only what the substance actually was, but whether it even existed – at least until a few years ago. Now, one of the greatest mysteries of plant biology seems to have been solved.

Julius von Sachs (pictured top-left) made other major discoveries besides the existence of florigen. For example, he discovered chloroplasts and the fact that they produce sugar. He also discovered that glucose is stored in the form of starch in granules. In other words, the mystery of florigen was as old as much of the fundamental knowledge in plant biology.

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Chloroplasts

The information superhighway

During the quest for florigen, it became clear that the sap flowing through the phloem (vessels) of the plant contained more than water and the sugars produced by photosynthesis in the leaves. As it turned out, many semiochemicals (substances that send signals to the plant) are dissolved in the phloem sap. These are mostly small molecules in very low concentrations. The phloem transports information from one place in the plant to another, including the signal to flower, in the form of these substances. This is why the phloem is also known as ‘the information superhighway.

Actually, florigen had been found a few years earlier, but its function had not been discovered until recently. You may wonder why it took so long to find florigen. Here’s the reason: Once the night length has crossed a certain threshold, the leaves produce a signal to start manufacturing florigen. The substance is only made in the growing points of the plant; a different substance, which reacts with the substance from the leaves, occurs only in the cells of growing points. The two substances together are actually the real florigen.

In addition, but no less important, is the fact that these are very tiny molecules, which were only discovered in the last few years. Until then, laboratory equipment was simply not advanced enough.

articles-florigen_text_4 The phloem (red) is the living vascular tissue of the plant, through which mainly sugars and water are transported from the top down. Besides the phloem there is also xylem (pink), dead tissue that transports nutrients and water up from the roots.

The future is smiling?

After more than 140 years, the quest for florigen is finally over. A great mystery has been solved. This is fine for science, but what does it mean for your average person? The answer is easy: a lot! Manipulating florigen has enormous potential.
Its discovery will bring about a revolution, in particular for conventional agriculture. Greenhouse horticulture will see increased yields from more hours of light. Scientists are especially thinking about growing crops in places where it was previously impossible, such as growing some tropical crops in Northern Europe. But a lot may change for tropical regions as well. The shortened growth time will mean that more crops can be grown in one growing season than is possible now.

Besides this immediate effect on food production, there will also be a revolution for seed companies. For example, fruit trees could be made to flower in the first year and so can be crossed with each other within months instead of the usual years breeders spend waiting for the first flowers.
For hobbyists, it will of course be the higher yields that make the applications of florigen most interesting.

We should not get carried away. It will be many years before we can make practical use of florigen. One thing is certain: its discovery will change agriculture and plant breeding forever.

From Gentech to hunger

Why the question mark after ‘the future is smiling?’ because florigen can’t simply be added to a plant. Biotechnology companies will have to provide crops with the information they need to make florigen themselves independent of day length, using genetic modification. Since this gene will initially be put into one or two crop varieties, these few varieties will quickly drive out local varieties (genetic erosion).

This will mainly be a problem for developing countries where agricultural production is now less than optimum, and where these new crops could greatly improve food production. At first, these monocultures of just a few varieties on such a huge scale will produce lots of food, but in the long term will lead to enormous problems from diseases. And what are you going to eat when the food crops are gone?

That’s the next challenge!

Plant hormones

Hormones are organic molecules that can influence the physiology and development of plants and animals even at low concentrations. Hormones play an important role in the growth and flowering of the plant and many other things. This article briefly explains how plant hormones work in plants and how hormones ensure that plants flower.

Hormones are produced and transported throughout the entire plant. Simply put, they are signals, chemical signals, that can be sent and received throughout the entire plant. A leaf can and will transmit a signal to the end of the stem telling it to form flowers for example. The most well-known plant hormones are auxin, gibberellin, cytokinin, ethylene, and abscisin (abscisic acid). In addition, it has been demonstrated that brassino-steriods, salicylates, and jasmonates also function in a similar way to hormones. Hormones can also occur bonded to sugars or amino acids. In this form, they are inactive and provide storage. The hormones can be released again and become active under various conditions such as the influence of gravity or light for example.

Auxin

In the 1880s Charles Darwin and his son Francis started experiments that finally confirmed the existence of plant hormones. They experimented with oats and the influence of light on the direction of growth. Auxin was the plant hormone whose processes were demonstrated during these experiments. Auxin is produced in the plant’s growing points both above the ground and in the roots. Auxin influences water absorption, cell division, and cell stretching (it softens cell walls) among other things. Because auxin promotes the formation of roots on stems it is used in a variety of forms in rooting hormones.

Experiments carried out by CANNA have shown that the effect of administering auxin depends very much on the concentration and method of application used for each plant type. With weak concentrations, flower formation is stimulated slightly and ripening takes longer. With high concentrations, there is an inhibiting effect on growth accompanied by deformities and tumour-like symptoms. Auxin that is produced in the tops of plants is capable of inhibiting the development of side shoots. This symptom is known as apical dominance. Removing the main tip stops the inhibiting effect and side shoots then develop which will eventually result in a broader plant. Where crop spacing allows only a few plants per square meter it is worthwhile removing the main tip as this makes it possible to use the light more efficiently. It’s also necessary to remove the tips regularly to achieve a good stock plant for propagation so that it will grow many more side shoots.

Gibberellin

Gibberellin was first isolated in 1935 in Japan by Yabuta. The gibberellin was acquired from a fungus that had been the cause of reduced productivity for Japanese rice farmers for centuries. The gibberellin initially gave better growth but later in the season, it caused sterile fruits. Generally speaking, gibberellins work as growth accelerators because of cell stretching and cell division. They ensure that seeds germinate and flowers form in plants that need long days. Gibberellin is often used in the cultivation of fruit to help unfertilized pears and apples develop fully.

Administering gibberellin to short-day plants, or autumn flowerers, as they are also known, very quickly gives clear effects even at low concentrations. Plants become light green in color and stem split open because of the fast growth (photo 1). The plant’s speed of growth can reach 10 cm per day! Administering gibberellin during the vegetative phase causes plants to start flowering more slowly. Gibberellin is for short-day plants as testosterone is for people. It stimulates the formation of typically male organs and longer plants; longer internodes and male flowers in dioecious plants. When the pollen from these flowers is used to fertilize female flowers, seeds are created that always produce female plants.

Certain environmental influences can also cause the production of extra gibberellin. Plants will make more gibberellin in poorly lighted conditions, which causes them to become long and look lanky. Another effect is seen when the lamp is too close to the plant. Buds that are flowering can start to shoot again if the lamp is too close. This will cause the tops to become long and thin. To prevent this, the distance from the plant to the lamp during flower formation must be at least 50 cm for a 600W lamp.

Cytokinin

Cytokinin activity was first demonstrated in 1913. 30 years later it was discovered that a natural substance present in coconut milk was capable of helping plant cells multiply. Cytokinin was the responsible hormone for this. Cytokinin is known as the hormone responsible for cell division. It stimulates the metabolism and the formation of flowers on side shoots and as such is a counterpart to auxin. The cytokinin concentration is highest in young organs (e.g. seeds, fruits, young leaves, and root tips). High concentrations in an organ or tissue will stimulate the transport of sugars to those tissues or organs.

Administering cytokinin leads to greater leaf surface area and faster flower formation. However, the time that flowering finishes is comparable to untreated plants. Cytokinin can be seen as a counterpart to gibberellin in this regard because it stimulates the formation of female flowers on male plants.

Ethylene

The practical use of ethylene comes from the time of Old Egypt when figs were scored to make them ripen faster. In 1934 it was discovered that plants produce ethylene themselves, which enables them to regulate fruit ripening. Ethylene is the least complex plant hormone from the molecular point of view and is produced by all organs. It is a gaseous hormone that is transported via the spaces between plant cells. It is responsible for fruit ripening, inhibition of growth, and leaf abscission (shedding). Ethylene has a stimulating effect on flower formation with certain types of plants (i.e. pineapples, mangoes, and lychees). Administering ethylene results in smaller plants and flowering finishes a lot quicker. The flowers ripen too quickly and consequently remain small.

Because plants can be very sensitive to ethylene, the concentration is expressed in parts per billion parts of air (ppb). Concentrations of just 10 ppb can cause abnormalities in tomatoes. In situations where ripening flowers come in contact with young plants, there is the risk of accelerated ripening in the young plants. The ethylene that is produced can reach the young plants via the air. Ventilating occasionally (once per day) will remove the ethylene that has formed. High concentrations cause leaves to turn yellow immediately.

Ethylene can also accumulate around roots if they are wet for too long. This can lead to leaf chlorosis, stem thickening, leaves bending towards the stem, and greater susceptibility to diseases.

In stress situations, for example, when there is disease present or damage to the plant, the plant produces more ethylene, which causes it to remain smaller and finish flowering faster. Mechanical stress such as air movement can also cause the plants to produce extra ethylene, which will result in smaller plants with thick, sturdier stems. When the fans are too close to the plants there will be too much stress and this will adversely affect the yield.

Abscisin

Abscisin was first isolated in 1963 and has the Latin word abscissio (breaking off) to thank for its name. This is because people thought that abscisin was responsible for the breaking off (shedding) of leaves and fruits, however, it was later shown that ethylene plays a much more direct role in this.

Abscisin is produced in the chloroplasts of older leaves and has both inhibiting (growth) and stimulating (protein storage) characteristics. When there is a large supply of abscisin to the growing points of the stem and roots, cell division stops and the plant enters a rest period.

Abscisin is an important hormone as far as stress situations are concerned. It is responsible for closing the stomata when the plant is under water stress due to continuing high temperatures, low atmospheric humidity, or an EC in the feeding medium that is too high.

Flower formation in short-day plants

Even though a lot of research has already been done into the changeover from growth to flowering in plants, it still hasn’t been explained how this mechanism works exactly. In the case of short-day plants, the formation and development of flowers depend on the precise length of the night. Short-day plants will flower when the nighttime period is longer than 12 hours. It is important that it is really dark during this period because the plant is only capable of measuring the period of darkness and not the period of light. Almost any light level during the dark phase will affect the cycle. This is measured in the leaves, which then send a signal to the furthermost ends of the branches instructing them to form flowers. The hormone that gives this signal is called florigen. So it is theoretically possible, for example, to use material from flowering plants to stimulate other plants to flower under 18 hours of light.

Different hormones play an important role in the phase following the first set of flower buds. So cytokinin and auxin play an important role in the further formation and growth of the flowers. Abscisin and ethylene are important during ripening.

Using hormone preparations

If you want to experiment with plant hormone preparations, pay close attention to how, when, and how much hormone you use. The final effect depends on many factors such as the time of administering (which phase, time of the day), the route chosen for administering (leaf or roots), and the concentration. The final effect of administering hormones can depend very much on the concentration used. For instance: weak concentrations of Auxin stimulate root growth while strong concentrations cause extra ethylene production, which, in turn, causes the plant to finish flowering faster.

Roots and root hairs

Roots are one of the most important parts of the plant for taking up nutrients and water. For some growers, they are so important that they always check the root system before watering. By Pieter Klaassen CANNA Research

The foundation

Plants need roots in order to stay upright and not to be blown over by the wind. Water and nutrients also enter the plant through the root system.

The root system will continue to increase in volume for as long as the plant as a whole, including the foliage, continues to grow. When a certain equilibrium has been reached, the plant will simply maintain its volume, and cease to grow. Even in this state of equilibrium, the roots continue to grow, but will die back partly as well. To understand this better, we will have to divide the root system into parts.

The root system

As mentioned, the root system will only increase in volume for as long as the rest of the plant continues to grow. However, transpiration from the leaves can also cause more roots to form in order to pump up the water needed. In the end, an equilibrium is established between the roots and the plant. A general rule of thumb is that the root system should comprise 30% of the total plant volume. Although this rule applies fairly consistently to plants in the open air, in substrate culture this does not always have to be the case. You can grow large plants in small pots as long as you supply them with water and nutrients and do not allow the pot to get too dry or too wet. To reduce the chance of this happening, we advise a large medium volume.

In hydro cultures you will also see that fewer roots are needed in order to grow a larger plant. This is because each root hair is able to absorb more water and nutrients. This is one of the reasons that hydro culture has the potential to produce higher yields.

Root hairs

Root tip and a cross section of the root tip

The root hairs are where most of the nutrients and water are absorbed. The root tip produces new cells on a daily basis, and thus also root hairs. When the plant is short of water and/or nutrients, it will devote more assimilates (photosynthesis energy) to producing more cells in the root tip. This also generates more root hairs until the root has found what the plant needs (more water or nutrients). The oldest root hairs will then die off.

In practice, as the medium gets drier, the root starts looking for water and will produce more cells, and thus more root hairs. Absorption capacity increases, because more root hairs are produced. But the youngest root hairs will enter even more “moist” soil. The plant can still take up water and sometimes even more! This is why the general advice is to grow on the dry side: when you water, some of the root hairs become redundant. To limit the energy-loss (dissimilation energy), the oldest root hairs will die off.

If you give the plant too much water, all the root hairs will die off. Effectively, the roots drown and it takes at least three days before a root tip has produced enough new cells with root hairs. The dying off of root hairs also happens after repotting the plants or after moving them very roughly. So when this is the case, go gently on your climate control the first 3 days.
And pay good attention to the watering: don’t overdo it but on the other hand, don’t let the plant dry out too much.

The root

Unlike the root hairs, the roots themselves are visible to humans. The root cells, without root hairs this time, will stretch to enable the propulsion of the youngest part of the root forwards. The outermost cells of the root suberise (form a hard surface, like the bark of a tree), after which they only serve as a pipeline to transport the water and nutrients absorbed towards the stem and the rest of the plant.

Plants in the vegetative phase will increase in weight, both above and below the ground. Even in the first stage of the generative phase, the leaf surface area will increase and an active climate will cause the roots to increase in volume. Eventually, an equilibrium will be reached. This maximum equilibrium usually comes when around 50% to 70% of the flowering period has elapsed (for example, week 6 of a 10-week growing cycle).

In potting mix cultures, the plant can absorb 5 to 6 litres of water/m2 per day. But in hydro cultures more water can be absorbed with fewer root tips (but not fewer root-hairs!).

Root tips

At the end of every root is the root tip. The root tip consists of a root cap and a growing point. The root cap is very hard and protects the growing point. It is so hard, in fact, that it can break and grow through asphalt if the cap has enough energy.

In the growing point behind the cap, new cells are created. The most important plant hormones are also produced here. These will not be discussed in this article. For more information on plant hormones, please see CANNAtalk 9. It is these new cells that cause the roots to grow further through the medium. The roots are able to do this not only because new cells are created, but also because the existing cells are stretched. The first cells also contain bulges, called root hairs.

Hard water and soft water

All over the world, questions pour in as to the distinctions between hard water and soft water and how these differences affect how and what plants are fed.

By Geary Coogler, B.Sc. Horticulture

The U.S. department of the interior and the U.S. Geological Survey (USGS) define 60 mg/l (60 ppm) or less of certain ions as soft water. Water with over 120 mg/l (120 ppm) is considered hard, and water in between is moderately hard. Other countries and agencies hold their own distinctions. (See Table 1) Strictly speaking, it is the concentration of dissolved positive multivalent metallic ions with a charge of +2 or +3, typically Calcium and Magnesium. This effect can be heightened by the presence of dissolved in water, can and will react with other elements added to the water, or with anything it comes into contact with. Hard Water is an issue for cleaning, for equipment, and increases the chemical activity of the water especially where pH is concerned; it is often considered healthier. Typically this comes from ground water that has been exposed for longer periods to mineral bearing rock. Well water is a prime example.

Soft Water on the other hand, allows soap to foam up and work better, has less issues for equipment, and provides more of a blank slate in chemical reactions; studies have shown a correlation between soft water and health issues including cardiac disease. Typically this comes through surface water, rivers, streams, and lakes that have not been exposed to mineral bearing rock formations for long periods. It can also be composed of treated water where most of all ions have been removed or replaced by single valance atoms such as Sodium from water softening equipment.

Table 1.

Recommended upper limits of chemical factors in irrigation water for greenhouse crop production (Based on 1 and 2, see bibliography)

The values can vary by EC meter. Here we have used the Truncheon Meter to calculate the values.

Problematic ions

Bad Water is bad water, whether because it has a high salt content or has undesired chemicals in it. It can be found anywhere especially in industrial areas, intense agricultural regions, and close to bodies of salt water. This has no bearing on water hardness.

Do not confuse ion or salt concentration with hardness or softness of water. Hardness is a function of multivalent ions like Ca2+ and Mg2+ not monovalent ions like Na+ or Cl+. Monovalent ions also show up in the Total Dissolved Solids (TDS) of a solution, so it is possible to have a TDS of 450 mg/L (1 ppm = 1 mg/L), derived from adding table salt to distilled water, but have soft water. There is no direct correlation between TDS or EC (electrical conductivity) and water hardness unless it is known with certainty that all EC derives exclusively from Ca, Mg or other positive multivalent metallic ions. Sugar water has EC but no hardness. Water softeners work this way by displacing the problem ions Calcium and Magnesium with Sodium ions. The EC stays the same or increases but the water goes from hard to soft; not a good thing for plants.

For us, the big question is “How does this affect the nutrients for plants?” One of the biggest effects for growing systems using hard water is the potential for insoluble deposits of Calcium or Magnesium carbonates. This combining of these ions is an endothermic reaction meaning that as heat is supplied to the solution, the process gets faster. The process of pumping water from a reservoir, through a pump, through smaller pipes, onto a table top and through a root system imparts increasing amounts of heat to the solution so the reaction is natural and persistent.

The EC value cannot tell you about the quality of your water. Sometimes hard water with an EC of 0.5 may still be high quality water for growing, while other water with the same EC could be bad or even very harmful to your plants because it contains the wrong salts and chemicals.

Reduced flows

At this heat enters the system, the combining of these elements increases, resulting in the deposition of insoluble materials on the inside of pumps, pipes, tubes and medium of the growing system. Ultimately this leads to reduced flows, blocked emitters, burnt out pumps and so on.

The effect on the chemical profile of the nutrient package can also be affected through various antagonistic relationships between individual elements and the overall affect on pH. The harder the water the more Calcium and Magnesium is being applied. The higher these elements get in relation to some other elements like Potassium and Phosphorus, the less available these elements become, effectively locking out these elements. These positive Ions will bring the pH of the solution up, and when hardness is also affected by carbonate levels, the pH effect will continue into the medium to which it is applied. The harder the water, the more acid is required to lower the pH.

There are a couple of commercial solutions to various situations of concentration and hardness. The first is Water Softening. Water softening involves flooding the water with a monovalent ion, typically Sodium, which drives out the Calcium and lowers the hardness of the water. This is great for clothes washing and baths but not so great for consumption by plants and humans, especially where the water is very hard.

The next is Reverse Osmosis (RO), a process where tap water is forced through a series of membranes with progressively smaller pores that block molecules and atoms of a certain size. This filters out the Calcium and other larger elements effectively lowering the water hardness. It also strips out most of all the elements including harmful molecules, Sodium ions, and most other ions thus effectively lowering overall Total Dissolved Solids and EC. It is also expensive to install and maintain and really not always necessary, at least to use pure Reverse Osmosis water.

Water sample

Decent nutrient companies should take the concerns of hard or soft water into account with the design of their products. Different lines have different needs in this area. Most of these differences are influenced by the medium the product is applied to. Potting mixes have a greater buffering capacity, the ability to hold elements, and should not be recirculated. Recirculating adds more heat to the system and allows deposits to form more readily. Potting mixes have natural buffers that hold pH changes down. The difference in content should be adjusted through the correct ratio of nutrients found in a fertilizer specially developed for potting mixes.

Only soft water is recommended for recirculating systems on inert mediums, so pure RO is acceptable in this system. Recirculating systems have to be able to adjust to, not only the hardness of the water, but also to the additional elements applied in the tap water over and above what is added or needed in the nutrient added. Controlling salt composition is critical because this also affects pH and pH is critical in signaling a plant’s flower response (in addition to photoperiod change). The best would be to use a nutrient which is designed to work with tap water EC values no greater than 0.3 – 0.4 mS/ cm while providing some buffering for pH control in the system (for example CANNA AQUA).

Another situation of current growing systems is the Run-To-Waste system where tank mixed nutrients are applied to a plant and the excess is allowed to drain away and not be re-captured. In this system, it is important to not only adjust the pH once it is mixed, but to maintain that pH across time as the product sits in a prepared tank. This keeps pH swings down while keeping insoluble compounds from forming. Also, there are less Calcium and Magnesium ions available in soft water and the amount needs to be augmented or replaced to achieve the correct disposition of ions. To make growing easier while allowing you to worry less about nutrient composition you should use a nutrient brand that has both a soft water and a hard water version (for RTW) to choose from (like CANNA SUBSTRA). How to know when to use the Hard Water version or the Soft Water version? Simple, see the above explanation and run a water sample.

So, what is gained with this knowledge: the appreciation of the fact that there are many aspects affecting water quality. Not only is the total amount of dissolved ions an issue, but also the composition of these elements and the effect they have on added nutrient packages and the post chemical reactions that can and will occur. Ultimately, it all affects the plant. Nutrients have to be designed and used based on the conditions of the water that the grower intends to utilize as source water. In the end, it is also about correctly designed nutrient packages that allow for both the plant’s nutrient requirements and the long term effect on plant development, and the effect of the medium on composition, storage, and reactivity. Testing is knowing, and knowing is growing; how much do you know?

Bibliography

Baily, D, T Bilderback, and D Bir. “Water considerations for container production of plants.” North Carolina State University Horticulture Information Leaflet 557. 1996.
Kessler Jr., J. R. “Water Quality Management for Greenhouse Production.” Alabama Cooperative Extension Service Publication ANR-1158. Alabama A&M and Auburn University, 2005.

Water types, quality and treatments

Good quality water is the foundation of all soilless growing, however not everyone is blessed with a suitable water source for hydroponics. Even crystal clear water may contain a range of minerals, water treatment chemicals and pathogens which can damage plants and slow growth. Luckily, water is relatively easy to treat and some growers choose to install small reverse osmosis (RO) units just to ensure their water is always top quality.

By Lynette Morgan, Suntec

Water types and potential problems

Water can be sourced from wells, or collected from roofs, streams, rivers or dams, but many growers are reliant on municipal or city water supplies and while these are usually safe to drink, they can sometimes pose problems for plant growth. The main quality problems encountered with different water types are as follows.

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Some water sources can carry plant disease pathogens such as Pythium which cause root browning and death if they take hold of a weakened plant.

Ground water (streams, rivers and dams)

Ground water sourced from rivers, streams or stored in dams/reservoirs typically poses the most problems for soilless growers, particularly if the water is not treated before use. Water which is continually exposed to air and soil becomes contaminated with organic matter, minerals leach from the surrounding area, and pathogen spore loading can be high. Many greenhouse operations use open air storage dams as an economic method of storing holding large volumes of water collected from greenhouse roofs or other surfaces, however this water is typically filtered and treated before use. River or stream water often has inconsistent water quality as operations being carried out up stream affect composition of the water and rainfall and flow rates also fluctuate throughout the year.

Well water

Water from wells in different locations around the world can vary considerably in quality. Very deep wells passing through certain soil layers will give an almost `filtered water’ although some minerals are always likely to be present in ground water. Some wells, particularly older types, or those which have been poorly maintained and are shallow can present problems with contamination from pathogens, nematodes and agrichemicals leached through the upper soil layers into the well water². Well water may be `hard’ and contain levels of dissolved minerals such as calcium and magnesium and other elements depending on the soil type surrounding the well.

High levels of sodium and trace elements are the most problematic for hydroponic growers, levels in excess of 2000ppm sodium have been found in inland well waters in some arid regions, although most well waters don’t pose such an extreme problem. Sodium is not taken up by plants to any large extent, hence accumulates in recirculating systems, displacing other elements. Trace elements in ground water, such as copper, boron and zinc may sometimes occur at high levels. Soilless growers utilizing well water are advised to have a complete analysis carried out on their water source to determine if any potential problems exist.

Rain water

Recirculating systems such as NFT can compound some water problems and unwanted elements such as sodium can accumulate over time.

Rain water is generally low in minerals, however acid rain from industrial areas, sodium from coastal sites and high pathogen spore loads from agricultural areas do still occur³. Much of this contamination has been found to happen when rain water falls on roof surfaces and picks up the organic matter, dust and pollutants which naturally collect there. In fact, numerous studies have shown that due to contamination following contact with catchments surfaces, stored rainwater often fails to meet the WHO guideline standards for drinking water especially with respect to microbial contamination³. In the USA, rainwater collected within 48km of urban centres is not recommended for drinking due to atmospheric pollution³. While drinking water standards don’t necessarily apply to hydroponic growing, the fact that high levels of microbial contamination often occur in stored rainwater means that common plant pathogen spores are also likely to be present. Rain water is best collected from clean surfaces with a ‘first flush’ device installed. which allows the first few minutes of rainfall to be discharged from the roof before any is collected for use.

Rain water may also contain traces of zinc and lead⁵ from galvanized roof surfaces or where lead flashings and paint may have been used⁴ and is a greater problem when the pH of the rain water is low. Generally, rain water collected from greenhouse roofs is free of zinc and lead problems.

Hard or soft water

‘Hard’ and ‘soft’ are terms used to describe the quality of many water sources. Hard water has a high mineral content, usually originating from magnesium, calcium carbonate, bicarbonate or calcium sulfate, which can cause hard, white lime scale to form on surfaces and growing equipment. Hard water may also have a high alkalinity and a high pH, meaning that considerably more acid is required to lower the pH in the hydroponic system to ideal levels. While hard water sources do contain useful minerals (Ca and Mg), these can upset the balance of the nutrient solution and make other ions less available for plant uptake. Smaller growers can counteract this by making use of one of the many ‘hard water’ nutrient products on the market. Soft water, by comparison, is a low mineral water source. Often rainwater is ‘soft’, while municipal water sources across the country range from very hard to soft, depending on where the individual city water supply is taken from.

Other water types

Some growers prefer to start with water which has been pre-treated to remove any chemicals, pathogens and other contaminates. RO (reverse osmosis), distilled water, filtered and bottled water are all options for small growing systems and those concerned with water quality.

City and Municipal water quality

Many city water sources are perfectly acceptable for soilless growers and hydroponic systems and can be used with no adjustment or treatment. However, the water treatment options used by city water suppliers change over time and with advancing technology. In the past, the main concern was chlorine in city water supplies. Chlorine is a disinfection agent which destroys bacteria and human pathogens, and residual chlorine can be detected by smell in a water source. High levels of chlorine can be toxic to sensitive plants, however chlorine dissipates rapidly into the air and can easily be removed by aerating the water or just letting the water sit or age for a few days before use.

Solution culture systems don’t have the buffering capacity of those using a soilless substrate so are more prone to problems with water quality.

While the chlorination of water supplies was easy to deal with, nowadays, city water treatment plants are moving more towards the use of other methods of treating drinking water. It has been found that some human pathogens were resistant to the action of chlorine, and consequently drinking water regulations were changed and alternative disinfection methods are being used more frequently. These days, water may still be chlorinated, but an increasing number of city water supplies have switched to use of ozone, UV light, chloramines, and chlorine dioxide. While many of these methods present no problem for hydroponics and soilless growers, the use of chloramines and other chemicals by many city water treatment plants can pose a problem for plants where high levels are regularly dosed into water supplies.

Chloramines are much more persistent than chlorine and take a lot longer to dissipate from treated water, hence they can build up in hydroponic systems and cause plant damage. Damage to plants caused by chloramines in city water supplies is also very difficult to diagnose as it looks similar to the damage caused by many root rot pathogens and growers are often unaware of what is causing the problem. Some plants are also naturally much more sensitive to chloramines than others, so determining levels of toxicity has also been difficult. One hydroponic research study has estimated that the critical level of chloramines at which lettuce plant growth was significantly inhibited was 0.18 mg Cl/g root fresh weight¹.

Hydroponic growers who have concerns about the use of chloramines in their city water supply can treat the water with specifically designed activated carbon filters or by using a dechloraminating chemical or water conditioners which are sold by the aquarium trade to treat the water for fish tanks. The chloramine carbon filters must be of the correct type that has a high quality granular activated carbon that allows for the long contact time required for chloramine removal. Growing systems that utilize substrates such as coco are a safer option than soilless culture or recirculating systems where water treatment chemicals are suspected to be a problem. Natural substrates provide a ‘buffering’ capacity in a similar way to soil and can deactivate some of the treatment chemicals contained in the water supply.

Other common water quality problems include the use of ‘water softener’ chemical either by city treatment plants, or in the home – these are often sodium salts which result in problematic sodium levels in the hydroponic nutrient. If sodium levels are too high, either through use of water softener chemicals or naturally occurring in the water supply, RO is the best option for sodium sensitive crops.

Tips and tricks for growers

How do you know if you have a water quality problem?

It can be very difficult to determine if a water quality issue is responsible for any plant growth problems which might be occurring. Many diseases and errors with nutrient management or incorrect environmental conditions will produce symptoms very similar to common water quality problems. Ideally, obtaining a full water analysis is useful for most growers, however detecting other issues such as chemical or microbial contamination is more complex. The simplest method of determining if water quality is the cause of growth problems is to run a seedling trial – growing sensitive seedlings such as lettuce using RO or distilled water as the ‘control’ or comparison will usually show up any problems originating from the water supply. Keeping all other factors such as nutrients, temperature and light the same between the plants in the different water samples and using a solution culture system will give the most accurate test. Comparing growth in the pure water to the suspected water sample will reveal any problems (if growth problems appear in both seedling treatment water samples, then something other than water quality is to blame). Water quality problems may show as stunted roots which don’t expand downwards, short, brown roots, yellowing of the new leaves, stunted foliage growth, sunken brown spots on the foliage, leaf burn and even plant death.

What to do about suspected microbial contamination

Zoosporic pathogenic fungi such as Pythium and bacteria can survive in and be distributed by water⁶. Water sources which may not have been treated and may contain disease pathogens such as ground, river or steam water can be relatively easily cleaned up by the grower before use. The safest options are UV, ozone and slow sand filtration as these won’t leave chemical residues which may harm young, sensitive root systems. Small UV treatment and filtration systems such as those used in fish ponds and aquariums are suitable for treating water for hydroponic use and will kill plant pathogens and algae. However these are best used for treating water only, not nutrient solutions as UV can make some nutrients unavailable for plant uptake.

Even clean, clear water may contain a range of minerals, water treatment chemical and pathogens which can damage plant growth.

What to do about other contaminates and treatment chemicals

Activated charcoal (slow) filters are still one of the more reliable and inexpensive ways of removing suspected contaminates from a water supply. Herbicides, pesticides, chlorine, chloramines, and other chemicals are reduced to low levels by suitable activated charcoal filters and these can be used by small and large growers alike. If chlorine alone is a problem, aerating the water for 48 hours by using a small air pump will dissipate this chemical. Using substrate-based systems incorporating a media such as coco fibre will give a greater degree of protection and ‘buffering’ capacity where chemical contaminates are suspected.

Aeration of chlorinated water supplies will cause the chlorine to dissipate, making the water safe to use in hydroponic systems.

What to do about excess minerals

Often it is possible to dilute a water supply which may have a slight excess in certain minerals, particularly trace elements, with a higher quality water source, however for water sources with a high natural salinity reverse osmosis or distillation are the only methods of demineralization. Some crops such as tomatoes are far more tolerant of excess minerals and salinity than others such as lettuce, so this factor should be taken into account.

What to do about ‘hard’ water with a high pH

Hard water is best treated with acid to lower the pH to 6.5 before adding any nutrients to make up the nutrient solution or before using the water to top up a nutrient reservoir. This will reduce the total amount of acid required in the system to keep pH under control. Hard water also contains minerals such as calcium and magnesium, so using a specific ‘hard water’ nutrient formulation or product in recirculating systems is advised, since these will keep nutrient ratios more in balance and also assist with keeping pH in check.

Micro-organisms in the growing medium

Micro-organisms are present everywhere; in the air, in water, on plants and in the soil. They can remain dormant for long periods in many different ways, as spores, mitochondrion (a membrane-enclosed organelle found in the cells of most organisms) or hyphae, and they can live up to several years and on a wide range of hosts. Although most people think of micro-organisms as being harmful by definition, life as we know it would not be possible without these minuscule life forms. In this article we will look at how they affect the growth of plants through their presence in the growing medium. By CANNA Research

Micro-organisms include a very broad range of species, and include bacteria, protozoa, algae and fungi. Most of these micro-organisms can multiply rapidly when the circumstances are right and they can have a major influence, both positive and negative, on the development of plants growing in the substrate. The kinds of micro- organism that appear in a given substrate will depend on a number of factors, such as the climate, the properties of the substrate, the plant species and the other organisms present in the substrate.

Micro-organisms can significantly affect the development of plants growing in the substrate, both positively and negatively. Many factors are important for micro-organisms, such as the type of growing medium. Others include the amount of oxygen, the temperature of the substrate and roots, the pH level and the salinity or EC level in the substrate. Another aspect that has quite an impact on the micro-life in the growth medium is the presence of pesticides.

Peat, coco coir, rock wool or perlite

One of the most important factors that influence micro-organisms is the type of growing medium available to them. The main differences can be found between organic substrates, such as coco coir, peat or soil, and inorganic substrates such as clay pebbles, perlite or rock wool. Micro-organisms that have a high saprophytic capacity (i.e. they live off dead organic matter) will do better in a substrate containing organic material such as peat or re-used substrate. But dead leaves lying on the surface of the substrate or infected plants with necrotic parts will do just as well.

The amount of oxygen in the substrate determines if aerobic or anaerobic micro-organisms will develop. Most plants need oxygen around their roots, so normally aerobic organisms will be present. Plants grown in a substrate with very little oxygen will usually be weaker and pathogenic anaerobic micro-organisms can benefit from that.

All organisms have an optimum growing temperature, so the temperature of the substrate and roots will affect the micro-flora around them. The average temperature and the temperature range (cold nights or a hot summer day) will determine which micro-organisms can survive, as well as the range and frequency of temperature fluctuations.

Fungus growth on rock wool cubes.

Just like plants, most micro-organisms prefer an acidity level of between pH 5.5 and 5.8. high and low pH levels, as well as pH fluctuations can disturb the development of micro-flora. still, some micro-organisms are able to grow or even flourish in extreme conditions. The salinity or EC level in the substrate will also affect the growth of the micro-organisms, and the type and composition of the salts in the growing medium also has an impact. This is especially true of salts such as Potassium chloride or Sodium chloride that can change the rhizosphere of plants and thereby the kinds of micro-life which will populate the root zone.

In inorganic substrates such as rock wool, clay pebbles or perlite, most micro-life is waterborne. These micro-organisms are introduced via watering systems, air or plants and will survive as long as the moisture level is sufficiently high, even if there are only small pockets of moisture. As with soil-borne micro-organisms, these can also be either beneficial or pathogenic.

Another aspect that has quite an impact on the micro-life in the growth medium is the presence of pesticides. These can build up in substrates or soils that are used for a prolonged period of time. Depending on the kind of pesticides – either herbicides, insecticides, fungicides or bactericides – these will influence the composition of microscopic life in the substrate.

One thing that one can rely on is the fact that micro-organisms can adapt to a range of circumstances. A well-known example is the resistance of some bacteria against antibiotics; it only takes a fractional change in their genetic material, but the effect on the resistance can be all-important.

The benefits of micro-organisms

The presence of micro-organisms can have both a positive and negative impact. As such, it is not necessary or desirable to get rid of all micro-organisms. The ability of a crop to defend itself against infections depends largely (albeit not exclusively) on the presence of micro-life in the substrate. It is difficult to quantify this benefit, however, since there are numerous factors on which the micro-life depends. The defense relates to the total microbial activity, the diversity of different groups of functional actinomycetes (rod-shaped bacteria), the total population of actinomycetes and the percentage of cellulose-decomposing actinomycetes.

If there is a good balance of micro-life in the substrate, there will most likely be less need to use pesticides and other measures, which will reduce costs. Not only are fewer and fewer pesticides actually permitted in horticulture and other applications, they are also very expensive. Plus, in a sterile substrate the most opportunistic micro-organisms will find a free space with no competitors and unlimited access to space and nutrients. These first opportunistic colonists will not necessarily be beneficial to the crop planted in the substrate. It is wiser to use the correct micro-life from the beginning to produce a healthy crop and good yield. Micro-organisms can even be used to improve the quality of reused substrates, where certain bacteria actually produce enzymes that can decompose accumulated salts.

Pythium infection on tomato roots in coco coir.

Substances that are exuded from the root system like sugars, amino acids or phenols can either attract or repel micro-organisms. The position of each species in the competition game that is continually going on between the bacteria can be influenced by these root exudates. Plants can use this to its advantage by exuding substances that attract beneficial organisms, establishing a symbiosis with a particular micro-organism. A prime example is the presence of symbiotic bacteria in the root nodules of leguminous plants such as peas or beans, which convert atmospheric nitrogen into a form that can be absorbed and used by the plant.

Another tactic is to introduce certain benign organisms to suppress pathogens. This works as follows. some micro-organisms are not very competitive and have a hard time colonizing a substrate that is already occupied by other micro-organisms. This can serve as a mechanism to get rid of pathogenic micro-organisms. By introducing beneficial micro-organisms such as mycorrhiza or trichoderma fungi into a clean substrate, the growing medium will become less inviting for pathogenic micro-organisms, thus protecting the plant from becoming infected.

Beneficial microbes compete with pathogens for nutrients or glucose and some antagonists have their own method of winning a competitive edge. For example, the fluorescent Pseudomonas bacteria can produce proteins that transform slightly soluble iron (Fe) into iron chelate, which it can then absorb much more easily. This then deprives the Fusarium fungi of the iron it needs to grow, preventing it from developing. competition for glucose can also cause microbiostasis which means that the spores of that pathogenic fungus germinate much more slowly due to a lack of energy from glucose.

Antagonistic micro-organisms can also block one or more stages of the propagation cycle of pathogens. Pseudomonas species P. Stutzeri, for example, interrupts the formation of conidia (asexual spores of several kinds of fungi) and the formation and germination of chlamydospores (thick-walled dormant spore of several kinds of fungi), but has no effect on mycelial growth (mycelial cords are capable of transferring nutrients over long distances). Pseudomonas can also produce antibiotics, which can be another tactic to remove pathogens, while other micro-organisms produce enzymes that attack the cell walls of competing species. Antagonists that produce chitinolytic enzymes have the potential to act against pathogenic fungi. It has also been found that several antagonistic organisms or closely related species can co-operate to fight a pathogen. Other antagonists simply overwhelm a pathogenic micro-organism by multiplying more rapidly and thus depriving all the competition of resources and therefore any chance of survival.

Algae development on rock wool cubes.

Pathogenic or harmful micro-organisms

Micro-life in the substrate also comes in the form of soil- or water-borne pathogens. some of these pathogens can attack over 80 different plant species and their resilience means they can be very important. There are many different harmful micro-organisms which result in a range of infections and symptoms (rotting fruits, fading, and necrosis to name but three).

Some pathogens produce micro-toxins which can attack the plant or the micro-life in the substrate. Pathogens can gain an advantage over other micro-organisms when they are able to germinate faster and are able to remain dormant for a longer period when conditions are not optimal. Pathogenic and non-pathogenic species can be very closely related, which makes it hard to use antagonists or other measures. Pathogens can fight back when they are attacked by antagonists. an example is fusarium, which can produce fusarium acid that affects the plant cells but can also suppress the production of antibiotics of Pseudomonas (this was discovered through research at Wageningen University in the Netherlands).

Balance your micro-organisms

Most infections by pathogens are in fact the result of a plant that was weak to begin with. Healthy plants are resilient and will be able to respond to an infection by micro-organisms. Provided the plant’s responses are quick and strong enough, it will be able to overcome such an infection. As such, ensuring an optimal climate and soil conditions for the crop is even more important than optimal growing conditions for (beneficial) micro-organisms.

It is crucial to get a good balance of micro-organisms in the substrate over a prolonged period of time, yet sometimes the growth cycle of a plant is simply too short to achieve this balance. Inoculating the substrate with antagonists is possible and, although the results are not always consistent, in some cases this can have a very positive effect on plant growth and health. In some cases, the results are comparable to the effect of using chemicals like fungicides, although these results may not last throughout the entire growing season.

Micro-organisms in the substrate can be a great help in suppressing plant diseases and a great deal of research is being conducted in this area of horticulture. Although this technology has still not entered the mainstream, research by the Louis Bolk Institute in the Netherlands has shown that introducing beneficial micro-organisms and or adding compost to increase the amount of micro-life can have a major effect on crop performance.

How air temperature affects plants

Most biological processes will speed up at higher temperatures, and this can have both positive and negative effects. For example, faster growth or fruit production is one benefit, in most cases. However, the excessive respiration that occurs is adverse because it means that there is less energy for fruit development and the fruits will be smaller. Some effects are short term, while others are longer term. The plant’s assimilation balance, for example, is influenced by the temperature and is affected immediately. Flower induction, on the other hand, is determined by the climate over a much longer period.

Spider Mite – Pest

Spider Mite – Pests & Diseases

Spider mites affect many crops worldwide. There are well over 1200 species of spider mite, of which more than a hundred can be considered as a pest, and about ten of those as a major pest. The most well known and problematic spider mite is Tetranychus urticae (common names include red spider mite and two-spotted spider mite). Their ability to reproduce extremely rapidly enables them to cause enormous damage in a short period of time. Spider mites have needle-like sucking mouth parts. They feed by penetrating the plant tissue with their mouth parts. Large populations can even cover entire plants with their web. These webs are used to move around. Because spider mites are so small they can easily move through ventilators.

About the pest in brief

What are spider mites?: Spider mites are not insects and are in fact more closely related to spiders. They belong to a class called Arachnida.

What can you see?: Spider mites usually spin a silk webbing. When spider mites infest plant leaves, they damage the plant tissue leaving yellowing and dead spots that coalesce until eventually the entire leaf is affected. The leaf will turn yellow, wilt and finally be shed. There are some varieties of mites that do not spin webs and live in the plants bud terminals, where the damage cannot be seen until the tip expands.

What can you do?: Spider mites have several natural enemies that can be used to control the population.

Biological cycle of spider mites

Each female two-spotted spider mite lays 10-20 eggs per day, and 80-120 altogether during its life cycle of up to four weeks. These are mostly attached to the silk webbing. The six-legged larvae hatch after 3-15 days. Newly hatched larvae are almost colorless and have bright red eyes. They moult three times within 4-5 days, becoming a protonymph, then a deutonymph and finally the adult form. Both adults and nymphs have eight legs.

Spider mite

Symptoms of the pest

The first visible symptoms will be small yellowish or whitish specks, mainly around the midrib and larger veins of the leaves. If these spots grow bigger and merge, the empty cells give some areas of the leaf a whitish or silvery-transparent appearance.

How to prevent the pest?

To minimize the risk and rapid spread of spider mite infestations, try to keep the temperature lower (60 %), since this will slow the rate of reproduction. Higher humidity is also needed for the predators of the spider mite. Keep your growing areas clean and remove all leaf litter. Adequate irrigation is important, because water-stressed plants are more likely to suffer damage.

Spider mite

Solutions for controlling the pest

When you see spider mites (recognizable from silk webbing on top of the leaves), remove the affected leaves. Spray the plant thoroughly with a mixture of alcohol and soap. Repeat this treatment several times a week.

You can also use natural enemies: predatory mites, ladybirds, predatory bugs and lacewings.

Whitefly – Pest

What are Whiteflies?
Whiteflies are hemipterous insects belonging to the Aleyrodidae family. They can cause considerable damage and loss of production.
What can you see?
Discolored patches on the parts of the leaf where the insects have been feeding.
What can you do?
One of the main objectives when controlling whitefly is to prevent the crop being infected by a virus that the insects can be carrying.

Whitefly are Hemiptera insects belonging to the Aleyrodidae family. They are considered a major pest for plumeria because they cause considerable damage. They feed by sucking the sap from the host plant. They are polyphagus, meaning that they feed on many different plants, and so they represent a hazard for the majority of crops, as well as feeding off wild plants and weeds that act as a reservoir for the pest.

The characteristic white color of these insects is due to a layer of white powder that covers both their bodies and their two pairs of wings.

The two species of whitefly that affect plumeria are Bemisia tabaci or tobacco whitefly and Trialeurodes vaporariorum or glasshouse whitefly. The main morphological difference that enables these insects to be distinguished from one another is the position of the wings. In B. tabaci, they are joined to the body and in T. vaporariorum they are parallel to the surface of the leaf. Furthermore, the adult and pupa of T. vaporariorum usually has a greater quantity of waxy powder than B. tabaci.

Biological cycle of Whitefly

The full life cycle of the whitefly lasts between 15 and 40 days, depending on environmental conditions, particularly the temperature, since eggs will develop into adults more quickly when the temperature is higher. The whitefly usually lays its eggs on the underside of the leaves, which the eggs stick to.

The whitefly usually lays its eggs on the underside of the leaves and the eggs stick to them by means of a pedicel. The larva or nymphs emerge from the eggs and in their first stage of development, they are mobile enough to move along the leaf until they find the right place to insert their stylus and begin to feed off the sap of the phloem, which is rich in sugars. The nymphs then pass through several more stages of development, during which they remain in the same place and continue to feed off the plant until the adult emerges from the last nymph stage. Non-fertilized eggs produce males while the fertilized eggs produce females.

Symptoms and Damage of the whitefly

The direct damage is caused to the plant when the whitefly feeds. The sucking of the sap causes discolored patches on the parts of the leaf where they have been feeding. Furthermore, as they suck out the sap, they release toxic substances into the phloem, which then spreads throughout the plant. This leads to metabolic imbalances in the plant and general weakening, chlorosis and changes to the flowers and fruit. In terms of indirect damage, the molasses excreted by the nymphs enables fungi, such as sooty mold (Capnodium sp.), to form on the leaves. This mold acts as a screen and reduces the photosynthetic capacity of the plant.

However, the most serious damage that the whitefly can cause to crops is the transmission of viruses. These include the TYLCV (Tomato yellow leaf curl virus), the ToCV (Tomato chlorosis crinivirus) or the TYMV (Tomato Yellow Mosaic Virus).

How to prevent the pest?

One of the main objectives when controlling whitefly is to avoid the crop being infected by any virus that the insects may be carrying. It is therefore important that, any weeds or remains of other plants, near the crop are removed because these can act as a habitat for whitefly. Furthermore, if a whitefly feeds off a weed that has a virus and then reaches your crop, the virus can easily spread. The use of protective barriers such as nets and covers are also a good option for preventing infestations.

Biological Control

A range of entomophagus insects, parasites, and some entomopathogenic fungi are used to control whitefly.

Most of the predators used feed on the eggs and nymphs of the whitefly. This category includes the Delphastus catalinae beetle. The chrysopidae larva and some bedbugs are also good biological controllers of this pest.

The small wasps of the Aphelinae family are parasites of the whitefly larva, where the wasps lay their eggs and they develop by feeding off their host. They are the most commonly used parasite wasps and are specific to the pest that they live off. This results in a quicker control of the pest, even though their specific nature means that they are not useful against other phytophagous insects.

Entomopathogenic fungi can also be used. This infects and grows inside the whitefly and eventually kills it. New spores emerge out from the corpse and infect other individuals. One example is the Verticillium lecanii fungus.

Whitefly (Bemisia tabaci) just emerged from its final nymphal stage, the fourth-instar nymph (pupa). Surface of hibiscus leaf. High magnification (5x) image showing the soft waxy appearance of this insect. This whitefly has a size of less than 1mm.

Cultural control measures

One of the main objectives when controlling whitefly is to avoid the crop being infected by a virus that the insect can carry.

Therefore, any weeds or remains of other plants that are near the crop should be removed as these can act as a habitat for the whitefly. Furthermore, if a whitefly feeds off a weed that contains a virus and then reaches your crop, the virus can easily be spread. The use of protective barriers such as nets and covers are also a good option for preventing infestations.

Phytosanitary treatments

The aim is to provide the plant with maximum protection during the earliest stages of the crop, thus preventing any whitefly from getting established. It is in these earliest stages that a viral infection will cause the greatest damage as the virus will spread throughout the plant and will show all its symptoms as the plant begins to produce blossom and fruit. This is why insecticides are applied to the seeds in some crops. These act systematically as soon as the seedling starts to grow and continue to protect it for several weeks.

In later stages, insecticides can be applied to the leaves to ensure the protection for the longest possible time. It should be noted that the use of non-systematic ingestion insecticides is not usually effective in combating whitefly in its larval stage, since many of the larva lack mobility. The use of insecticides that act by physical means are also a good choice to fight this larval stage.

Whitefly Pest

Hosts:

Recorded on plumeria and 38 genera of plants from 27 plant families and over 100 different species.

Common on plumeria, vegetables, ornamental, fruit and shade tree crops in Hawaii, including avocado, banana, bird-or-paradise, breadfruit, citrus, coconut, eggplant, kamani, Indian banyan, macadamia, mango, palm, paperbark, papaya, pepper, pikake, poinsettia, rose, sea grape, ti, and tropical almond.

Distribution:

Native to Central American and the Caribbean region. First reported in Hawaii in 1978 and now present on all of the major islands.

Damage:

Direct – damage caused by piercing and sucking of sap from foliage. Majority of feeding done during the first three nymphal stages. Usually insufficient to kill plants.

Indirect – damage due to accumulated honeydew and white, waxy flocculent material. The honeydew serves as a substrate for sooty mold, which blackens the leaf and decreases photosynthesis and plant vigor, and can cause disfigurement. The flocculent material is spread by the wind and can create an unsightly nuisance.

Virus transmission – damage from virus transmission can be considerable. These viruses cause over 40 diseases of vegetable and fiber crops worldwide.

Management:

This insect thrives in warm, dry weather. Heavy rains and cool temperatures may reduce populations.

Non-chemical control – five natural enemies were introduced into Hawaii from the Caribbean to control whitefly populations. One of the three coccinellid beetles (ladybugs) has proved effective with high population densities of whitefly. Two parasitic wasps have proven effective against low populations of whitefly. These biological controls generally provide adequate control to minimize damage to plants.

Chemical control – contact and systemic insecticides recommended for other pests on the same plant hosts may temporarily reduce whitefly populations. However, such insecticides may also harm whitefly predators and so should be avoided where possible.

The Spiraling Whitefly (Aleurodicus dispersus) has proven to be a nuisance and have caused damage to plumeria and native vegetation.

Root zone temperature and plant health

There are many aspects of crop and plant production that are critical for the success of the effort. One of the most often overlooked and seldom allow ed for aspects of production centers around the temperature of the root zone. After all, it is out of sight and there is not much that can be done about it. Besides, it must be OK to hold the entire plant at the same temperature, right? Wrong; and here is why.