Pythium
Pythium is a recurring problem for all horticulture and it has been of particular significance in the Hydroponic systems now used for the production of lettuce and tomatoes. From our observations, it seems to be an increasing problem and we hear the symptoms described quite regularly. If this is the case it may be due to the increased use of re-circulating systems or, more worryingly, to the emergence of stronger strains of the fungus which are more resistant to the normal prophylactics that have been used by commercial growers to date. It does also seem, from our phone calls, that hobby growers are encountering this problem more in recent months, although if this is so it will be difficult to advance any specific reason for it.
Pythium is a waterborne fungus and re-circulating Hydroponic systems provide it with an ideal environment in which to live and breed. Plants can survive and grow with high levels of inoculum in the nutrient solution. The fungus however will restrict the root system. A sudden rise in temperature will find the plants unable to increase their uptake of water and they will wilt. For many growers, this is the first sign that Pythium is active in their system.
What is Pythium?
Pythium root rot can be caused by several different species of the fungus Pythium. There is also a number of similar pathogenic organisms that can attack the roots of plants in the Hydroponic system. For the horticulturist and for the purposes of this brief article, the term Pythium is used generically, to describe fungal organisms of the Pythium family and others that behave in a similar way. These fungi are common in field soils, sand or sediment of surface water supplies, and dead roots of previous crops. Pythium is easily introduced into Hydroponic systems by using dirty tools, by introducing infected plant material, or, most commonly, by way of the raw water supply. When introduced into re-circulating Hydroponic system such as Flood & Drain or NFT, Pythium can cause severe root rot because it has few competitors to check its activity. It can procreate easily in warm conditions and transmit its spores very quickly throughout the system.
Almost all commercial crops are susceptible to Pythium root rot. When a plant is attacked by the colonizing fungal spores, the root tips, which are very important in taking up nutrients and water, can be damaged and eventually killed. Pythium can also rot the base of unrooted cuttings and attack small seedlings, a condition known as âdamping offâ.
The Hydroponic grower can encounter Pythium at any time and if he is unprepared he may well lose his crop. The fight against fungal root disease is ongoing and the careful grower will have it in mind from the day he starts to plan his system.
There are three avenues for the grower to explore in the battle against root rot disease. The first step, as in so many things is prevention. By maintaining a clean growing environment and by treating the incoming raw water the grower can minimize the opportunities for a fungal infection to enter the system. The second area of interest is in the optimization of system design. A well-designed system will be an unattractive environment for fungal spores to develop in and this represents the most important defense against them. Finally, we will need to look at the various prophylactics on offer in terms of tank additives and treatments designed to eliminate pathogenic organisms from the actual operational system.
Symptoms: Recognize the enemy
Hydroponic growers may well have at least one advantage over traditional ones as Pythium infection tends to show up in the roots before it has many effects on the upper part of the plant. Hydroponic growers usually see much more of their root system than conventional growers and are thus more likely to have some warning of the problem. This is especially true with NFT systems, as the roots are so visible and accessible. Roots that are subject to infection from Pythium or related fungal pathogens will begin to change from white to a light tan color. The roots will tend to lose their glossy appearance and become dull brown in color. Progressively the root mat turns browner and the roots will eventually soften and break up. It may be found that the brown tissue on the root surface can be pulled away exposing the stronger vascular tissue beneath. It will be a normal part of good management to check the root mat in an NFT system on a daily basis.
The condition of the roots can be the first indicator of other problems in the system as well as a warning of fungal problems. With a good microscope, it will be quite possible to see the thick-walled Pythium spores in the root cells. Not every Hydroponic system allows access to the root mat and Pythium may be harder to detect in Rockwool systems or in Perlite. Here is a list of symptoms that have been reported in connection with Pythium infection. ⢠Plants exhibit stunted growth. Leaves tend to curl downwards and become marked with necrotic spots.
⢠Root tips are brown and dead. Further browning and softening of the roots will follow.
⢠Plants wilt at mid-day and may recover at night. This is a common symptom associated with root problems in Hydroponic systems.
⢠Plants yellow and die. This is also, unfortunately, a symptom of many other problems.
If a pathogen problem is suspected it will be possible to get confirmation by sending samples to a good diagnostic laboratory. They will require a sample of the damaged root tissue and possibly of the nutrient solution as well. However, if you do not wish to use a laboratory then it will be possible to buy a test kit and do a bio-assay in the comfort of your own home.
Test kits are now available to make the identification of fungal problems much easier. The ALERT kits from The Scottish Agricultural College are based on a reliable serological procedure known as Enzyme-Linked Immuno Sorbent Assay. This is a very similar process to that used in pregnancy test kits and the AIDS test. Tests can be performed on any part of the plant and can be accurate enough to detect pathogen-specific protein concentrations as low as 10-100 mg/ml. That is to say the kits can detect 1% diseased tissue in 99% healthy tissue with 95% accuracy.
Cleanliness and sterilization
Prevention in this case is certainly better than cure. Pythium root rot is difficult to control once damage to the root mat is visible. Every effort should be directed toward preventing the disease before it begins. Prevention begins with a policy of total cleanliness. It must be remembered that in the greenhouse or grow-room the debris from past crops can remain dry and free from weathering and thus escape the chemical, physical and biological degradation that occurs in the open air. Virtually all crop disease organisms can survive in plant debris for months or years in a dry state, even in invisible dust on the floor. Organisms can also survive on tools, pots, implements such as meters, culture trays and clothing. Hygiene is therefore a primal requirement for overall disease control.
The entire growing area should be kept clean at all times. Dead leaves should always be removed from the plant and destroyed. Keep all soils and composts out of the Hydroponic grow-room or greenhouse. Do not convert plants from soil into Hydroponic systems. Do not enter growing area with outdoor footwear on. Do not bring any plant material in from outside if you can avoid it.
Sterilize all pots, gullies, trays and tanks between crops. Nutrient tanks should be kept scrupulously clean. Do not allow soil or plant material to fall into tanks at any time. Keep tanks covered except for airflow requirements.
Disinfect all surfaces, tools, and equipment that will contact the plants or the nutrient solution. Sterilize the entire system between crops.
Until fairly recently, horticultural sterilization was mainly carried out using Calcium Hypochlorite. This is the active agent in household bleach and is also widely used in swimming pools. Hypochlorite can be circulated through the entire system between crops. Effective concentration would be around 100-200 ppm and it should be circulated around the system for at least four hours. Hypochlorite is an effective sterilant but it is also very phyto-toxic in concentration so there needs to be plenty of flushing and rinsing of the system before re-planting.
Remember here that the water used for flushing must also be pre-treated or the pathogens can just be re-introduced to the system. Growers who have skimped on this step have lived to regret it as residual chlorine has caused many a crop failure in the early stages. For this reason Hypochlorite has to some extent been supplanted by horticultural preparations containing Hydrogen Peroxide H202. This chemical is highly oxidative and for the reason is as efficient a sterilant as Hypochlorite. It should be flushed through the system at a concentration of around 500-ppm. The attractive feature of Peroxide however is that it rapidly breaks down and the extra labor cost of flushing or rinsing the system can be avoided. The grower would simply allow a couple of days for the compound to break down before introducing plants into the system. Another effective sterilant is Formaldehyde but this is an unpleasant liquid to handle and, like Hypochlorite, it can leave a toxic residue in the re-circulating system.
There are now products on the market that are designed specifically to sterilize plant and equipment in horticultural situations. One such product is Antec Internationalâs broad-spectrum disinfectant Virkon which has shown a promising capacity to destroy infective organisms without leaving any dangerous residues.
Once the system is thoroughly disinfected it can be replanted. Nothing should be introduced at this stage that could be a possible source of re-infection. Recognized Hydroponic media such as RockWool, Perlite and expanded clay (LECA) are sterile when shipped but be sure to check that they are stored properly and not allowed to come into contact with potential sources of pathogens. These media can be re-used but only when they have been thoroughly sterilized. Perlite and clay can be soaked in a chlorine solution or in H2O2 and then thoroughly rinsed before use. Rockwool is difficult to sterilize and commercial growers find that steaming is the most effective method. Hobby growers are usually advised against re-cycling Rockwool at any time. The water supply.
Once the grower has a clean sterile system and non-infected plants it should be possible to maintain a disease-free environment. However many growers have been undone by not paying sufficient attention to the water supply. It has generally been accepted that town water supplies are free of infection because they have been treated with Chlorine. Some authorities however have begun to question whether the level of chlorination is adequate, as fungal organisms may become resistant to low levels. While town water is usually considered safe water from dams, rivers or bores is likely to contain bacteria and the free swimming zoospores of pathogenic fungi and should certainly be treated before use in Hydroponic system. There are a number of treatments available, none of them entirely satisfactory. Some of these treatments are suitable only for the raw water as it comes into the system. Filtration for example would remove nutrient ions from the solution if it were incorporated into the crop system. Most of these options however can be applied to the circulating nutrient as well as to the inlet water. This offers the advantage of both stopping most pathogens from entering the system and then collecting those that do in a secondary process.
Chlorination
Water can be held in a tank and chlorinated before being used in the Hydroponic system. Calcium Hypochlorite should be added in quantities to yield 10-20ppm Chlorine and mixed thoroughly before being allowed to stand until chlorine content has dropped to zero. Cheap kits to test chlorine levels are available from swimming pool supply shops. If using a Chlorine test kit be sure to use the DPD type and not the OTO type. This method is pretty effective but it can be inconvenient for the grower to have an extra tank of water standing around. If the raw water is treated in this way you can be pretty certain that it will reach the system in an abiotic state. Chlorine can also be added to the nutrient tank as an operational routine (see below).
Hydrogen Peroxide H202
Hydrogen Peroxide is widely used as a tank additive and is sold in the Hydroponic industry under a variety of trade names such as Oxy-Plus. It would certainly be useful as a treatment for raw water and should be added at around 500ppm and allowed to stand for a couple of days before use. Like Chlorine, it can be added to the nutrient tank on a regular basis and this would seem to be an attractive option for the grower. The breakdown of H2O2 in solution will release free Oxygen ions, which will circulate in the system and are very much available to the plant. In recent years this has become a very widely used tank additive and is considered valuable as a cleansing and oxygenating agent, much used by commercial growers.
Filtration
The raw water can be passed through 5-micron filter units, which will remove most, but not all, pathogens. Filters are expensive and will require regular maintenance or they will cease to function efficiently.
Ultra Violet
The raw water can be passed through special tubes in which it is bathed in UV light. UV is highly efficient at killing pathogens and, provided that exposure times are adequate, it will be quite an effective way of treating the inlet water. However the requirement for adequate exposure has a limiting effect on the flow rates and it can be quite a slow process if there are large tanks to be filled. Perhaps the UV option has more utility as a nutrient tank treatment, see below.
Slow Sand filtration
This is an efficient and low cost way of treating the inlet water but it will be unsuitable for the hobby or small commercial grower. In this system the inlet water is passed through a bed of fine sand and as it passes downwards it forms a filter skin on the surface of the bed. This skin is the key to the success of slow sand filtration as it is home to a wide variety of biologically active micro-organisms, which break down the organic matter.
System design
A well-designed Hydroponic system will be a less attractive environment for fungal organisms and it is in system design that the grower builds his most effective protection against pathogens. The key to successful design is in the nutrient tank and in the passage of solution around the system. Fungal pathogens seem to thrive in warm stagnant nutrient solutions so the primary requirement of design is to move the nutrient around quickly and to keep the oxygen levels as high as possible.
The Main Tank
Nutrient tank should be as large as possible. Commercial NFT system often have very large tanks but they are servicing large numbers of plants and the ratio is sometimes as low as 1 Litre per plant. The main problem with this situation is that liquid uptake through the day will be a large proportion of the tank and as the raw water comes in to replenish it there will be large variations in pH. Commercial growers will tend to use automatic controllers for pH so this is not a real problem but for the small grower it will be safer to use a proportionally larger tank. In a large tank, say 5 Liters per plant for example; the variations in pH will be slower and much less dramatic. A stable pH is very important, as extreme fluctuations have been shown to cause physical damage to the root mat, which can then provide an entry point for fungal pathogens to attack the plant.
The nutrient tank should be well aerated and the dissolved oxygen content of the solution should be as high as possible. The most effective way to introduce Oxygen into the tank is by dissolving it at the surface. The surface level of the water is called the meniscus and it has been demonstrated that about 90% of the dissolved oxygen, known as DO, in a system is introduced at this point. Contrary to popular belief, air-stones and bubble devices attached to air pumps are not very effective at all, in fact the only way in which they really add oxygen to the tank is by causing disruption of the surface layer allowing the dissolution of free oxygen from the atmosphere. The bubbles themselves are too small and too inhibited by surface tension to release significant amounts of oxygen into the solution. The Hydroponic grower can get far more agitation of the surface layer, and thus a much greater Oxygen gain, by using his existing pumping capacity. The main requirement is a decent-sized pump that has at least twice the required capacity. Excess pump capacity can then be used in a bypass, see fig 1. This bypass layout has many advantages for the grower. It allows him to turn of one tap B, shutting off supply to the plants. All the pump capacity will now be directed through the bypass causing considerable agitation in the tank. Now is the time to add nutrient concentrate and to check and correct pH. These jobs will be quick and easy as there is no nutrient in the system and the mixing is quickly performed inside the tank. Once the solution is correct the tap can be opened to send it out to the plants once more. It is usual to keep the tap to the plants wide open and then close down the bypass tap A until flow rate through the system is correct. This should leave plenty of flow through the bypass and a churning tank. If this does not happen we would suggest a bigger pump. To increase the agitation of the tank surface the return pipe should be situated well above the level so that it falls back in. This may be somewhat noisy but it will certainly improve the dissolution of oxygen at the surface level. Note that the bypass should be situated just above the pump. This will ensure that the pump is drawing water with the maximum possible Oxygen load.
Once there is adequate movement on the tank surface it is important to consider the air movement in the area. If the tank is sealed, or has a lid, there will be inadequate fresh air at the interface and thus no oxygen gain. Plant roots give off various compounds during the growing cycle, most commonly gases of the ethylene family which are heavier than air and which can flow down the gullies and collect in the tank. If the tank is sealed then the agitated nutrient could be dissolving ethylene rather than oxygen. This could possibly contribute to the kind of anaerobic environment favored by Pythium and other pathogens. It is very important to keep the tank uncovered, or only partially covered and make sure that there is adequate provision for air movement across the surface.
A small fan blowing across the top of the tank, or through an opening in the lid will be a very good idea indeed. Fresh air blowing across a highly agitated surface will be far more useful in terms of Oxygen than air stones could ever be.
We can now assume that the nutrient in the tank is holding as much oxygen as is possible for the conditions. If the grower is unsure about this it will be possible to purchase a DO meter to check the system. It will be especially instructive to compare DO readings in the tank with those obtained from the return supply. Remember as temperature rises so the capacity to carry oxygen is reduced. In our opinion nutrient tanks should not be heated under normal circumstances. There have been many attempts to demonstrate an advantage to a heated solution but the best results, in terms of improved growth or yield, have been pretty marginal. For the small grower it is possibly better to maintain a reasonable temperature in the growing area and keep the nutrient relatively cool to maximize DO potential. Obviously it should not be allowed to get to cold and a temperature between 18 and 22 0C will be ideal. Once the richly aerated solution leaves the tank it begins to lose its oxygen and it is a fundamental of system design to get that nutrient back into the main tank as quickly as possible. Thus the layout should try to make the journey as short and as quick as possible. This means that gullies or growing trays should not be too long. It also must seek to avoid any dams or areas of puddling or anything that holds up the progress of the nutrient solution. If nutrient solution forms puddles in the root area it will quickly become depleted of oxygen and root die-back can result. An early solution to this problem was the insertion of a fibrous spreader mat so that roots could grow laterally without concentrating on the root-gully interface. Unfortunately, most of the materials tested have proved too porous, allowing roots to continue growing along the bottom of the interface. For this reason the use of spreader mat in NFT system is not really to be recommended. It is not really used anymore to encourage lateral root growth but rather to cover up bad system design, to spread the nutrient across a tray or channel, when really this requirement should be an integral part of the channel design.
Soggy wet fabric will not in itself cause fungal problems but if Pythium spores are introduced into the system it may well prove a suitable habitation for them. A far better design would include a false bottom or collecting gully to allow the swift, unhindered passage of nutrient beneath the root mat. This fast moving solution will allow for adequate oxygen exchange beneath the roots and ensure that a large portion of the total solution will be returned to the tank in the shortest possible time. The careful grower will compare DO readings in the tank with those obtained from the return pipe and the fine tune the system to reduce the shortfall. This may mean a different layout or improved trays or gulleyâs. In any case it will be a fundamental requirement in the fight against Pythium as well as in the optimization of plant performance and yield improvement.
Tank additives and nutrient treatments
Despite the best efforts of the grower to start with a sterile system and the best system design there will eventually come a time when an infective presence is detected within the crop. The final resort will come from a range of treatments designed for the nutrient tank and supply system. In most cases there is a suggested level for the attempted elimination of an existing problem followed by a treatment at lower levels, designed to discourage re-infection.
