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Interactions between disease organisms, hosts and environment .
The number of NFT and bag crops severely affected by root rotting diseases is only a small proportion of all the crops grown hydroponically in spite of the ability of the fungi concerned to be extremely damaging in NFT systems. In many instances there may be many apparently healthy plants within a crop where random individual plants are showing severe root disease symptoms. Pythium has been consistently isolated from healthy lettuce root systems as well as from lettuce with diseased roots in experimental work with NFT (3). Pythium was also isolated consistently by baiting tests and isolations from NFT solutions on a commercial strawberry crop which was healthy (4). It has been suggested that the development of epidemics of Phytophthora root rot in tomatoes in NFT is the result of a complex interaction between root growth and pathogen development (5). The development of root disease in hydroponic systems may be due not only to introduction of the causal fungi into the system but also to the condition of the crop and the nutrient solution. Funck-Jensen and Hockenhull(3) have suggested that Pythium spp. can exist in NFT as saprophytes, and that their growth(as saprophyte or parasites) is dependent on food supply ( substrate availability). The substrate is the combination of root exudates and breakdown products of dead root tissue. Competition from other micro-organism in the NFT system for substrate must also affect the growth of the pathogens. Adding sucrose to the NFT solution provided Pythium with an alternate food source and resulted in much greater root death (3). Natural die back of tomato roots (root death) which commonly occurs about the time of first harvests, may also release large amounts of substrate for root rotting fungi and set off root disease. The abundance of root exudates can be influenced by a number of factors including light intensity, temperature and fruit load in crops such as tomatoes. Reduced substrate availability to the fungi may well trigger increased oospore production. The overall severity of root damage and effect on crop performance may well be the result of the balance between rate of root attack by the fungi versus rate of new root growth. Tomatoes in rockwool blocks maintained at 15° died following inoculation with Phytophthora, while similar plants with rockwool kept at 25° remained symptomless although the fungus could detected on their roots (6). Root growth rate is highly dependent on the supply of assimilates from the leaves to the roots, and this can be reduced by heavy fruit load, deleafing, poor light or other growth checks. Bacterial canker of tomatoes (caused by Clavibacter michaganensis) has been shown to be transmissible through NFT solutions (7), although in our experience the primary spread in tomatoes in NZ is not through the solution. Recent work has shown that the population of canker bacteria in NFT solution, and the incidence of the disease was much lower when the solution was kept at pH 5.0 than at pH 6.0, and that the survival lifetime of the bacteria in NFT solution was very short at low pH (8). The environment surrounding the root affects both the pathogen and host plant, and conditions unfavourable to the host (such as low root temperatures) may favour the pathogen. Any check to plant growth, or physical damage to the roots can weaken the host and increase susceptibility to disease. Checks can be due to a fruit overload and lack of sufficient assimilate to maintain root health, especially in dull weather, due to cold conditions, or sudden exposure of the roots to very high or very low CF or pH.
Sources of disease infection
Undoubtedly the most common source of disease is the planting of infected disease carrying (but often symptom free) plants into hydroponics systems. Pythium infections were found in from 1 to 76% of symptomless lettuce plants from two commercial nurseries in Belgium(9). Many transplants used in hydroponic systems in NZ are propagated in peat or bark based composts, which are not generally sterilised before use, and carry some risk of pathogen infestation. Rockwool cubes and blocks are sterile and are ideal for propagation.Rockwool slabs are a sterile growing medium. Peat and coir are not uncommonly infected by Pythium, while bark is frequently claimed to have some fungistatic properties. The infection risk depends very much on how hygienically potting composts are handled during preparation and use. Steam sterilisation of peat and bark is not desirable because of increased toxicity risks, methyl bromide sterilisation is safe and effective but its use is now prohibited . Rockwool should be nearly sterile when delivered, and the disease risks of new pumice and sawdust for bag filling should be very low provided that they are hygienically handled. Infected water sources and greenhouse flooding by surface water is very common cause of disease, as is carry over of disease within the greenhouse, on the hydroponic system, or on the floor or in the soil. Disease risks are quite high with water taken from rivers, streams and shallow bores. Even water collected from greenhouse roofs was found to infected with the tomato crown and root rot fungus (Fusarium oxysporum f.sp. radicis lycopersici) in Holland (10). Pythium, Phytophthora and Fusarium species can be brought into hydroponic systems by sciarids and shore flies. The spread of disease in hydroponic systems is not limited to root fungal pathogens but includes virus which can be introduced to the tops of the crop plants by many means including insect vectors, but which can then be carried via the nutrient solution to infect other plants through their roots. Cucumber green mottle virus is an outstanding and long recognised example of such transmission.
Spread of disease within hydroponic systems
Not all of the root diseases occurring in hydroponic systems are readily dispersed or transported by recirculating nutrient solutions, but those fungi that are well adapted to living in water provide the most severe disease problems. Pythium, Phytophthora and Rhizoctonia species are often referred to as water moulds as they are particularly well adapted to water borne dispersal with the first two having motile (swimming) zoospores as well as other spores and fruiting bodies that can be carried by water. The corky root fungus ( Pyrenochaeta lycopersici ) is not spread in NFT solutions, but Didymella lycopersici can be spread in NFT solutions, although such spread does not normally result in stem lesions (7). Most diseases caused by Fusarium species, and by some Verticilium species, and black dot disease of tomato roots (Colletotrichum sp.) appear to spread well through nutrient solutions, as do many bacterial and virus diseases. Nematodes are not general a problem in hydroponically grown food crops in NZ, but are readily spread in recirculating nutrient solutions.
DISEASE MANAGEMENT IN HYDROPONIC SYSTEMS
Disease management can be approached from two distinct directions, the first attempts to completely eliminate any disease organisms from the hydroponic system, and the second approach is to limit the crop loss from disease by a variety of methods. The first approach might be futile given the practically impossible task of maintaining completely sterile conditions in large scale greenhouses, and it seems that the best approach is one that manages disease incidence and disease effects on greenhouse crops. Root disease management plans must use consider the following:
1. It is essential that only clean disease free water is used. Water samples can be tested for the presence of pathogens by plant pathology laboratories. The water should be treated if pathogens are found, but a failure to find pathogens in any one sample cannot be taken as evidence that the water supply is disease free. All water from greenhouse roofs, dams, springs, rivers, and shallow bores should be treated and only water from deep bores should be considered as pathogen free.
2. It is essential that only disease free planting material is used.
3. Good hygiene and control of vectors such as sciarids and shore flies is essential.
4. Sterilisation of recirculating nutrient solutions. The Europeans are very strong advocates of some form of sterilisation of recirculated nutrient solutions, especially for crops in rockwool and pumice. However, there are still some European growers who recirculate nutrients solution without any form of sterilisation treatment, and the usual philosophy is that sterilisation treatments are a form of insurance to reduce the risk of disease spread through recirculated solutions.
5. Use biological controls. There is considerable research on microbiological antagonists of root pathogens and the use of micro-organisms for cross protection against specific diseases. Organisms used include a variety of bacteria and fungi including Pseudomonads, Fusarium species and Trichoderma species. There are a number of commercially available products containing Trichoderma. While encouraging results have been obtained in some instances, no practical control methods giving complete control have so far emerged.
6. Root growth promoters are commercially available. There are a variety of products with claims for improving weak root growth, including inorganic materials and organic material reputedly containing hormones and other growth promoters ( including seaweed extracts), but few if any of these products have been subjected to any rigorous scientific testing. Never the less some growers have obtained good results from some of these products.
7. Fungicide incorporation in nutrient solutions. Treatments of recirculating systems with chemical fungicides is sometimes recommended, more commonly for ornamental crops than food crops, but few chemicals have specific activities against pathogens without harming the competing microflora.
Water and solution disinfection methods
Disinfection methods are easier to apply to solutions recirculated from hydroponic systems using solid media (rockwool, pumice, scoria, sawdust etc.) than to solution cultures (NFT and deep flow systems) because of the relatively small volumes recycled from solid media and the huge volumes circulating daily in solution cultures. The usual system collects the drain water for recirculation in a holding tank. Batches of solution from this tank are passed through the disinfection unit and into another tank holding the clean (disinfected) recirculating nutrient solution. This clean solution is blended with clean (pathogen free) fresh water, more nutrients are added to the correct EC before delivery as feed to the crop through trickle irrigation.
Heat sterilisation is accomplished by passing the solution through a plate heat exchanger where the solution is kept at 97° for 10 seconds. A second plate heat exchanger is used to cool the solution and recover heat to improve the efficiency of the process. The process kills all bacteria, fungi and bacteria (12). Unfortunately the process is very expensive to operate and uses 1 m³ of natural gas for heating each m³ of drainwater sterilised. More recently it has been found easier to use a lower temperature of 85° for 30 seconds, as this allows the hot water required to be drawn from the normal hot water boilers used for greenhouse heating. Heat sterilisation of nutrient solution has the highest operating cost of any treatment method, but is the most effective.
Ultra violet radiation can be used for treating nutrient solution. Ultra violet radiation (light with a wavelength of 254 nanometers) damages cellular nucleic acids in all living organisms, and organisms receiving a large enough dose of UV radiation are killed. Water treatment by UV is achieved by shining light from either low or high pressure UV lamps through a thin layer of flowing solution. The solution must be clean and clear or light penetration through the solution is limited. The effectiveness in killing micro-organisms depends on entirely on the dose given, and the dose required to kill varies for each species. In general bacterial are killed readily at relatively low doses, pathogenic fungi require higher doses, and different parts of any fungus may have different lethal doses, so that spores might be killed more easily than pieces of mycellium; the highest doses are required to kill viruses(13). A recent practical recommendation is that 100 mJ/cm² should be used to treat recirculating solution for fungal pathogen control or 250 mJ/cm² for virus control (14). It is very important that the best UV treatment equipment be installed and operated within the manufacturers specifications if the proper dose is to be obtained. Practical requirements include prefiltering the treated water to obtain best light transmission and using the correct water flow rate as either too low or too high a flow rate will reduce the efficiency. UV water treatment equipment is usually specified in term of light intensity (mW/cm²) and the dose is then the intensity multiplied by the exposure time (expressed either as mW.seconds/cm² or as mJ/cm²). Complete treatment of all the drainage water in media systems is quite feasible with UV, but with NFT systems only partial treatment is feasible. A number of clients have used UV in this way for NFT systems, resulting in reduced root disease, but not completely preventing root disease in all cases. UV treatment completely eliminated Pythium from the solution in an experimental NFT lettuce system, but accidentally introduced Pythium was not controlled (15). In another experiment with NFT lettuce UV sterilisation treatment did not result in complete sterilisation but did result in 4 consecutive lettuce crops without disease (16).
Ozone (O 3 ) is a very powerful oxidising agent, and treatment of water or nutrient solutions with ozone can result in the elimination of bacterial and fungal pathogens and viruses. Disease control is complete if the redox potential of the treated solution is increased to 750 mV. In commercial installations in Holland this required treatment of 1 m >SUP< 3 of drainwater with 10 g ozone for one hour. The reliability of redox potential as a guide to zone concentration may be questionable, but it is a simple and effective measurement (17 and 18) using relatively low cost redox meters. Effective ozone treatment is not easy. Ozone treatment systems available in NZ use a venturi installed in pipe through which there is a steady flow of solution to draw air through a an ozone generator. Some of the oxygen in the air flowing through the generator is converted to ozone , and the ozone enriched air is discharged as a stream of bubbles into the solution flowing through the venturi. The ozone has to dissolve into the solution from these bubbles, and the contact time for solution is critical. The discharge needs to pass into a deep solution tank or an absorption tower for maximum efficiency. Much of the oxidising effect of the ozone is spent on organic matter and other materials in the solution, and when solutions contain many reducing agents it is difficult to achieve a high redox potential. The efficiency of ozonation can be increased very considerably by lowering the treated solution to pH 4.0 by adding nitric acid before treatment (17). Ozone treatment systems must be installed in a way that avoids ozone air pollution as ozone is dangerous to human and plant health.
Some clients, and particularly clients growing lettuce in NFT systems have installed ozone for partial treatment of NFT solutions. Only in very few cases have growers been able to achieve high redox potentials in the nutrient solutions, but in spite of this growers believe that disease incidence is reduced. Lettuce growers also, and probably wrongly, believe that ozone is carried over in the solution flowing down the gullies, but ozone has a very short half life, and this effect seems unlikely.
Other effects of UV and Ozone. Both UV treatment and Ozone breakdown some of the iron chelate in the solution and may cause some precipitation of manganese compounds(19). They also breakdown many complex organic compounds including insecticides and fungicides in nutrient solutions. Some insecticide and fungicide breakdown products can be extremely phytotoxic. Ozone can be quite damaging to some plastic components of NFT systems.
Slow Sand Filtration Slow sand filtration is being rapidly developed as a means of treating drainage water from hydroponic systems. In the original systems the drainage water percolated through a deep bed (800-1200 mm) of very fine filter sand (O-2 mm particles with an effective median diameter between 0.15 and 0.3 mm). A flow rate of 100-300 litres per hour per m² of filter surface area was achieved when the water depth above the sand was about 800 mm. (20-23).Early filters were effective in in controlling bacteria, Pythium and Phytophthora but less effective for Fusarium (13). Continuing research has found that slightly coarser sand and other media can be used and special rockwool granules may be the most effective filter medium. The effectiveness of slow sand filtration was originally believed to be due to the build up of particular microflora within the filter bed but it is now believed that both biological and mechanical filtration effects are involved. Slow sand filters are more effective after they have aged for some time. Some research workers believe that slow sand filtration will reduce bacteria, fungi and viruses, but others have found Fusarium and tomato mosaic virus in the effluent from slow sand filters. (20-23)
Disinfection by chemical dosing. Chlorination is a very old method of disinfecting water, but it is usually recognised that doses effective in eliminating fungal pathogens are phytotoxic to greenhouse crops, although it has been recommended as a treatment for fresh water in Australia (18). Hydrogen peroxide has also been suggested for treatment of raw water and nutrient solutions. Hydrogen peroxide is a much weaker oxidising agent than ozone, and relatively large amounts of hydrogen peroxide have to be used (100 ppm for 5 minutes to kill condia of Fusarium oxysporum f.sp lycopersici) and these rates are phytotoxic to crops,, but research is still in progress (24). Iodine has recently been used for disinfecting recirculating NFT solution growing lettuce ( )
Disinfection methods for raw water It is vital that the raw water used for hydroponics is free from pathogens. All surface water roof water and water from shallow bores or bores know to be contaminated with pathogens should be treated. Water from reverse osmosis plants and water treated by membrane filtration will be free from fungal and bacterial pathogens. Water treatment by UV is relatively simple and effective.
Fungicide treatments applied to nutrient solutions.
In many countries overseas fungicide treatments of nutrient solution used for food crops are illegal, and while not illegal in NZ, safety is somewhat questionable. The effectiveness of most fungicides is also very limited, particularly as primary pathogens and especially Pythium readily mutate and become resistant to repeated applications of the same fungicide. There is also considerable variation in susceptibility to specific fungicides between different species of Pythium, and the usual diagnoses do not identify Pythium species.
Treatment with biological competitors and control agents.
A number of commercial products are offered for this purpose but there has been little research to prove their efficacy. Trichoderma species are widely mentioned in the literature and can readily survive in NFT solutions and in solid media. Many bacteria with disease suppressing properties have also been shown to be capable to forming stable populations in both nutrient solutions and on rockwool slabs.
At present there is no definite solution to the problems of plant disease in hydroponic systems. What is clear however is that planting infected plant material and using infected water sources must be avoided, and that root contact with drainage water in bag and other solid systems should be avoided. Recirculation probably generates greater population of competing and controlling microbes than run to waste systems, even though the risk of disease spread by recirculation is higher. Practical applications of disinfection systems cannot guarantee freedom from disease, and are expensive, either in capital or operating costs. The major risk of disease introduction to the crop from sources outside the crop is always present, whether or not the recirculating solutions are disinfected. When the solution is disinfected there may be less microbial buffering against disease, than when the solutions have not been disinfected. Good hygiene in the greenhouse and in the surrounds remains imperative. Good crop management to provide steady growth, without stress is probably a vital factor in avoiding disease. Enrichment of the natural microflora with known disease antagonists such as Trichoderma may be helpful.
1) Berklmann,B., Wohanka,W., and G.A. Wolf (1994) Characterisation of the bacterial flora in circulating nutrient solutions of hydroponic system with rockwool. Acta Horticulturae 361:372-381.
2) Tu,J.C., Papadopoulos,A.P., Hao,X. and J.Zheng. (1999) The relationship of Pythium root rot and rhizosphere micro-organisms in a closed circulating and open system in rockwool culture of tomato. Acta Horticulturae 481:577-583
3) Finck-Jensen,D and J.Hockenhull.(1983) The influence of some factors on the severity of Pythium root rot of lettuce in soilless (hydroponic) growing systems. Acta Horticulturae 133:129-136 .
4) Price,T.V and P.D.Nolan.(1984) Incidence and distribution of Pythium, Phytophthora and Fusarium sp.. in recirculating nutrient film hydroponic systems. ISOSC Proceedings 1984:523-529.
5) Pegg G.F and M.Holderness.(1984) Infection and disease development in NFT-grown tomatoes. ISOSC Proceedings 1984:493-507.
6) Kennedy,R and G.F.Pegg.(1989) The effect of root zone temperature on the control of Phytophthora cryptogea in rockwool-grown tomato plants. Acta Horticulturae 238,165-171 .
7) Staunton, W.P. and T.P.Cormican.(1978) The behaviour of tomato pathogens in a hydroponic system. Acta Horticulturae 82: 133-135.
8) Huang,R. and J.C. Tu. (1999). Effect of the NFT solution pH on root transmission of tomato bacterial canker (Clavibacter michiganensis subsp.. michaganensis). Acta Horticulturae 481: 569-575.
9)Vanachter,A. (1995) Development of Oplidium and Pythium in the nutrient solutions of NFT grown lettuce and possible control methods. Acta Horticulturae 382:187-196.
10) Rattink,H. (1991) Epidemiology of Fusarium crown and root rot in artificial substrate systems. Med.Fac.Landbouww. Rijksuniv.Gent,56/2b,423-30 (quoted by Runia 1994).
11) McPherson,G.M., Harriman,M.r., and D.Pattison. (1995). The potential for the spread of root diseases in recirculating hydroponic systems and their control with disinfection. Med.Fac. Landbouwkundige en Toegepaste Biologische,Wetenschappen,Univ.Gent 60(2b):317-379.
12) van Os,E.A., van de Braak,N.J., and G.Klomp. (1988). Heat treatment for disinfecting drainwater, technical and economic aspects. Intnl Soc. Soilless Culture Proc. 1988: 353-359.
13) Wohanka,W. (1992) Slow sand filtration and UV radiation: low cost techniques for disinfection of recirculating nutrient solution or surface water. Intnl Soc. Soilless Culture Proc. 1992:497-511.
14)Runia,W.Th. (1994). Elimination of root infecting pathogens in recirculation water from closed cultivation systems by ultra violet radiation. Acta Horticulturae 361:361-371
15) Jamart.G., Bakyoni, J., and O. Kamouen. (1994). UV disinfection of recirculating nutrient solution in closed horticulture systems. Med.Fac. Landbouwkundige en Toegepaste Biologische, Wetenschappen,Univ.Gent 59(3a) 1071-1078.
16) Vanachter,A. (1995). Development of Oplidium and Pythium in nutrient solutions of NFT grown lettuce and possible control methods. Acta Horticulturae 382: 187-196.
17) Runia, W.Th. (1994) Disinfection of water from closed cultivation systems with ozone. Acta Horticulturae 361:388-396.
18) Mebalds,M., Bankier,M. and D. Beardsell. (1998) Disinfection of water for hydroponic systems. In Best of Practical Hydroponics & Greenhouses, Casper Publications, Narabeen NSW, Australia: 126-132.
19) Acher,A,. Heuer,B., Rubinskaya,E and E.Fischer.(1997). Use of ultraviolet disinfected nutrient solutions in greenhouses. J. Horticultural Science 72(1):117-123.
20) Wohanka,W. Disinfection of recirculating nutrient solutions by slow sand filtration. (1995). Acta Hortiulturae 382:246-262.
21) Wohanka,W. (1999) Optimisation of slow sand filtration as a means for disinfecting nutrient solutions. Acta Horticulturae 481: 539-544.
22) van Os,A.E., van Kuik,F.J., Runia,W.Th., and J. van Buren. (1998) Prospects of slow sand filtration to eliminate pathogens from recirculating solutions. Acta Horticulturae 458:377-382.
23) van Os,A.E., Amsing, J.J., van Kuik,A.J. and H. Willers. (1999). Slow sand filtration: A potential method for the elimination of pathogens and nematodes in recirculating nutrient solutions from glasshouse grown crops Acta Horticulturae 481:519-525.
24)Runia W.Th. (1995) A review of possibilities for disinfection of recirculation water from Soilless cultures. Acta Horticulturae 382:221-229.
25) The Isan system Iotec Australasia Pty Ltd.
All Acta Horticulturae papers and Practical Hydroponics & Greenhouses are available online - see the links page for website addresses.
This client advice note replaces an
earlier advice note “Root diseases in NFT systems” and supplements the
advice note “Hydroponics- Clean water” Revised June 2007
©: R.A.J.White June 29,2007