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Contents
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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.
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.
Conclusions
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.
References
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 .
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and Fusarium sp.. in recirculating nutrient film hydroponic systems.
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6)
Kennedy,R and G.F.Pegg.(1989) The effect of root zone temperature on
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16)
Vanachter,A. (1995). Development of Oplidium and Pythium in nutrient
solutions of NFT grown lettuce and possible control methods. Acta
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17) Runia, W.Th. (1994) Disinfection of water from closed cultivation systems with ozone. Acta Horticulturae 361:388-396.
18)
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21)
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22)
van Os,A.E., van Kuik,F.J., Runia,W.Th., and J. van Buren. (1998)
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Os,A.E., Amsing, J.J., van Kuik,A.J. and H. Willers. (1999). Slow sand
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24)Runia W.Th. (1995) A
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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.
July 1999