Learn about how to control and manage diseases of greenhouse vegetables.
Several diseases in greenhouse vegetable crops are caused by fungi, bacteria and viruses. The main fungal disease agents that affect plants in greenhouses are classified into foliar pathogens and soil-borne pathogens. The most frequent greenhouse air borne fungal pathogens are species of Alternaria, Botrytis, Bremia, Cladosporium, Erysiphe, Leveillula, Phoma, Phytophthora, Pseudoperonospora, Puccinia, Sclerotinia, Septoria, and Sphaerotheca. The most common soil borne fungal pathogens includes the species of Fusarium, Phytophthora, Pythium, Rhizoctonia, Sclerotinia, Sclerotium and Verticillium.
Bacterial pathogens cause leaf and fruit spots, soft rots, wilts, galls, hairy roots and scab. The main bacterial groups in greenhouse vegetables are Agrobacterium, Clavibacterium, Erwinia, Pseudomonas, Streptomyces and Xanthomonas.
The viruses which are often found in greenhouse vegetables cause mainly yellow mosaic and other symptoms on the leaves or fruits, as well as stunting. Severe symptoms caused by virulent strains may end in stunting, leaf defoliation and even wilt. The major viral diseases which are common in greenhouses are – Tobacco mosaic virus (tomato, pepper), cucumber green mottle mosaic virus (cucumber and melons), potato virus X and Y (tomato), tomato bushy stunt virus (tomato, brinjal) and melon necrotic spot virus (melons, cucumber).
Some of these diseases are yearly problem, while others appear with frequency depending upon season, variety, production conditions, etc. In addition, normal field use of multipurpose fumigants for control of soil-borne pathogens is not followed in greenhouse production due to design of these facilities. Thus, hydroponic or ground systems where root or wilt disease occur are often difficult to disinfest.
Management of the pathogen involves the practices directed to reduce inoculum, eradicate inoculum and prevent inoculum (legislation such as quaratine). Management of the host involves the practices directed to improve plant vigour and induce resistance through nutrition, induction of resistance through breeding, and providing protection against attack by chemical means. The integrated use of different approaches in controlling the diseases is known as integrated disease management (IDM).
For the successful management of plant diseases in greenhouses, the crop production system need to be closely associated with goals of disease and insect management practices. Some insects such as white flies, aphids and thrips are vectors of vital diseases. Many of the General Pest Management principles and tactics that apply to control of insects are also relevant in the management of diseases.
The major emphasis of a plant disease control programme for greenhouse vegetable production should be given on prevention. It is far more expedient to prevent the introduction of a plant pathogen than to control. The major reason for this is that few fungicides have been or are likely to be recommended for use on vegetables in the greenhouse.
The main factors that discourage fungicide manufactures from using greenhouse vegetable names on their product are:
1. Greenhouse vegetables acreage is insignificant, hence there is no profit incentive for the manufacturers.
2. Worker exposure and subsequent manufacturers liability is much higher for fungicides cleared for use indoors.
3. Pesticide formulation is more phytotoxic within the protected environment of the greenhouse.
4. Pesticide residues are higher in greenhouse vegetables compared to field vegetable produce due to the absence of environment eroding forces such as UV light, rain etc.
Plant disease control in the greenhouse should better be taken up on preventive basis, or at the most with appearance of first symptoms of the disease. Methods of prevention include the use of pathogen free seeds and planting material, resistant cultivars or rootstocks, soil disinfestations by heat, steam, methyl bromide or formaldehyde, elimination of pathogen sources (sanitation), chemicals, pathogen-free soil-less systems to create conditions un-favourable for the development of diseases, or application of fungicides as protectants.
One of the most reliable methods is the use of resistant or tolerant cultivators. After disease appears, hygienic practices and environmental control can slow down the dispersal of the pathogen. Fungicides may then be used to protect the plants and thus suppress the rate of disease development.
The vascular wilt pathogens are the most difficult to eradicate even when beds are steamed/or fumigated. Pathogen-free propagation programme are followed, employing clean sources of propagation material, and seeds treated by heat chemicals.
The important principles of disease management in greenhouse are described as under:
(a) removal or destruction of infected plant parts or plant as a whole, and their dispose,
(b) uprooting of alternate hosts within and outside a greenhouse which harbour the pathogens to complete their life cycle and abridge the two seasons,
(c) crop rotation is effective against soil-borne pathogens, and
(d) sanitation (Pre-plant and production sanitation).
(i) House Preparation:
Workers should remove as much of the previous crop as possible. All houses should be cleaned of fallen debris and surface should be made free of soil, media, etc. Production systems that have had incidence of root and stem diseases should be rigorously sanitized. Solarization during the non-cropping period in summer should be done. Structures can be sealed completely after wetting media, surfaces, etc. All air intakes should be covered with insect-proof and soil-proof screens.
(ii) Media Preparation:
Infested bags should be discarded at the end of the season or immediately after plant death. Infested media in troughs should either be discarded or sterilized in place. New media should be mixed upon disinfested concrete slabs and should be thoroughly disinfested prior refilling. All troughs should be sterilized between crops. The ground system of production of vegetables in the greenhouse also required sterilization between cropping seasons.
It is a source of plant pathogenic bacteria and fungi. Therefore, it is advisable to use pathogen free clean water of deep wells (60-75 feet) to minimize the load of microorganisms in greenhouses. Filters should be used for the irrigation water, mainly to avoid pathogenic bacteria.
(iv) Cleaning of Tools:
Tools used for pruning, media transport, pollination, etc. should be disinfested between cropping seasons.
(v) Production of Seedlings:
The production of seedlings should utilize only new seed and new sterile media. They should be produced in a separate greenhouse from ongoing crop production to minimize water contact with these plants. Isolation of seedlings will reduce the likelihood of disease spread from production areas.
A number of recommended sanitation measures have been adopted by successful growers.
Some of the more common ones are given below:
i. Addition of an air lock type entrance to each production house so that, wind-carried insects, soil, etc. do not enter the production area directly from the outside or along with the workers.
ii. Use of foot baths to prevent unsterile soil from being carried into the production space.
iii. Restricted access by visitors to production and transplant houses.
iv. Raising transplants at a height of at least one foot above the ground to minimize dust or splashing soil contamination of plants.
v. Prohibition of cigarette, chewing tobacco, etc. used by workers so as to minimize contamination by viruses present in tobacco.
vi. Rigid hand-scrubbing rules for personnel involved in pruning, pollination, tying and harvesting activities.
vii. Filters on all air intakes to restrict air-blown soil and insect vectors entry.
viii. Rigid vegetation control around the periphery of houses to avoid insect and pathogen buildup of weeds.
ix. Periodic disinfestations of tools and walkways.
2. Cultural Control:
Soil disinfestations can be done by steam and by fumigation or application of chemicals. It is practiced in hot countries to control soil-borne pathogens and the elimination of weeds. In greenhouses, solarization is carried out in summers in the absence of a crop.
Mulching of the soil with polythene or polyvinyl is done inside closed greenhouses to intensify soil heating. Formaldehyde is one of the commonly used chemical to sterilize the soil. Drenching the soil with formaldehyde @ 25 ml/1 of water and then covering it for a week with plastic film. It is found very effective to check the attack of damping off disease.
(b) Environmental Control:
Environmental control of diseases can minimize the need to use chemicals. The factors which affect development of diseases in the greenhouse are temperature of both soil and air, vapour pressure deficit, dew, soil water content and light (quality, day length and intensity). All these factors can be controlled to a certain extent depending on the facilities available. Interplay among these factors affects sporulation, dispersal, germination of conidia, penetration of the germ tubes and lesion development.
(c) Soil-Less Cultures:
The use of soil-less media eliminates the need for regular soil solarization, thus avoiding a very costly and laborious task. The plant roots are no longer exposed automatically to the pathogens which almost inevitably build up in intensively cropped greenhouse soil. Therefore, there is a major advantage in soilless cultures. Soil-less culture offers an opportunity to employ non-conventional means of disease control.
Soil-less culture systems, particularly those without a soil substrate can readily be made pathogen-free before planting, but there still remains the possibility of re-infestation during cropping. If, a root disease sets in a bed after the plants are grown, or is transmitted in propagation material, then the possibility exists of rapid spread of a pathogen throughout a hydroponic system.
Sand and gravel culture for re-use is sterilized after roots and plant debris have been removed from the heavy substrate. Where rock wool or organic substances such as peat are to be used, steam sterilization is effective.
(d) Resistant Varieties:
Several problems common to greenhouse crops are overcome by growing vegetable varieties resistant to diseases. Several diseases, mainly Fusarium and Verticillium wilts of tomato, Fusarium wilt of cucumber and melons; Phytophthora capsici and tobacco mosaic virus (TMV) on pepper and leaf mould and TMV on tomato are effectively prevented either by employing resistant varieties, by grafting desirable varieties on resistant rootstocks or by both techniques.
3. Chemical Control:
For a given disease problem in the greenhouse, there may be several means of chemical control. Treatment before the pathogen enters the host plants is carried out mainly with ‘Protectant’ fungicides. However, due to the intensive growth of greenhouse plants, the protectants have to be applied at short intervals in order to cover new growth as well as old plant surfaces, on which residues of earlier applications are not effective.
Systemic fungicides can provide a solution to this problem which are absorbed by the plants and trans-located to entire system of plants. Systemic fungicides eliminate the internally seed borne pathogens and used as seed treatment e.g. Seed treatment with Bavistin @ 2g/kg of seed.
Surfactants (spreaders, wetters or stickers) improve the performance of a fungicide or pesticide in the greenhouse by lowering the surface tension between the droplets of the spray and the waxy cuticle covering the host. These polymers themselves may have a fungistatic effect.
Eradication of Soil Borne Inoculum:
Stem (80-95°C for 1-2 hours) is a common means of control of soil borne pathogens in several countries. It gives effective sterilization down to a depth of 40 cm. In places where stem is too expensive to produce, methyl bromide fumigation is used, however, it leaves chemical residues in the soil. Formaldehyde is replacing methyl bromide in certain greenhouses as it has no residual effect and effective against both bacteria and fungi.
As it is biodegradable to alcohol and CO2, it does not pose hazards to the environment. “Structure solarization” can be carried out in summer by increasing the air temperature in the greenhouse dramatically by closing all openings for 1-2 months which help eradicating some pathogens not from the growth medium but also from all solid materials left in the greenhouse. Water may carry pathogens such as bacteria into the greenhouses. Ultraviolet irradiation or chlorination is an effective treatment in such cases.
4. Biological Control:
The use of pesticides is necessary at present but not a long term solution to crop, human and animal health. Besides, because of their non-target effects and hazardous nature, they are becoming more expensive and some are losing their effectiveness resistant strains are coming up. Due to these limitations of pesticides, the use of biological agents is becoming popular in disease control.
For diseases of roots, foliage, flowers or fruits, the greenhouse crops are often more attractive possibilities for biological control because the population level of antagonists is easier to maintain. Antagonist can be a micro-organism that adversely affects another (e.g. a target pathogen) growing in association with it and the mechanism involved is called antagonism.
The most common fungal genera used for biological suppression are Trichoderma spp. The use of these bio-control agents effectively manages Rhizoctonia solani, Pythium spp. and Fusarium spp. in several greenhouse vegetable crops. Among the bacterial antagonists, Pseudomonas fluorescens is the widely studied agent in India and is used to control the diseases caused by the fungal pathogens such as Fusarium oxysporum, Pythium spp, Rhizoctonia solani, Phytophthora spp. and Sclerotium rolfsii and bacterial pathogens like Xanthomonas spp. and Pseudomonas spp. in both greenhouses and open fields.
Biological control of plant pathogens can be achieved through soil application, seed coating, foliar spray and others like wound dressing. Seed coating by the antagonistic Trichoderma spp is aimed at the most susceptible site of the plants and affects pre-and post-emergence diseases.
Combining Trichoderma spp. with methyl bromide or soil solarization in the greenhouse prevented re-infestation of the fumigated soil with pathogens and had a long term effect. Bio- control agents applied to the soil may also increase plant growth, which may promote their use in greenhouses.
Advantages of Bio-Pesticides:
The availability of microbial pesticides has opened up highly potential avenues:
1. Many of these are as effective as broad spectrum chemical pesticides and, thus, can replace them to a considerable extent.
2. Source of these is extremely target specific and hence has no effect on non-target organisms including man.
3. As these are bio-degradable, they do not pose any environmental hazards.
4. Due to their nature of action, the possibility of resistance development is less.
5. Some of them are known to enhance growth after application.