Vegetable Program
Soil, Crop and Pest Management

Disease Management of Hydroponically-grown Vegetable Crops

The Plant Disease Diagnostic Laboratory

Prepared by Dr. Robert L. Wick, Department of Plant Pathology, Fernald Hall, Room 111, University of Massachusetts, Amherst, MA 01003. Tel. (413) 454-1045 Fax (413) 545-2532

Revised October 20, 1993 V5

Introduction

This publication addresses diseases and control measures for tomatoes, lettuce, cucumbers and herbs grown in hydroponic systems; however, the information is applicable to greenhouse crops grown in soil or bag culture.

Tomatoes are the most commonly grown hydroponic crop in Massachusetts. To a lesser extent, lettuce, cucumbers and various herbs are also grown hydroponically. Greenhouse crops are subject to many of the same diseases that occur in the field; however, the greenhouse environment can favor diseases such as leaf mold, Botrytis blight and Pythium root rot, that are relatively minor diseases outdoors.

With regard to disease management, there are advantages and disadvantages to growing plants in the greenhouse. An advantage is that the environment can be controlled to a great extent. This is important because the fungi and bacteria that cause diseases of the foliage, stems and fruit are dependent on fairly specific environmental conditions such as temperature and humidity. Diseases of above-ground portions of the plant occur during specific environmental conditions, often with seasonal regularity. Keep records of when these diseases occur to help predict their occurrence in the future. Pathogens that attack the root system are less dependent on environmental conditions and may cause disease at any time.

A disadvantage of most hydroponic systems is that the nutrient solution is recycled throughout blocks, or throughout all of the plants. If a root pathogen enters the nutrient solution it has the potential of spreading throughout. Growers that use bag culture in place of the recycling nutrient solution avoid this problem; however, bag culture does not insure protection from plant pathogens, particularly if the bags and media are used for more than one year.

General Management Recommendations

The accurate identification of the cause of disease is paramount to effective and economic control. Bring specimens to the nearest county extension office, or to the Department of Plant Pathology in Amherst. If possible, select plants with various stages of disease. There will be a charge for the diagnosis.

Important principles of plant disease management include the use of resistant cultivars (varieties), sanitation, cultural practices and fungicides. A holistic approach to the management of plant disease is always the best approach.

Resistant cultivars. The most desirable way to control plant diseases is to grow resistant cultivars. If a particular disease is prevalent in your geographic area, determine if resistant cultivars are available. It is also important that the cultivar has the genetic characteristics necessary to be a good greenhouse plant, provide adequate yield, and be marketable. Consult with your extension agent or seed supplier for this information.

Sanitation. Sanitation is a general term for the removal of diseased plant material, and for the surface disinfection of tools and working surfaces. When disease occurs, diseased plants or plant parts should be removed and taken away from the greenhouse. Do not pile the plant debris outside the greenhouse door. Plant litter should be periodically removed from the floor. When water-molds (Pythium and Phytophthora) cause root rot in nutrient- circulating systems, thorough disinfection should be carried out. Household bleach diluted 1:9 is very good for this purpose and there are other disinfectants available.

Cultural practices. Different fungal pathogens have different environmental requirements for disease development (see Appendix I). With respect to greenhouses, the most important environmental factors that influence disease development are temperature and relative humidity. For example, a film of moisture on the surface of the plant is necessary for fungal spores to germinate. In the case of Botrytis, the plant surface must be wet for 8-12 hours. Therefore if humidity and condensation can be reduced, diseases can be prevented. Excessive condensation often occurs during the cooler months. To remedy this problem, open the vents just before sunset, adjust the fans to low speed, and turn up the thermostat a few degrees. The heat will drive the moisture laden air out through the vents. This may have to be repeated several times into the evening. A horizontal air flow system will also help keep plants drier and thus reduce the occurrence of foliage diseases. The system is described in the publication Horizontal Air Flow, and is available for $1.00 from: Agricultural Publications U-35, 1376 Storrs Rd. University of Connecticut, Storrs, CT 06269-4035.

Fungicides. Too often it is assumed that disease management is synonymous with fungicides. Fungicides can provide excellent control of some diseases, but for others they may be ineffective, unavailable or illegal. A holistic approach to disease control should always be the course of action.

The greenhouse operator is limited to those chemicals that are specifically labeled for a particular greenhouse crop. The label is a legal document and the pesticide must be used according to the label. When fungicides are to be used, it is important that the application method results in thorough coverage. Most of the fungicides act as protectants on the surface of the plant by killing spores after they germinate. In the absence of the appropriate fungicide, the fungus can penetrate into the plant. When significant amounts of spores are present, fungicides may not provide adequate control. Fungicides currently labeled for use on greenhouse tomatoes are listed in Appendix I. At this time, there are no fungicides labeled for use on herbs in the greenhouse.

Diseases of Tomato

Botrytis blight. Botrytis cinerea is a common resident in greenhouses, and when conditions favorable for disease development occur, the fungus can be very destructive. Optimum conditions for spore germination are: air temperatures between 55 and 65 F, moisture present on plant surfaces for 8 to 10 hours, and humidity greater than 93%. Optimum temperature for disease development is between 68 and 75 F.

Botrytis may cause leaf blight, fruit rot, fruit ghost spot and stem canker. The disease can be recognized by the profuse gray mold which develops on the lesion. During conditions unfavorable for disease, Botrytis remains alive in plant refuse and soil. Because of the relatively low temperatures necessary for disease development, disease most commonly occurs in greenhouses during the fall and spring.

Management. Practice thorough sanitation. Remove plant debris and weeds from the greenhouse. During periods when excessive condensation occurs, heat and ventilate as outlined on under cultural practices. Chemicals for controlling Botrytis blight and other fungal diseases are listed in Appendix I.

Early Blight. Early blight is caused by the fungus Alternaria solani. The disease may occur on stems, leaves and fruit. Lesions are dark brown and usually display a target-like pattern. Seedlings and young transplants may be girdled and killed. The fungus survives on plant material in or on the soil and may also be seed-borne. Weeds in the tomato family such as black nightshade can harbor the disease.

Alternaria may cause disease at a wide range of temperatures (between 60 and 90 F) but 75 to 85 F is optimum. A free film of moisture and relatively high humidity are necessary for spore germination.

Control. Plants of low vigor are most susceptible to early blight, therefore a balanced fertility program should be maintained. Magnesium deficiency has also been correlated with increased susceptibility. Seeds should be planted in a soil-less medium or in pasteurized soil. Seedlings should be inspected carefully for stem cankers before they are moved into the greenhouse. If disease occurs at any time, practice thorough sanitation. Chemicals for controlling early blight are listed in Appendix I.

Gray Leaf Spot. Gray leaf spot is caused by the fungus Stemphyllium solani and occurs during warm, moist weather. The disease occurs on the foliage of plants as spots and results in withering. The fungus overwinters on dead plant material.

Control. Seeds should be planted in a soil-less medium or in pasteurized soil. If disease occurs at any time, practice thorough sanitation. Chemicals for controlling gray leaf spot are listed in Appendix I.

Late Blight. Late blight is caused by the fungus Phytophthora infestans. The fungus attacks stems, foliage and fruit. During humid conditions, a downy growth of fungus may be visible on the lesions. Leaves will appear water-soaked as if frozen and thawed. Fruits will have a firm decay that will usually start at the stem end. Very high humidity for 15 hours is necessary for Phytophthora to produce spores. This disease occurs during periods of cool nights and warm days. Optimum temperature for disease development is between 70 and 80 F.

Control. Heat and ventilate the greenhouse during evening hours when environmental conditions are favorable for disease. See cultural practices, page 2. Do not allow condensation to drip from the ceiling onto the plants. Water that collects on the stem end of the fruit provides an ideal environment for Phytophthora to cause disease. Do not plant potatoes in the vicinity of the greenhouse. If disease occurs at any time, practice thorough sanitation. Chemicals for controlling late blight are listed in Appendix I.

Leaf Mold. Leaf mold is caused by the fungus Fulvia fulva (previously known as Cladosporium fulvum). It is one of the most common disease of greenhouse tomatoes in Massachusetts. The disease occurs primarily on the foliage but may occasionally be found on fruit stems and blossoms. Symptoms first appear on the upper surface of the leaves as yellowish spots. A felty brown or greenish-brown growth of spores develops on the underside of the leaves. Diseased foliage will wither and die and yield can be reduced considerably. Air temperature between 65 and 80 F and relative humidity of 90 to 100% must be present for this disease to occur.

Control. Do not allow humidity to go above 90% when the air temperature is between 65 and 80 F. To decrease humidity, see cultural practices, page 2. Resistant cultivars are available (see below). Chemicals to control this disease are listed in Appendix I.

Fulvia Resistant Tomato Cultivars

Pith Necrosis. Pith necrosis is caused by the bacteria Pseudomonas corrugata and Erwinia carotovora. The bacteria primarily colonize the pith resulting in a hollow stem. Several feet in length of the stem may be affected before wilt occurs. External symptoms include the development of adventitious roots and external cankers.

Control. Remove affected plants from the greenhouse. There are no effective chemicals to control this disease.

Bacterial Canker. Bacterial canker is caused by Clavibacter michiganense subspecies michiganense (formally Corynebacterium michiganense subsp. michiganense). Bacterial canker is the most destructive diseases of tomato in our region. The bacterium is seed-borne and can also survive in soil, on wooden stakes, flats and some solanaceous weeds. Pruning and tying plants, and fungicide sprays will spread the bacterium.

There are a number of different symptoms and a plant may have one or more depending on the age of the plant and the length of time it has been diseased. The canker symptom, for which the disease is named, is not always present or obvious and occurs after the plant has been diseased for some time. The cankers are dark and water-soaked in appearance and occur on the stem or branches. The bacterium may exude and drip from the cankers allowing for spread to healthy plants. Young plants that become diseased develop a wilt of the lower foliage. At first, wilt may be restricted to one side of the plant or to only one half of a leaf. Partial wilting occurs in early stages of disease because only a portion of the vascular system is affected. A discoloration of the vascular system may be observed by tearing a petiole down the stem. When the stem is split open, a light to brown discoloration may be seen just inside the woody tissue. In early stages of disease, sections of the pith may appear mealy. Later, the pith will become chambered or hollow, suggestive of the disease tomato pith necrosis.

In addition to wilt, a scorching of the foliage may occur on older plants. The margins of the leaves will be necrotic and will often have a yellow border to the inside. Scorch is less likely to occur in the greenhouse than outdoors. Scorch occurs when the bacterium is spread to the foliage by sprays or wind driven rain. Scorching of the foliage may occur in the absence of wilt or stem canker.

Control. Buy certified seed from a reputable source. The use of hot water-treated seed (127-130 F for 20 to 25 minutes)is encouraged. Discard plants that show signs of wilting or other symptoms of disease. Wash hands with soap and water after handling diseased plants.

Diseases of Lettuce

Botrytis Blight. Botrytis cinerea can attack all parts of the lettuce plant including the roots but the leaves are most commonly affected. For a description of conditions favorable for disease development, see Botrytis blight under tomato.

Control. Practice thorough sanitation. Remove plant debris and weeds from the greenhouse. During periods of excessive condensation, heat and ventilate as outlined under `cultural practices'on page 2.

Pythium Root Rot. Various species of the fungus Pythium can cause a destructive root rot of lettuce. Pythium is a water mold and a recirculating hydroponic system is ideal for extensive root rot to develop. Pythium rots the cortex (outer tissues) of the root, leaving the thin vascular strand remaining.

Control. Avoid contamination of the nutrient solution with soil. Do not allow dirty hands or tools to come in contact with the plants or nutrient solution. Currently, there are no fungicides registered to control this disease. Tools should be sanitized (see page 2) before coming in contact with the nutrient solution. Ultraviolet sanitizing units for water are effective in destroying Pythium and are available for installation into hydroponic operations. However, these systems are of little value if the disease causing organism is present in the nutrient troughs and substrate. See Appendix III for a listing of companies selling ultraviolet sanitation equipment.

Diseases of Cucumber and Herbs

Water molds such as Pythium are the most likely cause of disease in hydroponically-grown cucumbers and herbs. For control of water molds, see Pythium root rot of lettuce.

Botrytis can be particularly destructive to basil. To control Botrytis blight, practice thorough sanitation and reduce excess humidity as outlined under `cultural practices'on page 2.

Fusarium wilt has also been noted on basil. Fusarium can be seed-borne, and once disease develops in production, spores are easily disseminated throughout the crop. Currently there are no fungicides available for use on herbs in the greenhouse or in the field. Bacterial wilt of cucumber, caused by Erwinia tracheiphila, is spread by the striped cucumber beetle. It can be prevented by controlling the beetle. Botrytis can be a problem on cucumber, and Sclerotinia may occur if plants are planted in soil. Currently their are no fungicides registered for use on greenhouse cucumbers.

Appendix I

Summary of Greenhouse Tomato Diseases and Their Control

*Many strains of Botrytis are resistant to Benlate

Appendix II

Fungicides Referred to in this Publication

Exotherm Termil
Chemical name. Tetrachloroisophthalonitrile
Common name. chlorothalonil
Tradename. Exotherm
Toxicity. LD₅₀ l0,000 mg/kg
Signal word. Caution
Company. Rigo Company, Jct. 1-71 & Highway 146, Buckner, KY

Maneb+Zinc
Chemical name. Manganese Ethylene- bisdithiocarbamate. Contains 7.6% manganese.
Common name. Maneb
Tradename. Maneb+zinc F4
Toxicity. Oral LD₅₀ 7,990 mg/kg
Signal word. Caution
Company. Elf Autochem North America.

Maneb 80
Chemical name. Manganese Ethylene- bisdithiocarbamate. Contains 16.5% manganese.
Common name. Maneb
Tradename. Maneb 80
Toxicity. Oral LD₅₀ 7,990 mg/kg
Signal word. Caution
Company. Elf Autochem North America.

Appendix III

Sources of Ultraviolet Water Purifying Systems*

Allanco International
692 Derwent Way
Annacis Industrial Estate
New Westminster, B.C., Canada
V3M 5P8
Phone (604) 526-1742
FAX (604) 526-1753

Atlantic Ultraviolet Corp
250 North Fehr Way
Bay Shore, New York 11706
Phone (516) 586-5900
FAX (516) 595-2609

International Specialty Supply
820 E. 20th St.
Cookeville, TN 38501
Phone (615) 526-1106 1-(800) 277-7688
FAX (615) 526-8338

Ster-O-Lizer Manufacturing Corp
P.O. Box 27488
Salt Lake City, Utah 84127
Phone (801) 532-6500
Fax (801) 973-6463

Trojan Technologies Inc
845 Consortium Court
Logndon Ontario
NGE 2S8
Phone (519) 586-6660
Fax (519) 681-8355

*This is not a complete list. No product endorsement is implied, nor is discrimination intended against suppliers not listed.