Diseases of vegetable bedding plants

Some diseases of vegetable bedding plants include damping-off, Botrytis blight, powdery mildew, tospoviruses and bacterial leaf spot. The first step to manage these diseases is proper identification. Cultural mistakes caused by high soluble salts or nutrient imbalances often mimic disease symptoms. The use of resistant cultivars, sanitation, sound cultural practices and the proper use of fungicides are all needed to manage diseases of vegetable bedding plants.

RESISTANT CULTIVARS
Seed catalogs often feature disease resistant and tolerant varieties of vegetables that should be used when they are available.

Start and stay clean
Start with a clean, weed-free, disinfected greenhouse. After the greenhouse has been cleaned, care must be taken to avoid recontamination with disease pathogens. Growing media is easily re-infested by dirty hose nozzles or tools and unsanitary growing conditions. Grow transplants off the ground in a well-ventilated greenhouse.
 

Seed treatments
Seed treatments are useful for many vegetable crops to prevent root diseases, as well as certain diseases carried on or within the seed. Eradicative seed treatments use hot water or chlorine to kill disease-causing agents on or within the seed. Protective seed treatments use fungicides on the seed surface to protect the seed against decay and soil-borne organisms such as damping off.

Damping-off
Damping-off is a disease of germinating seeds and young seedlings. Several fungi are capable of causing damping-off including Rhizoctonia, Alternaria, Sclerotinia and the water molds Phytophthora and Pythium. Young seedlings are most susceptible to damping-off. However, later in the crop cycle, the same pathogens may cause root and stem rot.

Symptoms
In pre-emergence damping off, seeds rot before they break thru the growing medium. There will be bare spaces and poor stands.

In post-emergence damping off, newly emerged seedlings topple over and wilt. There may be a water-soaked dark, sunken lesion at the soil line.

Damping off usually spreads radically from a central point of origin so plants often die in a circular pattern in seed flats. Vegetable seeds that are germinated in a poorly drained, cool medium are especially susceptible.

Cabbage, cauliflower, tomato and pepper seedlings may be girdled by brown or black sunken cankers. Stems of these plants may shrivel and become dark and woody (wire stem or collar rot). The plants may not collapse, but remain stunted and die after transplanting.

Management
Damping off is best managed preventively because it is difficult to stop once symptoms occur. Use only certified disease-free seed from reputable seed companies. Use pasteurized growing media, compost-based or soilless mixes. Fill flats with pre-moistened growing medium to avoid compaction. To avoid compaction, do not stack or “nest” filled trays or pots. Apply preventive biological fungicides to suppress disease pathogens.

Use new flats and pots and disinfect all used flats, pots and tools. Do not pack young plants into containers, use pre-dibbled holes for transplants.

Germinate seed under conditions that will ensure rapid emergence and vigor by providing bottom heat. Avoid over watering, excessive fertilizer, overcrowding and planting too deeply. Promptly rogue out infected plants from flats. If needed, apply appropriate fungicides.
 

BOTRYTIS BLIGHT
Botrytis can cause leaf blight, cankers, damping off and root rot. The fungus attacks tender, young, wounded or dying plant parts and flowers.

Symptoms
Look for soft, tan to brown areas on leaves that spread under humid conditions. Tan or brown stem cankers can also occur. Under humid conditions, Botrytis blight produces the all-too-familiar brown to gray fuzzy masses of spores on the surface of infected tissues.

Management
Management of environmental conditions such as temperature, humidity and duration of leaf wetness, plus sound cultural practices and appropriate fungicides help prevent Botrytis blight. Control weeds and remove plant debris between crop cycles and during production. Dispose of diseased plants and cover trash cans to prevent the airborne spread of spores.

Reduce humidity and leaf wetness duration to prevent spore germination. Provide good air circulation and reduce humidity within the plant canopy. Water in the morning, never late in the day to allow water to evaporate from plant surfaces.

Avoid growing ornamental hanging baskets above vegetable bedding plants. Spent flowers dropping on plants are often a source of Botrytis infection.
 

POWDERY MILDEW
Powdery mildew may occasionally occur on tomatoes, eggplants, squash, pumpkins and cucumbers. Growers who produce cucurbit transplants and verbena should be especially careful to separate these two crops.

The powdery mildew that affects certain cultivars of ornamental verbena can also infect cucurbit seedlings including squash, cucumbers and pumpkins. It is possible that this powdery mildew could affect the cucurbit transplants that may not have otherwise become infected until the fruit was beginning to form in the field.

Symptoms
Powdery mildew is easily recognized by its white talcum-like growth. Faint, white mycelium may develop on leaves and stems with yellow margins.

Management
Maintain proper plant spacing to reduce relative humidity levels within the plant canopy. Heat and ventilate in the late afternoon and early morning to reduce high relatively humidity at night. Thoroughly clean greenhouses between crops, removing all weeds that could be potential hosts.

TOSPOVIRUSES
Tospoviruses, including impatiens necrotic spot virus and tomato spotted wilt virus, are primarily spread by western flower thrips. Tospoviruses have a wide host range including many different ornamentals, vegetables such as tomatoes, peppers and eggplant and weeds.

Tospoviruses are not seed borne. They may be introduced into a greenhouse on vegetatively-propagated ornamental plants or seedlings that have been exposed to the virus or on virus-infected thrips. Once thrips in the greenhouse become infected, they can transmit the virus to susceptible crops and weeds.

Symptoms
Symptoms are highly variable but may include stunting, foliar ring spots and black lesions on stems. On pepper, small, dark brown spots, necrotic streaks or line patterns may appear on the leaves. Fruit from infected transplants will have chlorotic spots.

On tomatoes, bronzing may appear on young leaves. Infected tomato transplants in the garden may be stunted.

Management
Discard infected plant material, including weeds and plants and start a thrips management program. Since impatiens necrotic spot virus and tomato spotted wilt virus are not seed-borne, vegetable transplants may be kept free of tospoviruses if they are not brought into contact with other infested crops or thrips carrying the virus.

Growers attempting to produce all their warm temperature crops in a single house run the risk of mixing tospovirus-free vegetable seed crops with infected plants. If possible, vegetable bedding plants should always be grown in separate greenhouses from ornamentals.
 

BACTERIAL LEAF SPOT
Bacterial leaf spot is caused by Xanthomonas campestris pv. vesicatoria and is found primarily on peppers, and above ground parts of tomatoes are also susceptible.

Symptoms
Spots on pepper leaves are chocolate-brown and irregularly shaped with areas of dead leaf tissue. At first, the spots are less than ¼-inch in diameter. Severely spotted leaves appear scorched and leaf drop may occur.

Management
Bacteria can be introduced on infected seeds or infected transplants purchased from another operation. Bacteria can also survive on weeds in the same family as the host crop.

Buy certified seed from a reputable source. Use hot water-treated seed. Ideally, the seed should be custom-treated by the seed company. There is a risk that germination percentages will be reduced if the seed crop is grown under stressful environmental conditions.

Promptly remove infected plants and adjacent plants to prevent further infection and avoid unnecessary handling of plant material. Avoid overhead irrigation, splashing or periods of extended leaf wetness. Disinfect all benches, equipment and flats.
There are many resistant varieties of bell peppers available, but there are fewer choices for resistant specialty peppers.

Leanne Pundt is extension educator, University of Connecticut, (860) 626-6240; leanne.pundt@uconn.edu

For more: Pest Management for Vegetable Bedding Plants. Pundt, L. and T. Smith. 2007. NESARE. See Table 2: Selected fungicides, bactericides and biological fungicides labeled for vegetable bedding plants. Available online at www.hort.uconn.edu/ipm or www.nevegetable.org