Botrytis blight

Botrytis can cause leaf blight, cankers, damping off and root rot. Plants may be attacked at any stage, but the new tender growth, freshly injured tissues and dead tissues are most susceptible.

Symptoms: Botrytis blight produces characteristic gray fuzzy appearing spores on the surface of infected tissues. Air currents and splashing water can easily disseminate the spores. In general, germination of spores and infection is dependent on a film of moisture for 8 to 12 hours, relative humidity of 93% or greater and temperatures between 55° and 65°F. After infection, colonization of plant tissues can occur at temperatures up to 70°F.

Management: Management of environmental conditions such as temperature, humidity and duration of leaf wetness, sound cultural practices and fungicides will help prevent disease development.

• Control weeds and remove plant debris between crop cycles and during production. Dispose of diseased plants and debris in a plastic trash bag. Keep the bag closed to help prevent spreading spores to uninfected plants as the bag is removed from the greenhouse. Cover trash cans to prevent the airborne spread of spores from diseased plant tissue.
• Reduce humidity and leaf wetness duration to prevent spore germination. See techniques for reducing relative humidity.
• Provide good air circulation and reduce humidity within the canopy. Proper planting dates, fertility, watering and height management will prevent overgrown plants, thereby reducing humidity within the canopy.
• Water in the morning, never late in the day. Rising temperatures during the day will cause water to evaporate from the foliage and dry the leaf surface.
• Avoid growing ornamental hanging baskets above vegetable bedding plants. Spent flowers dropping on plants below cause Botrytis infection.

Damping-off of Seedlings

Damping-off is a common 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. Soil-borne fungi generally do not produce air-borne spores but are easily transported from contaminated soil to pathogen-free soil by infected tools, hose ends, water-splash and hands. Young seedlings are most susceptible to damping-off. However, later in the crop cycle, the same pathogens may cause root and stem rot.

Symptoms: Symptoms of damping-off include seedlings failing to emerge or wilting, often with a stem lesion that appears water-soaked or dark, necrotic and sunken at the soil line. The fungus disease usually spreads radially from a central point of origin so plants often die in a circular pattern. Vegetable seeds that are germinated in poorly drained, cool soils are especially susceptible. Young plants that do emerge are weak and often wilt at or below the soil line. 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 (wirestem or collar rot). The plants may not collapse, but remain stunted and die after transplanting.

Management: Damping-off must be prevented because it is difficult to stop once symptoms occur. There are several strategies to prevent damping-off.

• Use only certified disease-free seed from reputable seed companies.
• Use fungicide-treated seed. Certain fungicides are labeled for damping-off for selected vegetable crops.
• Use pasteurized soil, compost-based or soilless mixes.
• Disinfect all flats, cold frames, pots and tools.
• Germinate seed under conditions that will ensure rapid emergence, such as with the use of bottom heat.
• Avoid overwatering, excessive fertilizer, overcrowding, poor air circulation, careless handling, and planting too deeply.
• Fill flats with pre-moistened growing media to avoid compaction.
• Lightly fill and brush containers
• Do not pack young plants into containers; use pre-dibbled holes for transplants.
• To avoid compaction, do not stack or nest filled trays or pots.
• Provide adequate light for rapid growth.
• Promptly rogue out infected plants from flats.

Powdery Mildew

Powdery mildew may occasionally occur on vegetable transplants including cucurbit and solanaceous crops such as tomato and eggplant. Faint, white mycelium may develop on leaves and stems with yellow margins.

Most growers are all too familiar with powdery mildew when it develops on cucurbits in the field. The powdery mildew that affects certain cultivars of ornamental verbena can also infect cucurbit seedlings. Greenhouse growers who produce cucurbit transplants as well as verbenas should be especially careful to separate these two crops. It is possible that this powery mildew could affect the cucurbit transplants that may not otherwise become infected until fruit is forming in the field.

Tospoviruses

Tospoviruses are a group of viruses that include impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV). They may infect hundreds of plant species including tomatoes, peppers and eggplant. These viruses are primarily spread by the western flower thrips. Tospoviruses are not seedborne but are brought into the greenhouse on vegetatively propagated ornamental plants or seedlings that have been exposed to the virus. Once the thrips in the greenhouse become infected, they can transmit the virus to susceptible crops and weeds.

Symptoms: Symptoms include stunting, foliar ringspots and black lesions on stems. Symptoms of INSV and TSWV will vary depending upon the host.

Management: To manage tospoviruses, it is necessary to discard infected plant material, including weeds and to manage thrips. Infected vegetable transplants planted into the garden or field will be stunted and will not produce a harvestable crop. Since INSV and TSWV 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 concentrate all their warm temperature crops in a single house run a risk of mixing tospovirus-free vegetable seed crops with leftover ornamental stock plants or new cuttings that may carry the virus. Pre-finished or vegetatively propagated ornamentals from another producer could be infested with thrips or infected with a virus. Therefore, vegetable bedding plants should always be grown in separate greenhouses.

Bacterial diseases

Bacterial diseases of vegetable bedding plants, such as bacterial leaf spot of peppers, bacterial speck and bacterial canker of tomatoes and black rot on cole crops are introduced into a greenhouse through infected seed and transplants.

Bacterial leaf spot, Bacterial speck

Symptoms: Bacterial leaf spot is caused by Xanthomonas campestris pv. vesicatoria and is found primarily on peppers although all aboveground parts of tomatoes are also susceptible. Spots on leaves are chocolate-brown and irregularly shaped with areas of dead leaf tissue. At first, the spots are less than 1/4 of an inch in diameter. Severely spotted leaves will appear scorched and defoliation may occur. This disease is most prevalent during moderately high temperatures and long periods of leaf wetness.

Bacterial speck occurs on tomato but not pepper. The bacterium, Pseudomonas syringae pv. tomato, causes small black spots to develop resulting in chlorosis (yellowing), necrosis (dead tissue) and blighting of the foliage. Bacterial speck can usually be distinguished from bacterial spot by the size of the lesions, however, in some cases, the symptoms look similar.

Bacterial canker

Bacterial canker of tomato is caused by Clavibacter michi-ganensis (formerly Corynebacterium michiganense). In New England, bacterial canker occurs less frequently than other tomato diseases but it is potentially more destructive. The bacterium is seed-borne but may survive on plant debris in soil for at least one year. It can also survive in the greenhouse on wooden stakes and flats. Wilt, leaf scorch, canker, pith necrosis and fruit spot may occur singly or in combination depending on the circumstances. When the bacterium is carried in the seed, the vascular system becomes colonized, resulting in wilt, pith necrosis and external cankers. Wilt initially occurs on one side of a leaf or one half of a plant because only a portion of the vascular system is blocked. Cankers and pith necrosis occur in later stages of disease development. Cankers are dark and water-soaked in appearance and often exude bacteria that are easily spread to adjacent plants. Pith necrosis is first evident as a darkening of the center of the stem that soon becomes chambered or hollow. When leaf scorch occurs, the petioles usually bend downward while the leaf edges curl up. The margins of the leaves become brown with a yellow border to the inside. Scorching of the foliage often develops in the absence of wilt or stem canker. Transplants may not express symptoms until six to eight weeks after infection and initial symptom expression is accelerated by environmental stress.

Black rot

Black rot, caused by the bacterium Xanthomonas campestris pv. campestris occurs whereever cruciferous plants are grown. Cabbage, cauliflower and Chinese cabbage are often severely affected. The bacterium enters the leaves by colonizing the hydathodes (water pores) and moves from the leaf margins inward. Lesions may also begin at wounds. Diseased tissue is often V-shaped; flaccid, tan to yellow and with blackened veins. The blackened veins are diagnostic and are best seen by holding the leaf up to the sun. When the lesions reach the petiole and stem, the bacterium moves systemically through the plant, resulting in premature leaf drop. At this stage of disease, a cross-section of the stem will reveal a ring of discolored vascular tissue.

Management of bacterial diseases: These bacteria can be introduced on infected seeds, infected transplants purchased from another operation, or in the field on crop residues. These bacteria can also survive on weeds in the same family as the host crop. The management of these bacterial diseases is similar and includes the following strategies:

• Buy certified seed from a reputable source.
• Use hot water-treated seed. Ideally, the seed should be custom-treated by the seed company. Seed companies may treat the seed upon request. 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, flats and stakes.
• Follow sound practices for weed and insect control.

Prevent bacterial leaf spot on peppers by choosing resistant varieties whenever possible. There are many resistant varieties of bell peppers available, but there are few choices for resistant specialty peppers.