Disease Management

Introduction

There are a limited number of fungicides labeled for greenhouse-grown vegetable bedding plants compared to ornamental bedding plants. Integrated pest management (IPM) offers a practical way to effectively manage pests on vegetable bedding plants and transplants. Through the use of sound cultural practices, monitoring techniques, accurate problem identification, and timely implementation and evaluation of appropriate management strategies, growers can improve their production while minimizing their reliance on routine pesticide applications. IPM utilizes many different management options; genetic, cultural, physical, mechanical, biological and chemical. Routine crop inspection alerts growers to developing pest and cultural problems while they are still minor and can be easily managed. Early detection and intervention is the foundation of an IPM program.

Diseases of vegetable bedding plants include Botrytis blight, damping-off, Alternaria blight, late blight, powdery mildew, downy mildew, bacterial diseases such as bacterial leaf spot, bacterial canker, and black rot, and viral diseases such as Cucumber Mosaic Virus (CMV), Tobacco Mosaic Virus (TMV), and Tospoviruses. Effective control of diseases requires accurate identification. Failure of disease control is often because the cause was not accurately identified. Symptoms caused by poor cultural practices can also mimic disease symptoms. Fungicides cannot correct problems caused by high soluble salts, poor aeration or a nutrient imbalance. An integrated approach to disease management involves the use of resistant cultivars, sanitation, sound cultural practices and the proper use of the correct pesticide.

Resistant Cultivars

Seed catalogues often feature disease resistant and tolerant varieties of vegetables. Utilize resistant varieties where feasible, but take some time to research the diseases that are giving you the most trouble to find other strategies to incorporate into the disease management plan.

Seed Treatments for Disease Management

Seed treatments are useful for many vegetable crops to prevent root diseases, as well as certain diseases carried on or within the seed. There are two general types of seed treatment: eradicative and protective. Eradicative seed treatments use hot water or chlorine to kill disease-causing agents on or within the seed. They are useful in controlling certain seed-borne bacterial diseases such as bacterial leaf spot on pepper and tomato and bacterial canker on tomato. Protective seed treatments use fungicides on the seed surface to protect the seed against decay and soil-borne organisms such as damping off caused by Pythium, Phytophthora and Rhizoctonia. For more information regarding seed treatments, contact your seed sales representative, Extension vegetable specialist, or plant pathologist.

Sanitation

Pest management on vegetable bedding plants and transplants begins with a clean, weed-free, disinfected greenhouse. Before growing the crop, the greenhouse should be cleared of plant debris, weeds, flats and tools. Empty benches, potting tables, storage shelves, tools and cell packs should be washed and disinfected with a sanitizing agent. It is important to thoroughly clean or power wash to remove organic debris from plastic containers before using a sanitizing agent. Bits of organic debris can be difficult to remove and the organic matter can be a source of disease causing pathogens if the containers are reused.

After the greenhouse has been sanitized, care must be taken to avoid recontamination with pathogens. Purchase certified, disease-free seed from reliable sources. If possible, purchase seed that has been disinfested by chemical and/or heat treatment by the seed company. Potting media is easily re-infested by dirty hose nozzles or tools and unsanitary growing conditions. The floor of the greenhouse is a good source for diseases. Use a hook to keep the hose nozzles off the floor. Grow transplants off the ground in a well-ventilated greenhouse. To prevent root rot diseases, avoid over-watering and over-fertilizing. Water early in the day to allow foliage to dry quickly to help prevent foliar diseases.

Use separate greenhouses for vegetable seedlings and ornamental bedding plants. Separate greenhouses: 1) will protect vegetable seedlings from insect pests that may migrate from ornamentals and plants that are held over; 2) will help protect vegetable seedlings from Tospoviruses (i.e. Tomato spotted wilt virus) due to migrating infected thrips; 3) protect vegetable transplants from diseases that ornamentals may also be susceptible to (i.e. curcurbit seedlings from powdery mildew on calibrachoa and petunia and some cultivars of verbena) or tomato transplants from late blight on petunias); and 4) facilitate treatment of the vegetable seedlings if pesticides are needed.

Keep tomato transplant production separate from greenhouse tomato fruit production. Greenhouses with both young transplants and mature plants increase the risk of perpetuating diseases.

Techniques to Reduce High Humidity

High relative humidity is one of the major contributing factors to Botrytis blight and powdery mildew, common fungal diseases of bedding plants. Warm air holds more moisture than cool air. During warm days, the greenhouse air is more humid. As the air cools in the evening, the moisture-holding capacity drops until the dew point is reached. Water then begins to condense on surfaces. Humidity can be reduced by exhausting the moist air and replacing it with cooler outside air that is drier. The method and time it takes to heat and vent depend upon the heating and ventilation system in the greenhouse. In greenhouses with vents, turn the heat on and crack the vents open about one inch. The moist humid air escapes from the vents. In greenhouses with fans, activate the exhaust fans for a few minutes and then heat the greenhouse to raise the air temperature. Then, shut off the fans. A clock can be set to activate the fans. The cooler, outside air will lower humidity levels as it is warmed in the greenhouse. A relay may be needed to lock out the furnace or boiler until the fan shuts off so that flue gases are not drawn back into the greenhouse. This will also help to prevent air pollution damage (ethylene or sulfur dioxide) to sensitive seedlings. Heat and vent two or three times per hour in the evening after the sun goes down and early in the morning at sunrise. Heating and venting can be effective even if it is cool and raining outside.

Air movement, even in a closed greenhouse, helps reduce moisture on the plant surfaces and surrounding the plants. Using horizontal airflow (HAF) can also reduce condensation. HAF fans keep the air moving in the greenhouse, helping to minimize temperature differentials and cold spots where condensation occurs. Air that is moving is continually mixed. The mixed air along the surface does not cool below the dew point so does not condense on plant surfaces.

HAF fans are more efficient than low-cost residential fans which are generally not designed for greenhouse conditions.

In addition, cultural practices can be used to reduce humidity within the plant canopy. These include proper watering practices and spacing of plants. Since most vegetable bedding plants are grown in flats that are spaced flat to flat, reducing humidity within the canopy is difficult. Proper planting dates, plant nutrition, watering practices and height management techniques help to prevent lush, overgrown plants thereby reducing humidity within the canopy.

Always water in the morning to reduce the length of time the leaves stay wet after irrigating to prevent foliar diseases. Rising temperatures during the day will evaporate water from the foliage, so the leaves stay dry. Avoid watering late in the day or when water will sit on leaf surfaces for long periods of time.

Fungicides and Bactericides

Fungicides can provide excellent management of some diseases, but for others they may be ineffective. In general, to control root diseases, broad-spectrum fungicides should be applied as a drench on a preventative basis. Read directions for application on pesticide labels. An application of additional water may be necessary. For foliage diseases, obtain thorough spray coverage and treat when disease is first evident.

Biological Disease Control Products

Biofungicides are biological fungicides that contain living organisms such as fungi, bacteria, or Actinomycetes (a group of bacteria that form branching filaments) that attack plant pathogens and the diseases they cause. They can be used as part of an integrated disease management program to reduce the risk of pathogens developing resistance to traditional fungicides. Currently, there are no pathogens that are resistant to biological fungicides.

Biological fungicides may suppress diseases in a number of different ways including direct competition or exclusion, antibiosis, predation or parasitism, induced resistance and plant growth promotion. Many biological fungicides work in multiple ways, such as by competition and parasitism, so are less likely to develop resistance than conventional fungicides that often work in a single way with a specific mode of action. 

Direct Competition/ Exclusion: Before root infection can occur, pathogens must gain accesses to the zone closely associated with the root, known as the rhizosphere. For foliar diseases, the pathogen must make contact with the leaf or flower zone. The biofungicide grows a defensive barrier around this root, leaf or flower zone. The beneficial microbes compete with plant pathogens for nutrients, infection sites and space, excluding the pathogen. 

Antibiosis: The biofungicide produces chemical compounds or secondary metabolites such as antibiotics or other toxins that kills the target organism.  The biofungicide creates compounds that inhibit fungal or bacterial spores from germinating and causing plant disease, or the compounds restrict the pathogen’s growth.

Predation or Parasitism: The biofungicide attacks and feeds on the pathogen, producing cell wall degrading enzymes, inhibiting or killing the pathogen. 

Induce Resistance to the Host Plant: The biofungicide triggers the host plant to turn on its own defense mechanisms. Plants produce salicylic acid (a derivative of aspirin) which travels to other parts of the plant and signals these tissues to activate their natural defense mechanisms.  This is also known as systemic acquired resistance (SAR), which improves the plants' response to pathogen attack by priming the metabolism of plant defense compounds.

Plant Growth Promotion: The biofungicide promotes root and shoot growth in the absence of disease causing pathogens. There may be increased nutrient availability of iron and other micronutrients by changing the pH or enzymes to help dissolve insoluble elements. 

Benefits of Biological Fungicides

  • Reduced risks to applicators and the environment.
  • Shorter re-entry intervals and days to harvest intervals than many conventional fungicides.
  • Many are labeled for use on edible crops including herbs and vegetables.
  • Most (not all) are OMRI approved for organic production. Check company labels or websites or see the OMRI website at www.omri.org
  • Less chance of plant damage, but not always, so consult product labels.
  • Generally compatible with beneficial  predators and parasites (natural enemies), beneficial nematodes (check company websites for more information).
  • Improved nutrient uptake of certain elements.
  • Can be used in rotation with conventional chemicals to reduce the risk of pathogens developing  resistance to conventional fungicides (especially systemic fungicides).
  • Not genetically modified.

Limitations of Biological Fungicides

  • Must be used preventively, for they will not cure diseased plants.
  • Must be used with proper cultural controls for plant growth, including clean starter material. 
  • Must use strict sanitation protocols.
  • Have a shorter shelf life  of 1 to 2 years (consult labels) than conventional fungicides and need to be stored under proper conditions.
  • May need to be re-applied more often than conventional fungicides.
  • May need to integrate traditional fungicides in rotation for more aggressive pathogens.

A number of products are commercially available for use on vegetable bedding plants and transplants. See Table 19 for information on labeled crops and diseases for these biological fungicides.

Fungal Diseases

Botrytis blight

Botrytis can cause leaf blight, stem 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 leaves and stems. Young leaves may become infected and then progress to the stem with tan stem cankers developing on basil and tomato.

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: Botrytis diseases can only be managed by a combination of methods including manipulation of environmental conditions (temperature, humidity and duration of leaf wetness), sound cultural practices and use of fungicides. Fungicides alone cannot control Botrytis and this pathogen has a long history of fungicide resistance development.

  • Control weeds and remove plant debris before 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.

Late Blight

Late blight is caused by the water mold Phytophthora infestans. The fungus-like organism typically overwinters in potato cull piles or in soil where plant tissue has not completely frozen and is not considered a problem for locally grown tomato seedlings. However the disease can be a problem on potato bedding plants. Late blight is not seed borne in tomatoes but can be carried on potato tubers used for bedding plant production. Petunias and tomatoes are in the solanaceous family and are susceptible to late blight. Using drip irrigation of petunia hanging baskets helps to minimize long periods of leaf wetness which is conducive to late blight.  In addition, in order to decrease the possible spread of late blight from one host to another, petunias and tomatoes should not be grown in close proximity (avoid placing hanging basket petunias over tomatoes and grow bench crops in separate greenhouses). 

Symptoms: Common symptoms on tomatoes are sunken, dark green or brown, water-soaked lesions on leaves and brown lesions on stems. White fuzzy growth sometimes develops under moist conditions. Leaf lesions begin as irregularly shaped olive green to brown spots and quickly grow larger – spots that are consistently small are most likely Septoria leaf spot. Confirm late blight by submission of a sample to a diagnostic laboratory.

Management: Many of the same fungicides used for Botrytis blight will help protect tomato seedlings from late blight.

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. Pathogens usually spread 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, properly produced compost-based or soilless mixes. Apply biological fungicides as a drench at planting or incorporate.
  • Disinfect all flats, cold frames, pots and tools.
  • Germinate seed under conditions that will ensure rapid emergence, using 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.
  • Apply biological fungicide as a drench at planting or incorporate into the growing media
  • Discard entire infected flats.

Downy Mildew

Downy mildew (Peronospora belbarhii) is a recent problem on basil (grown in the greenhouse and in the field). It was first reported in Florida in 2007 and has been found in New England since 2008. All sweet basil cultivars are very susceptible to downy mildew. The least susceptible basils include the lemon and spice types. Efforts are being made to breed new basil varieties with resistance to downy mildew.

Symptoms: Infected leaves develop a diffuse yellowing that is easily confused with nutrient deficiency. Distinct vein bounded patches on the underside of the leaves develop that produce dark purple-brown sporangia. The fuzzy, dark growth makes leaf undersides appear dirty.

Management: Management of environmental conditions such as temperature, humidity and duration of leaf wetness, sound cultural practices and fungicides will help prevent disease development. The pathogen needs at least 6 hours of leaf wetness and at least 12 hours for severe infections to develop. The optimum temperature for basil downy mildew to develop is 68F, with no basil downy mildew growth below 53F or above 77F.

  • Start with disease free seed. Ask if your supplier is steam treating their basil seed. (Basil seed produces a gelatinous exudate, so it is difficult to use hot water seed treatments).
  • Buy seed from a trusted source.  Talk to your supplier about how the seed was produced, if it has been tested. The pathogen may be seed borne, but the mechanisms involved are not well known and testing is difficult.
  • Select less susceptible varieties.
  • If you purchase plugs or transplants, inspect them carefully upon arrival.
  • Monitor plants at least once a week. Inspect plants in areas where the air movement is the lowest, such as the central part of the greenhouse or the middle of benches.
  • It is vital to 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.
  • Consider use of sub-irrigation to keep leaves dry.
  • If fungicides are used, they need to be applied preventatively on a regular schedule, before plants are infected. If contact fungicides are used, thorough coverage is needed to the underside of the leaves.
  • If you see symptoms of downy mildew, immediately destroy the infected plants, clean and sanitize the greenhouse.
  • After you discard the infected plants, protect adjacent plants with fungicides.
  • Plan on planting and harvesting basil early.

For Garden Retailers:  Dr. Meg McGrath, Cornell University suggests encouraging home gardeners to grow some plants in containers that can be brought inside when humidity outside is high (overnight and on rainy days). The pathogen needs at least 85% humidity to be able to infect.

Powdery Mildew

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

Most growers are familiar with powdery mildew when it develops on cucurbits in the field. The powdery mildew that affects certain cultivars of calibroachoa, petunia and verbena can also infect curcurbit seedlings including squash, cucumbers and pumpkins. Growers who produce curcurbit transplants as well as calibroachoa, petunia and verbena should be especially careful to separate these crops.  Fortunately, powdery mildew resistant verbenas are commonly available in the trade. 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.

Bacterial Diseases

Bacterial diseases of vegetable bedding plants, such as bacterial leaf spot of peppers and tomatoes, bacterial speck & 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 with yellowing at lesions' margins, 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 michiganensis pv. michiganensis (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 where cruciferous plants are grown. All Brassicas can be 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 light. 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. For example, black rot can survive on weeds in the same family as the host crop especially, mustard, shepherd's-purse and cruciferous weeds. Bacteria enter wounds created by insects, so keep insect pests under control. The management of these bacterial diseases is similar and includes the following strategies:

  • Buy certified disease-free 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 few resistant specialty peppers.

Viral Diseases

Some viral diseases of vegetable bedding plants include cucumber mosaic virus (CMV), tobacco mosaic virus (TMV) and the Tospoviruses, INSV and TSWV. There is no control for plants infected with a virus. It is important to have the virus disease accurately identified. Serological techniques such as ELISA (enzyme-linked immunosorbent assay) are now available to accurately identify a wide range of viruses. On-site grower kits using this same technology are also available from Agdia (www.agdia.com) to test for viruses such as CMV, TMV, INSV, and TSWV.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) has a wide host range of over 400 species of plants including vegetables, ornamentals and weed hosts.

Symptoms: Infected plants may show mild mosaic patterns and mottling, flecking, and fern leaf distortion.

CMV is primarily spread by aphids that can acquire the virus in as little as 5 to 10 seconds. Aphids then move the virus from plant to plant for a few hours.

Management: Rogue diseased plants. Eliminate weeds such as common pokeweed, chickweed, field bindweed, yellow rocket, and bittersweet nightshade that may be reservoirs of CMV.

Tobacco Mosaic Virus (TMV)

TMV has a wide host range but is especially a concern on solanaceous crops. In recent years, TMV has been reported on pepper, calibrachoa, petunia, and tomato. TMV is not transmitted by insects! It is a very stable virus that can be spread by contact. Workers can easily spread TMV when they handle plants or when cutting tools become contaminated. TMV can persist in dried tobacco leaves, so tobacco products can also be a source of TMV.

Symptoms: Symptoms include yellow mottling, upward leaf curling and overall stunting. Some infected plants may not show any symptoms at all.

Management: Discard infected plants including roots, plant debris, potting media and associated plastic tags. Wear disposable gloves and discard gloves immediately afterwards. Carry the sealed plastic bags directly out of the greenhouse. Do this at the end of the working day.  Disinfect hands by washing with milk, or tri-sodium phosphate and then thoroughly with soap and water. Smokers need to wash their hands before entering the greenhouse so they do not infect plants. In greenhouses, hard surfaces such as door knobs, or flats can become contaminated after handling virus-infected plants and remain a source of infection. Thoroughly disinfect the growing area with a commercial disinfectant. A 20% solution of non-fat dry milk can be used to wash contaminated hands or tools. Control perennial weeds in the Solanaceous family such as ground cherry and horsenettle that could be reservoirs of TMV.

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 basil, 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 a virus. Therefore, vegetable bedding plants and ornamentals should always be grown in separate greenhouses.