Hundreds of thousands of dollars worth of crops have been destroyed by Impatiens Necrotic Spot (INSV), and Tomato Spotted Wilt Virus (TSWV), once called the I-strain and L-strain of tomato spotted wilt. However, growers aggressively attacking the problem can avoid crop losses by controlling thrips, carefully inspecting new plants brought into the greenhouse, and maintaining the health of plants already in the greenhouse.
Below are the steps required to aggressively attack INSV and western flower thrips in greenhouse production.
Impatiens Necrotic Spot
The virus causes a wide variety of symptoms including wilting, stem death, stunting, yellowing, poor flowering, 'chicken pox-like' sunken spots on leaves, etches or ring spots on leaves, and many others. In other words, symptoms will not tell you anything except that there is something wrong with the plant. That something could be virus or any one of 1000 other things. Virus symptoms depend upon what time of year the plant is infected, its age when infected, its physiological state when infected, growing conditions in the greenhouse, the strain of virus present, and other factors not fully understood at this time. In fact, some infected plants do not exhibit any symptoms.
Positive diagnosis is made by submitting the plant to a plant disease clinic or commercial company capable of running chemical tests to determine if the virus is in the sap.
Management of INSV-caused diseases:
Inspect all incoming plants for symptoms. Those found with suspicious symptoms must be immediately isolated until the presence or absence of the virus is confirmed.
Isolate incoming plants from all other plants in the greenhouse until certain they are free of the virus. If the plants are free of the virus, maintain their isolation unless certain all other plants in the greenhouse are free of the virus. In other words, don't put all your plants in one greenhouse.
Destroy infected plants. Infected plants cannot be cured. Do not vegetatively propagate infected plants. The only way the virus is maintained and spread in a crop is through vegetative propagation if western flower thrips are not present.
Western Flower Thrips
Detection Of Thrips Activity
Inspect all incoming plants for thrips. Especially check white and yellow flowers. Isolate incoming plants from all other plants in the greenhouse until certain they are free of thrips. If the plants are free of thrips, maintain their isolation unless certain all other plants in the greenhouse are free of thrips. Use a hand lens. Tap the plant with a pencil while holding a sheet of white paper to catch any falling insects.
Since the thrips are very small and stay hidden most of the time, they are difficult to detect. Yellow or blue sticky cards placed at crop height are excellent for monitoring thrips. Blue is effective for thrips but not a good all around color for other insects. One to 3 cards per 1000 sq. ft. is recommended. Effectiveness depends on number used per sq. ft. and placement rather than the size of the card. Place some near vents, doors, and other openings. If you are uncertain of the identity of the insects you have trapped, contact your state Department of Agriculture plant inspector, your County Extension Agent, or a Department of Entomology at your landgrant university for assistance. Change the cards regularly. For ease of handling, cover used cards in one layer of cellophane or similar clear plastic wrap when they are removed. Record the number of thrips trapped each week to determine if the population is increasing or decreasing.
The thrips-virus relationship and controlling the thrips:
A single application of any material is not adequate. Applications are made at 5-day intervals if allowed as stated on the chemical label. Use one pesticide for one generation of thrips. That means, apply one pesticide 2-3 times over a a 14-15 day time. Then switch to a different class of pesticide. Do not use tank mixes of different chemicals.
Thrips lay their eggs inside the plant tissue where they are protected until they hatch and emerge 3 to 4 days later. The first enstar (larva) is clear. Feeding larvae acquire the virus but do not transmit it from plant to plant. The larvae retain the virus until they mature into feeding adults. The second larva is yellow at first. Both the first and second larval stage hide among the bud and flower parts of the plant. The second larva turns white just before it molts and moves to the soil or leaf litter. Adults emerge from the soil 2 to 5 days later and may be yellow or dark brown. They hold their wings flat over their backs. Adults can live 30 to 45 days and transmit the viruses to plants throughout their life. The viruses do not pass into new eggs. Each new generation of larvae must feed on an infected plant to acquire the virus. The first chemical application should kill winged adults while the second and third should kill newly emerged wingless individuals. Thorough coverage is required. It is also suggested that methods of application be rotated. Fogging and spraying should be employed.
If there are no INSV or TSW virus-infected plants in the greenhouse, no crops will be lost to this virus even if western flower thrips are present.
Be Certain the Chemical is Registered for use on the Plants to be Treated.
Consult with a Department of Agriculture plant inspector in your state or an entomologist at your landgrant university to determine what materials are registered and recommended for thrips control on your specific crop.
Maintain strict thrips control on all plants kept in the greenhouse. Eliminate all weeds in the greenhouse and ones close to the building and all plants not being carefully tended from inside the greenhouse. Such plants may harbor both the thrips and the viruses. Western flower thrips are known to survive outdoors in Pennsylvania.
Partial list of plants found infected with tomato spotted wilt and/or impatiens necrotic spot virus:
|Aubrietta||Eggplant||Plectranthus (Swedish ivy)|
|Begonia||Exacum||Primrose (annual, Primula)|
|Browallia||Gaillardia (Blanket flower)||Salvia|
|Calendula||Geranium (ivy, diploids, and tetraploids)||Shamrock (Oxalis)|
|China aster||Impatiens (New Guinea, balsam)||Stock|
|Cineraria||Lily, Asiatic hybrid||Tiger lily|
INSV on impatiens.
INSV on impatiens.
INSV on coleus.
INSV on cyclamen leaves and flowers (Cyclamen photos by F. E. Gildow.)
Prepared by Gary W. Moorman, Professor of Plant Pathology