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Impatiens Necrotic Spot Virus by Paul Pilon

June 08, 2010

Impatiens necrotic spot virus (INSV) is type of plant virus that infects a wide range of ornamentals including hundreds of commonly grown bedding plants, potted crops, perennials, and shrubs. INSV is a tospovirus that is primarily transmitted or vectored to crops by Western flower thrips (Frankliniella occidentalis), but can also be vectored with vegetative propagation.

Western flower thrips can only acquire INSV when the first instar larvae feed on virus infected plants and/or weeds. Other life stages cannot acquire INSV, but do vector the disease if it was acquired in the first larval stage. Winged adult thrips are primarily responsible for spreading viruses to uninfected plants.

The symptoms observed vary with the type of plant infected, the plant’s age, how long it has been infected, the environment, and the amount of stress it has undergone. The primary symptoms include black, brown, reddish or yellowish concentric rings, necrotic streaks, rings or spots on the leaves and stems, distorted growth, stunting, and bud drop.

Growers should not rely solely on visual observations to properly diagnose INSV as the symptoms observed often resemble other plant viruses, fungal diseases, or other plant disorders. Symptomatic plant tissues should be sent to virus testing laboratories for more accurate testing and identification procedures. Agdia, Inc. (www.agdia.com) has developed practical and cost effective ImmunoStrip® test kits for identifying INSV and other viruses; growers can use ImmunoStrip® on-site to provide quick and reliable results.

There is currently no cure for plants that have become infected with viruses. Therefore, any symptomatic or infected plants should be removed from the production facility and discarded after plant symptoms appear and are confirmed to have INSV. Failure to remove symptomatic plants from the crop area will likely result in significantly more infected plants down the road as thrips vector INSV from infected plants to non-infected ones.

The first strategy in managing INSV and other viruses is to ensure clean, virus free starting materials are entering the production facility. This involves inspecting any incoming plants for insect vectors (thrips) and virus symptoms.

Scouting programs can help growers detect the presence of thrips and help identify plants expressing virus symptoms early. Once a plant has been diagnosed with a virus, it should be removed from the production facility immediately.

Indicator plants, such as the fava bean, which rapidly expresses INSV symptoms, can be inter-dispersed within the crop as a tool to help monitor and detect the presence of INSV early. Virus infected or symptomatic indicator plants should be removed from the production area to prevent the transmission of INSV to uninfected crop plants.

With no known controls for INSV, the best management strategy involves controlling the vector – Western flower thrips. Thrips exclusion screening on greenhouse openings such as vents, sidewalls, and air intake fans will prevent thrips from entering the production site. It is also beneficial to control all weeds that are growing within and around the production facilities as they are also frequently host plants for INSV and a source of food for Western flower thrips as well; removing weeds that may be infested will reduce the chance of thrips from vectoring viruses from the weeds to the crops.

Thrips can be controlled biologically using Beauveria bassiana, predatory mites (Amblyseius), parasitic wasps (Thripobius), or pirate bugs (Orius). Several insecticides for thrips control include abamectin, acephate, azadirachtin, chlorpyrifos, chlorfenapyr, cyfluthrin, fenpropathrin, horticultural oil, insecticidal soap, methiocarb, pyrethrum, s-kinoprene, and spinosad. Successful thrips control usually entails applying insecticides at 5-7 day intervals until thrips populations have reached acceptable levels.

Paul Pilon
Perennial Solutions Consulting
paul@perennial-solutions.com

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