Extension offices and state universities are warning growers of the dangers this year of Impatiens Necrotic Spot Virus (INSV) and thusly a lack of availability of Impatiens this growing season. INSV is closely related to the Tomato Spotted wilt Virus (TSWV) and was once called I-strain and L-strain of TSWV.
INSV causes a wide variety of symptoms including wilting, stem death, stunting, etches of ring spots on the leaves and sunken spots on the leaves as well as other symptoms. The virus has the ability to “compartmentalize” itself so that it affects only one area of the plant. Regardless of the situation, all affected plants should be destroyed immediately as there is no cure for this virus. Other plants affected by this virus are gloxinia, cineraria, chrysanthemums (all of which you will see now for the Easter season), begonias, tomatoes, other vegetables and grasses. There have even been reports of the virus showing up in specimens of salvia.
Other than the propagation of infected plants, the virus is only known so far to be spread by thrips, which are tiny insects only about 1/5” long and may be seen as “animated” lines running along veins on the undersides of plant leaves. Signs of thrip damage may be brown or mottled silver wilted leaves and the damage is more easily seen than the thrips themselves. Other damage includes folding of the leaves, leaf roll, leaf blisters, discoloration of petals and scarring of flowers.
What can you do as a home gardener? Inspect all plants prior to purchase for any signs of disease. If you are not sure, ask the grower to look at it with you. Once you are satisfied with the plants you are purchasing, be sure to feed and water your plants regularly to keep them healthy. A healthy plant is less prone to disease. Dried out, wilted plants, since they are under stress, are more prone to infection and attack by thrips. Remove any faded or dead blooms right away as well as dying foliage. You should also keep your garden and yard free of weeds. Thrips thrive in weedy locations and both the larvae and adults feed on flowers leaves, twigs or buds. They affect many plants, not just impatiens.
Thrips, though they have wings, are not very good fliers. However, they can readily be spread over long distances by floating with the wind or being transported by other infested plants. After a storm has passed, it is probably a good idea to inspect your garden for evidence of thrips or other insects that may have been brought in by the storm. If you suspect an infection, contact your local extension office for advice. If pesticides are recommended, do not rely on them solely to treat your problem. Good cultural habits must also be used. The best practice is to keep your area clean and the plants healthy, because unless you can use a systemic pesticide (thrips burrow and hide), you may not be able to effectively treat the plants.
For more complete information and photos on thrips and their control, please visit the UC Davis website.