A wide range of colors and a preference for difficult shady areas has made impatiens one of the most popular bedding plants in the United States. Two species are readily available at most garden centers: Impatiens wallerana, Busy Lizzy or Touch-me-not, and New Guinea impatiens (Impatiens hawkeri). Both of these species are hardy in U. S. Department of Agriculture plant hardiness zones 9 through 12. Impatiens are vigorous plants but do have some disease and pest problems that can usually be avoided through good cultural practices.
Fungal diseases that affect impatiens include root, crown and stem rots, powdery mildew, gray mold and downy mildew. Dull foliage, wilting and yellowing are the first aboveground symptoms of root and crown disease. Stunting and death quickly follows. Stems turn dark and soft and decay, break off easily and have brown tips. Seedlings don't emerge and rot in the soil. Powdery mildew covers the leaves with a soft, white fuzzy surface. Likewise with gray mold except the spore mat is gray. Downy mildew appears as a white, lavender, or purple dusting on leaves.
Bacterial soft rots are caused by a number of organisms. Affected plants turn brown and mushy near the soil. Seedlings collapse and plants grow slowly. Bacterial leaf spots start as small, water-soaked areas that may encompass the entire leaf. These same bacteria may also cause vascular wilt if they become systemic. Fasciation on impatiens is a condition that it not completely understood but is thought to be caused by bacteria, viruses, or possibly genetics. Stems are abnormally flattened and appear to have grown together. Shoots are short and grow in swollen clumps. Leaves are plentiful and abnormally small. The bacteria that cause fasciation live in dead and decaying leaf and flower material.
The two main viruses that affect impatiens are the impatiens necrotic spot virus and tomato spotted wilt virus. Symptoms include black ring spots, mosaic patterns on the leaves and dead tissue that is brown or tan. Flower color is mottled and the leaves become distorted and yellow. Stems develop black areas or lesions and plants become stunted. There is no cure for either of these viruses once they have infected the plant.
A number of insects are found on impatiens. Most pernicious is the black vine weevil. This pest attacks most pot plants leaving notches on leaf edges. These slow-moving weevils are nocturnal and feed on the foliage. The larvae live in the soil and devour the plant's root system. Greenhouse whitefly adults suck the plant juices, leaving yellow stippling marks in the leaves. Red spider mites are tiny, sap-sucking mites that are neither red nor spiders. They are found on the undersides of leaves and are most prevalent in warm, dry conditions. Thrips, another tiny sap-sucking insect, is among the worst of greenhouse pests and is also found on the undersides of leaves. Both whiteflies and thrips can spread diseases. Mealybugs are sap-sucking insects found on the upper surfaces of the plants. Other insects that may be found on impatiens are melon mites, broad mites, cyclamen mites, and leaf miners.