Botrytis blight, also called tulip fire, is the most common disease problem on tulip. Damp, overcast weather favors the growth of the causal fungus, Botrytis tulipae. This fungus commonly attacks tulips that have been damaged by frost or hail.
All parts of the plant may be infected. Leaves show small, slightly sunken oval spots that usually have a dark margin. With moist conditions, these spots may enlarge and coalesce, sometimes involving entire leaves. Stems may rot off completely. Flower lesions also appear as small oval spots that are light in color. The entire flower may eventually become blighted. The outer scales of the bulb develop blackish fungal structures called sclerotia. These resistant structures allow the fungus to survive the winter and remain viable in the soil for years.
Botrytis, also called the gray mold fungus, can be seen on diseased plant parts in moist weather. Under these conditions, the fungus produces large numbers of spores, causing a grayish mold to appear. These spores are spread to neighboring tulips by air currents. Plants that grow from diseased bulbs are a common source of infection.
Certain cultural practices can help control Botrytis blight. Examine bulbs before planting and discard any that appear damaged or moldy. Because infection is more likely to occur on bulbs that have been bruised or cut, take care to prevent injury to bulbs. Diseased plant parts should be removed as soon as symptoms are noticed. Working around plants during dry conditions will help reduce the risk of spreading the disease. If desired, protection from disease can be achieved by applying a fungicide starting when the leaves emerge from the soil and spraying several times until the bloom stage. Because the tough sclerotia produced by the fungus can survive for a number of years in the soil, it is best to rotate planting site for tulips.
This article originally appeared in the April 16, 1999 issue, p. 41.