I. Introduction: Thread blight is caused by the fungus Corticium stevensii Burt. The disease is primarily a problem in poorly managed orchards in the southeastern United States.
II. Symptoms: Thread blight symptoms are readily noticed in early summer. Leaves wilt and turn brown, usually in the interior or shaded portions of the tree. Dead, curled leaves cling to blighted branches, frequently in mid-branch, with unaffected leaves still appearing on both sides of the diseased area. In well managed orchards, the disease is usually not seen until after harvest, when the fungicide spray program has been discontinued. Positive diagnosis in the field is made by observing signs of the fungus. A sparse, white mycelial fan can be observed in advance of dead areas on partially blighted leaves. This mycelium can frequently be traced as fine white threads back to the leaf petiole and twigs. Abscised leaves may be tied to twigs and leaves by this network of threads and mycelium. The fungus is present on twigs and branches as silvery-tan rhizomorphs and white to tan sclerotia, which become hard and dark brown with age. Rhizomorphs may be up to 1/16 inch wide and sclerotia up to 3/16 inch wide and 1/8 inch thick. These fungal structures are superficial and can be scraped from the bark. The bark and wood of blighted branches do not appear adversely affected by the disease. Sclerotia and rhizomorphs can also grow superficially on fruit.
III. Disease Cycle: Information on the disease cycle of thread blight is limited. Sclerotia retain viability over the winter under eastern Kentucky conditions. Leaf blight usually appears first in the late spring on branches having sclerotia from the previous season. During the season, the fungus may grow from blighted to adjacent healthy leaves. The disease is more severe when trees are growing in a moist, shaded environment.
IV. Monitoring: Monitor for thread blight in early summer in areas of the orchard where the disease was present in previous years. Monitor again after harvest and before leaf abscission and record the location of any new centers of infection.
V. Management: Thread blight, once established in an orchard, is difficult to control with fungicides. Preventative fungicide sprays applied to trees prior to infection may protect the orchard from thread blight. Avoiding "hollows" and other shaded and poorly ventilated areas when selecting an orchard site should help prevent the disease. Under light disease pressure, pruning out blighted twigs and branches may provide adequate control. Pruning to promote better penetration of sunlight and air may also help.