Thread blight is a fungus disease caused by Corticium stevensii. Primarily, the disease is a problem in poorly managed orchards in the southeastern United States.
Thread blight symptoms are noticeable in early summer. Leaves wilt and turn brown (fig. 1), usually on the interior or shaded portions of the tree. Dead, curled leaves cling to blighted branches, frequently mid-branch, with unaffected leaves remaining on both sides of the diseased area (fig. 2). In well-managed orchards, the disease usually does not occur until after harvest, when growers have discontinued their fungicide spray programs.
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Figure 1. Leaves killed by the thread blight fungus. Photo: Alan R. Biggs, West Virginia University. |
Figure 2. Leaves killed by the thread blight fungus and healthy leaves nearby. Photo: Alan. R. Biggs, West Virginia University. |
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Positive diagnosis in the field results from observing signs of the fungus. A sparse, white mycelial fan is evident in advance of dead areas on partially blighted leaves. Frequently, this mycelium connects through fine, white threads back to the leaf petiole and twigs. The network of mycelium and threads may tie abscised leaves to twigs and other leaves. 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 (figs. 3, 4). Rhizomorphs may be up to 1/16 inch wide, and sclerotia may be 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 to be adversely affected by the disease. Sclerotia and rhizomorphs can grow superficially on fruit as well.
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Figure 3. Silvery rhizomorphs, or “threads,” and round sclerotia of the thread blight pathogen on an apple twig. Photo: Alan R. Biggs, West Virginia University. |
Figure 4. Closer view of figure 3. Photo: Alan R. Biggs, West Virginia University. |
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Information on the disease cycle of thread blight is limited. Under eastern Kentucky conditions, sclerotia retain viability over the winter. Usually, leaf blight appears first in late spring, on branches having sclerotia from the previous season. During the season, the fungus may grow from blighted leaves to adjacent healthy leaves. The disease is more severe when trees are growing in a moist, shaded environment.
Thread blight, once established in an orchard, is difficult to control with fungicides. Applying preventative fungicide sprays to trees prior to infection may help protect the orchard from thread blight. You may be able to prevent the disease through certain cultural practices. For example, avoid hollows and other shaded and poorly ventilated areas when selecting an orchard site. Under light disease pressure, prune out blighted twigs and branches. Also, consider pruning to promote better penetration of sunlight and air.
Resource
http://www.caf.wvu.edu/kearneysville/disease_descriptions/threadblight.html
Original text prepared by A. L. Jones and T. B. Sutton.