Plectosporium Blight

Plectosporium blight (Microdochium blight), caused by the fungus Plectosporium tabacinum (Microdochium tabacinum), is an important disease of pumpkin and squash. Plectosporium blight could cause yield losses of up to 100% (Fig. 1).

Symptoms. Plectosporium blight fungus infects stems, leaf veins, petioles, and fruit. The disease is characterized by the production of light tan “bleached,” sunken, spindle-shaped lesions on the stems, petioles, main leaf veins, and peduncles (Fig. 2). Initially, the lesions are small, but they quickly coalesce, causing the entire surface of the stem or leaf vein to turn white (Figs. 3and 4. Because leaf lesions are restricted to the veins and do not spread to the interveinal tissue, they may be overlooked in the early stages of disease development. Infected stems are dry and brittle. Leaves on the severely affected vines die and complete defoliation may occur in severe infections. On fruit, the fungus causes white, tan, or silver russeting on the surface (Fig. 5).

Disease Cycle. P. tabacinum occurs in soil and decaying plant material. The fungus produces two-celled, ellipsoidal to cylindrical and slightly curved spores. The spores are likely spread by rain-splash and wind and initiate infection upon landing on host tissues. Warm, wet weather favors disease development.

Disease Management. No resistant pumpkin or squash variety to Plectosporium blight has been reported. Rotation with noncucurbit crops should help reduce disease incidence. The disease is readily controlled by application of fungicides. Chlorothalonil (e.g., Bravo) and pyraclostrobin (e.g., Cabrio) provide effective control of Plectosporium blight in pumpkin fields. For up-to-date information on management of Plectosporium blight, refer to the current edition of the “Midwest Vegetable Production Guide for Commercial Growers”.

Fig. 1. A pumpkin field with Plectosporium blight. Fig. 2. Spindle-shaped lesions of Plectosporium blight on a petiole of pumpkin.

Fig. 3. Stem lesions of pumpkin, caused by Plectosporium tabacinum. Fig. 4. Plectosporium blight lesions on leaf veins of pumpkin.

Fig. 5. Silver russeting on fruit and stem of pumpkin, caused by Plectosporium tabacinum.