Plectosporium tabacinum infects stems, leaf veins, petioles, and fruit. Symptoms of Plectosporium blight are very distinctive. The disease is characterized by the production of light tan “bleached,” sunken, spindle-shaped lesions on the main stems, petioles, main leaf veins, and peduncles. Initially, the lesions are small, but they quickly coalesce, causing the entire surface of the stem or leaf vein to turn white. 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. Individual lesions are less that 1/4 inch in diameter, but often coalesce to form a continuous dry, scabby surface. Fruit stems may become entirely white at harvest.