Edward J. Sikora, Extension Plant Pathologist, Associate Professor, Plant Pathology, and Joseph M. Kemble, Extension Horticulturist, Assistant Professor, Horticulture, both at Auburn University Plant Disease Notes Late Blight of Potato Late blight, caused by the fungus Phytophthora infestans, is an extremely serious potato disease that predisposes tubers to severe damage and storage losses. Late blight is visible as pale green, water-soaked spots on the leaf's edges or tips. These circular to irregular lesions are often surrounded by a yellowish border that merges with healthy tissue.
Lesions expand rapidly, taking on a purplish, brownish, or blackish color.
During high moisture periods, lesions are often ringed with grayish white fungal growth. Foliar destruction can be severe, with large crop areas defoliated in as little as one week from the time of initial symptom development. Secondary infections, caused by bacteria or other fungi, can result in the slimy breakdown of additional tuber tissue in storage. Persistence And Transmission. Late blight most commonly occurs under cool, wet conditions.