Verticillium wilt

Foliar symptoms first appear as chlorosis and necrosis beginning in the lower leaves. On warm, sunny days, leaves may appear limp and flaccid. Sometimes symptoms occur on only one side of the leaf or the plant. In severely diseased plants, medium-tan discoloration of the vascular tissue is evident, and the plants may be stunted. Tubers of some cultivars may develop a light brown discoloration of the vascular ring, although other factors may cause this symptom. Tuber yield is reduced because of the decreased rate of photosynthesis and premature death of foliage. The optimum temperature range for potato growth is 18-20 C (64-68 F). When the temperature rises above 20 C (68 F), plant stress increases and symptoms of Verticillium wilt are more severe.