Fusarium wilt – Fusarium spp., fungus
Fusarium wilt can result in a variety of symptoms on tubers, ranging from surface decay to vascular discoloration. Several species of Fusaria can infect potato and cause wilt symptoms in the plant, and each causes slightly different symptoms on tubers. The most important of these pathogens can cause sunken brown necrotic areas at the stem attachment, firm brown circular lesions on the tuber surface, and brown discoloration of the vascular tissues. A shallow cut through the stem end reveals the streaky vascular discoloration referred to as "stem–end browning." This disease may also cause a light brown to tan discoloration, extending a short distance on each side of the vascular system in the tuber, which may resemble symptoms of ring rot (14). However, this tissue is firm and does not produce a cheesy exudate when squeezed as does tissue affected by bacterial ring rot. Vascular discoloration resulting from net necrosis (19) and vine killers may also be confused with Fusarium wilt. It is difficult to distinguish between Fusarium and Verticillium wilts (20) based on tuber symptoms alone, and isolation of the causal pathogen is necessary for positive diagnosis. Infected tubers can be an important source of inoculum.