Epidemiology of Potato Gangrene in Tasmania

In Tasmania, Phoma exigua Desm. var. exigua accounts for 90% of potential inoculum on tuber surfaces and 90% of gangrene lesions in stored potato tubers. P. exigua Desm, var, foveata (Foister) Boerema accounts for the remaining 10%. Tubers lifted from pre-senescent crops contain negligible levels of surface-borne inoculum and develop few lesions in storage. Washing removes only a small percentage of potential inoculum on tuber surfaces, which initiates infections if tubers are wounded and held in an environment unfavourable for wound healing. Tuber resistance to gangrene disease is determined by the interaction between tuber cultivar, tuber maturity and the nature of the fungal isolate within both varieties of P. exigua. The resistance response is greatest at high temperatures and is characterized by three phases: lesion retardation, lesion arrest and lesion rejection. Lesion rejection represents the end product of tuber resistance response and is associated with the development of post-infectional periderm.