The Effect of Wounding, Temperature, and Inoculum on the Development of Pink Rot of Potatoes Caused by Phytophthora erythroseptica

Pink rot, caused by Phytophthora erythroseptica Pethybr., is an important disease of potato (Solanum tuberosum L.) (20). The name of the disease describes a characteristic and diagnostic pink color that appears after infected tissue has been cut and exposed to air for 20 to 30 min (8,12,17). Other symptoms of pink rot include dark discoloration of the skin and lenticels of infected parts, purplish discoloration of the eyes, and firm leathery texture of the rotted tissue (8,12,17). Pink rot was first described on potatoes in Ireland by Pethybridge (17) in 1913, and first reported in the United States in Maine in 1938 (2). Pink rot is widely distributed in North America (22) and other parts of the world (25). Potato farmers often refer to pink rot as “water rot” (1) without distinguishing the pathogen(s) involved. In 1991 we examined tubers with “water rot” from 45 potato storage sites in seven states and one Canadian province (22). P. erythroseptica predominated in tubers from 32 (71%) of the sites, while Pythium spp. predominated in four locations. In locations where it was present, P. erythroseptica was recovered from 57% of rotted tubers. P. erythroseptica was recovered from samples from all regions included in the survey. A representative sample of isolates of P. erythroseptica was tested for pathogenicity. All caused typical pink rot symptoms when inoculated into potato tubers. Similar studies were done on a more limited regional basis in 1992, 1993, and 1994 with similar results (16). Although the firm leather texture and the pink color of the infected tissue are diagnostic symptoms of pink rot, confirmation that the disease is caused by P. erythroseptica must be accomplished by culturing symptomatic tuber pieces on semiselective media (28) or on water agar (pH = 5.5). Since P. erythroseptica is homothallic, it forms its characteristic oogonia with amphigynous antheridia abundantly after 7 to 10 days of culture at 15 to 24oC (4,9,25). Zoospores, sporangia, and oospores are the natural inoculum sources of P. erythroseptica (15,17,18,21). Inoculum sources for laboratory and greenhouse studies have included: Replacement of excised tuber tissue with colonized agar plugs (8,12), dipping of wounded tubers in a suspension of mycelium and oospores (14), pouring mycelia and/or zoospore suspensions onto injured roots (29) or uninjured roots, stems, stolons, tubers, and leaves (15), or the use of infected tissue pieces from artificially inoculated tubers (8,12,13). Published information about factors influencing pink rot development relate to occurrence of the disease in the field or at harvest. Soil moisture is an important factor in the development of pink rot (1,5). The disease is frequently found in lowlying areas, wet fields, or in fields under ABSTRACT Salas, B., Stack, R. W., Secor, G. A., and Gudmestad, N. C. 2000. The effect of wounding, temperature, and inoculum on the development of pink rot of potatoes caused by Phytophthora erythroseptica. Plant Dis. 84:1327-1333.