VIEW ARTICLE

Resistance

Interrelationships of Seedling Age, Inoculum, Soil Moisture Level, Temperature, and Host and Pathogen Genotype in Phytophthora Root Rot of Alfalfa. R. G. Pratt, Former Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address of senior author: Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 66:81-85. Accepted for publication 11 July 1975. DOI: 10.1094/Phyto-66-81.

Resistance of two susceptible cultivars and of two resistant lines of alfalfa was compared by transplanting 2-, 4-, 6-, and 8-week-old seedlings into soil infested with cornmeal-sand inoculum of Phytophthora megasperma. Taproot disease was usually most severe in seedlings transplanted at 2 weeks of age and least severe in those transplanted at 8 weeks of age. Lines and cultivars differed in resistance at all ages tested. Disease severity in seedlings transplanted when 6 weeks old was greater at low dilutions of inoculum (1/8 or 1/32, v/v) than at the highest dilution (1/2,048), but four-fold or greater dilutions did not always result in less disease. Resistance of lines and cultivars did not differ when disease was severe at the lowest inoculum dilution (1/8, v/v). With few exceptions, disease was more severe at 20 and 24 C than at 16 or 28 C. Periodic flooding of soil caused greater disease in all lines and cultivars at one or more temperatures. Differences in resistance were evident at all temperatures. Growth of roots of cuttings from field-resistant plants was generally greater than growth of roots of susceptible plants in the presence of P. megasperma. Differences in virulence of P. megasperma isolates were not related to geographic areas of origin and do not suggest the existence of different pathogenic races.