SUSCEPTIBILITY OF STRAWBERRY GENOTYPES TO INFECTION AND COLONIZATION BY RACES OF PHYTOPHTHORA-FRAGARIAE AND THE GROWTH-RESPONSES OF INOCULATED GENOTYPES

Nineteen strawberry cultivars maintained in the North Carolina strawberry certification program and Cambridge Favourite, a cultivar widely grown in Europe, were evaluated for susceptibility to infection and colonization by seven races of Phytophthora fragariae. Darrow, Delite, Earliglow, and Midway were susceptible to infection and' colonization by only one of the seven races tested. Partial resistance (i.e., incompletely effective) was detected for 12 strawberry cultivars in response to five races of P. fragariae and appeared to be race-dependent. In controlled environment chambers, the susceptible cvs. Tennessee Beauty, Atlas, and Earlibelle had fewer crowns per plant, lower foliar and root dry weights, and higher percent root necrosis when inoculated with isolates NC-1 and NC-2 of race Pf-2 than noninoculated controls. However, no significant reduction in the number of crowns per plant, foliar and root dry weights, or percent root necrosis was observed for the susceptible cultivar Sunrise when plants were inoculated with isolate NC-2 of P. fragariae. Inoculated plants of Cardinal, a cultivar with a low disease severity index (an experimental measure of infection and colonization) to race Pf-2, had reduced foliar and root dry weights and a four- to sixfold increase in percent root necrosis relative to the noninoculated controls. Growth characteristics of resistant Earliglow plants inoculated with P. fragariae were not reduced relative to noninoculated controls. Some of the mechanisms that could be responsible for partial resistance in the cultivars Sunrise and Cambridge Favourite are discussed.