Effect of Host Plant Resistance and Reduced Rates and Frequencies of Fungicide Application to Control Potato Late Blight

Late blight of potato (Solanum tuberosum, L.) caused by Phytophthora infestans (Mont.) de Bary, is a major worldwide threat to the production of high quality potatoes (12). Unchecked, P. infestans can rapidly defoliate plants in the field and can infect potato tubers when spores are washed into the soil (15). Prior to the early 1990s, the population of P. infestans throughout much of the world was clonal, mefenoxam/metalaxyl sensitive, and US1/A1 (genotype/mating type; 13). Potato late blight control strategies changed following the migration of mefenoxam/ metalaxyl-resistant populations of P. infestans from Mexico to North America in the 1990s (12) and necessitated cultural control methods and crop protection strategies that rely primarily on protectant foliar fungicide applications (12,18). Although fungicides have been used to manage late blight, both the efficacy and availability of commonly used fungicides have been threatened. This problem is compounded by the demand to reduce chemical input in agricultural systems (9) and the potential loss of commonly used protectant fungicides such as chlorothalonil (14) and metalaxyl/mefenoxam (17). In addition, the cost of protecting potato crops in the United States against late blight is estimated at $155 million annually (19). Therefore, crop production economics would suggest that more economical, yet still effective methods of disease control need to be developed. There are several potential methods for reducing fungicide inputs in potato crop management. These include the use of fungicides with less active ingredient, reduced application rates, longer application intervals, and a combination of any of these strategies. In addition, Fry (10,11) observed that a combination of cultivar resistance and regular applications of protective fungicides reduced foliar late blight infection in potato. There are currently no late blight resistant potato cultivars that meet commercial standards in the United States. However, controlled environment and field trials at Michigan State University have identified certain foreign cultivars and advanced breeding lines (ABL) that are less susceptible to foliar late blight in the absence of fungicides than important cultivars grown and developed in the United States (e.g., Snowden, Atlantic, and Russet Burbank; 4–6,8). Typical fungicide application programs use a 5to 7-day spray interval, depending on environmental conditions and grower preference. The frequent fungicide spray intervals and rates currently used by growers to control late blight are expensive and more economical control measures are needed. Therefore, the objective of this research was to determine if acceptable control of foliar late blight can be achieved by using increased fungicide spray intervals and reduced application rates of residual contact fungicides on potato germplasm with a range of susceptibility to late blight.