The impact of oospores of Phytophthora infestans on late blight epidemics

Several aspects of the ecology of oospores of Phytophthora infestans were studied in the Netherlands using both observational and experimental methods. Following the introduction of a genetically variable late blight population in Europe during the 1970s, P. infestans epidemics have become more severe, leading to an increase in fungicide use in many potato production areas in Northwestern Europe. In the Netherlands, oospores are readily produced in unsprayed crops and volunteer potatoes and their incidence varied from 78% to 15% of sampled leaflets with two or more lesions, for the northeastern and southwestern region in 2000, respectively. A fungicide application following infection of plants with an A1 and A2 mating type strain significantly reduced the number of oospores produced as well as oospore viability. Several alternative hosts facilitating oospore formation have been identified in the Netherlands: S. nigrum, S. dulcamara and S. sisymbriifolium. The impact of oospores on late blight epidemics is discussed.