Factors affecting the survival of metalaxyl-resistant strains ofPhytophthora infestans (Mont.) de Bary in Ireland

Summary Five metalaxyl-resistant and five sensitive isolates ofPhytophthora infestans were compared for sporangium production per unit leaf area, sporangium germination and virulence to foliage and to tubers. Differences between the two groups of isolates were significant for sporangium production per unit leaf area only. The metalaxyl-resistant strains produced fewer sporangia per unit leaf area than the metalaxyl-sensitive strains. The implications under field conditions are discussed.