Inheritance of mefenoxam resistance in Phytophthora nicotianae populations from a plant nursery

Three sexual crosses involving isolates of P. nicotianae with differing sensitivity to mefenoxam were established to study the inheritance of mefenoxam resistance. Mefenoxam sensitivity was determined by measuring the mycelial growth on both mefenoxam-amended clarified V8 agar and non-amended agar and then calculating the relative growth. When both parents had the same phenotype (both were resistant or both were sensitive), all F ₁ progeny had the parental phenotype and no segregation for a major effect gene was observed in sensitivity to mefenoxam. However, variations in the mycelial growth of progeny indicated the segregation of minor-effect genes. When the cross involving the mefenoxam-resistant isolate 3A4 and a sensitive parent, the F ₁ progeny segregated for mefenoxam resistance in a ratio of 1:1 (resistant: sensitive), indicating that the mefenoxam resistance is controled by a single dominant gene. Mating type was not linked to the mefenoxam-resistance gene locus. One RAPD marker linked in trans to the MEX locus was obtained by bulked segregant analysis using RAPD markers and was converted to a sequence characterized amplified region marker (SCAR). The SCAR maker identified in this study is a limited but useful tool for differentiating homozygous resistant isolates from sensitive isolates of P. nicotianae.