Oospore survival and pathogenicity of single oospore recombinant progeny from a cross involving US-17 and US-8 genotypes of Phytophthora infestans

Oospores of Phytophthora infestans produced in vitro and in planta, from a cross between US-17 and US-8 genotypes, were exposed to a variety of enviro nments and their survival was assessed. Additionally, the pathogenic charac teristics of some resultant progeny isolates were assessed. Viability of oo spores as measured by plasmolysis declined slightly over a period of 18 mon ths whether they were stored in water at 4 degreesC, in soil at 18 degreesC , or in soil under natural field conditions. In comparison, viability as me asured by germination was lower overall but appeared to increase after stor age in soil. Oospores produced in planta were buried in the field in the fa ll of 1998, and were capable of infecting both tomato and potato leaflets w hen recovered in May 1999. Single oospore progeny (n = 53) from the in vitr o cross were analyzed individually for genetic and pathogenicity characteri stics. All 53 progeny tested for restriction fragment length polymorphisms with probe RG57 were hybrids. All but one progeny produced sporulating lesi ons on detached potato or tomato leaflets in growth chamber tests, but most lesions were smaller and developed more slowly than those produced by eith er parental isolate. In a further test of pathogenicity, under field condit ions, none of a subset of 10 A2 progeny was capable of initiating a detecta ble epidemic in small plots of either potatoes or tomatoes.