Effect of Some Factors on Detectability of Virulence Genes in Phytophthora infestans (Mont.) de Bary

Abstract Inoculum concentration, length of incubation period and passages on leaves of potato R–genotypes were found to influence the detectability of virulence genes in Phytophthora infestans.
At lower inoculum concentrations, only a few virulence genes were detected, these were usually 1 and 4. Coupled with an increase of the inoculum dose a gradual increase was found in the number of detected genes and their multiple combinations. An increased incubation time also resulted in the detection of additional virulence genes and their combinations. Two successive passages on respective potato R-genotypes produced a 2--3 fold rise in the number of detected genes.
To explain this phenomenon, a hypothesis has been proposed which suggests that for a virulence gene to be detected, certain infection thresholds have to be reached, which may be different for particular differential host genotypes or virulence genes.