Characterization of mating type and the diversity of pathotypes of Phytophthora infestans isolates from Southern Brazil

Late blight is caused by the oomycete Phytophthora infestans, and it is the most devastating disease of potato in Southern Brazil. The objective of this work was to evaluate the variability of P. infestans isolates collected from potatoes in Rio Grande do Sul (RS), Parana (PR) and Santa Catarina (SC) states for their mating type (MT) and virulence. One hundred and twenty-eight isolates of P. infestans were collected between 2010 and 2012. They were characterized for mating type and subsequently evaluated for virulence/avirulence using a differential series of 11 potato clones. The virulence study was conducted in vitro by the leaf disc method, where the potato clones were inoculated with each isolate. Seventy-six isolates were identified as MTA2, 24 as MTA1, 17 as MTA1A2 and 11 as self-fertile. In addition, 79 pathotypes of P. infestans were detected in the survey and the largest number of races was identified in RS. The most frequent pathotypes identified were ‘14’ and ‘21’ and the vast majority of isolates overcame the R7 resistance gene, followed by the R3, R1 and R11 genes. Smaller numbers of virulent isolates were detected to the clones carrying the R5, R2 and R9 genes. Higher percentages of virulent and complex isolates were identified in the MTA2. High indices of diversity were observed in populations grouped by mating type and by state of collection. The highest value of the Gleason index HGR = (0.95) was obtained for the isolates from PR. Higher complex P. infestans populations were found in RS, and the least complex population was in PR.