Emergence of self-fertile Phytophthora colocasiae is a possible reason for the widespread expansion and persistence of taro leaf blight in Japan

Since 2015, the leaf blight of taro caused by Phytophthora colocasiae has resulted in severe economic loss in Japan. In order to investigate the causes of disease expansion and persistence, we need to clarify the mating type distribution of this pathogen, and to characterize each mating type. We collected 317 P. colocasiae isolates from 99 agricultural plots in seven prefectures from 2014 to 2020. We examined the mating type of each isolate and the distribution of mating types in each region and location. Five mating types were identified: heterothallic A1 and A2 and self-fertile (SF) A1, A2, and A1/A2. We found complex mating type distributions in some plots, and some leaves with multiple lesions carried more than one mating type. In addition, the variability of each mating type was checked after growth of single colonies from hyphal tips, zoosporangia, and zoospores. The SF type isolates were genetically unstable and segregated into both heterothallic and SF types after propagation. On the other hand, the heterothallic A1 and A2 isolates were stable. In pathogenicity tests, the heterothallic A1 isolates were less pathogenic than the heterothallic A2 and SF isolates. The SF strains can self-fertilize or mate with the heterothallic strains and produced abundant oospores. Therefore, the SF strains have the ability to reproduce at high rates and survive long term in the environment. The characteristics suggest that one possible causal factor for the rapid expansion and persistence of this disease is the appearance of the SF mating types in Japan.