Pathotypes and metalaxyl sensitivity of Phytophthora sojae and their distribution in Heilongjiang, China 2011–2015

A total of 395 single-zoospore isolates of Phytophthora sojae that were obtained from 467 soybean fields in Heilongjiang, China from 2011 to 2015 were identified for pathotypes using differential soybean cultivars with Rps1a, Rps1b, Rps1c, Rps1d, Rps1k, Rps2, Rps3a, Rps3b, Rps3c, Rps4, Rps5, Rps6, Rps7, and Rps8 resistance genes. The results showed that P. sojae was widespread in Heilongjiang, but not evenly distributed. A heavy infestation of P. sojae in eastern Heilongjiang coincided with the serious diseases caused by this pathogen in that region. Among 135 pathotypes were identified, 20 isolates matched races of P. sojae based on published race definitions. To our knowledge, this is the first report of races 2, 8, 23, 25, 26, 29, 33, 39, 42, 43, and 48 in Heilongjiang. Race 1, previously considered as the dominant race in Heilongjiang, only comprised 1.52 % of the isolates in the present study. Virulence frequencies of 395 isolates to 14 Rps genes ranged from 17.85 to 82.41 %. Less than 30 % of the isolates were virulent on cultivars containing Rps genes 1k, 1c, or 3a, which indicated that these Rps genes were more effective than other genes in Heilongjiang. All isolates could defeat more than two Rps genes. Ninety-six percent of the isolates were virulent against more than four Rps genes, which indicated that multi-virulence isolates existed in Heilongjiang. All 223 tested isolates were sensitive to metalaxyl, but EC50 for this fungicide against the pathogen increased 18-fold in the last 20 years. The results of the present study suggest that incorporating Rps genes 1c, 1k, and 3a into soybean cultivars and the use of metalaxyl-coated seeds should provide integrated approach to manage Phytophthora root and stem rot of soybean in Heilongjiang, China.