Metalaxyl-resistant mutant strains of Phytophthora boehmeriae are as aggressive and fit as their metalaxyl-sensitive wild-type parents

Phytophthora boehmeriae Sawada is an important pathogenic oomycete, and causes Phytophthora blight of cotton and ramie, which are main limiting factors to the production of cotton and ramie in China. The aggressiveness and fitness of the metalaxyl-resistant (MR) mutants of P. boehmeriae obtained by inducing on LBA amended with sublethal dose (10 µg·mL −1 ) of metalaxyl were studied in this paper. The results showed that there were no significant differences between the MR mutants and their metalaxyl-sensitive (MS) wild-type parents in temperature for mycelial growth, zoosporangium production, sensitivity to malachite green, and the pathogenicity to cotton seedlings. However, the oospore productions of the mutants were much lower than those of their MS parents. The growth rates and the colony morphology of MR mutants and their MS parents all displayed obvious variation or separation, and were inherited unsteadily in their single-zoospore and single-oospore progenies. At the same time, the homothallic character of MR mutants could inherit steadily as well as that of the MS wild-type isolates did in asexual and sexual progenies. It was suggested that the MR mutants of P. boehmeriae possessed equal aggressiveness and fitness with their MS parental isolates, and would develop the MR population easily in a short time as long as the MR mutants formed. Thereby, there would be a high risk of development of metalaxyl resistance in populations of P. boehmeriae in the field, when metalaxyl was long and continuously applied for the control of the diseases caused by P. boehmeriae.