Diversity among Single Zoospore Isolates Derived from Single‐zoosporangia of Phytophthora sojae Kauf. and Gerd

Abstract Phytophthora sojae Kauf. and Gerd, a host specific pathogen to soybean, causes pre and postemergence damping‐off and root and stem rot on soybean. The pathogen evokes severe yield losses in most soybean growing areas worldwide. The objective of this study was to determine phenotypic and genotypic diversity among single zoospore isolates (SZIs) originating from two single zoosporangia (Ps411‐1 and Ps411‐2) derived from the same parental isolate of P. sojae Ps411. Results showed that colony morphology and growth rate of 32 SZIs derived from sporangium Ps411‐1 and 35 SZIs released from sporangium Ps411‐2 did not significantly differ from the parental isolate Ps411. Pathogenicity of the SZIs was tested on three resistant and three susceptible Chinese soybean cultivars. While the majority of SZIs derived from sporangium Ps411‐1(59.4%) and sporangium Ps411‐2(71.4%) retained the same virulence spectrum as the parental isolate, the other SZIs of both progenies demonstrated either a higher or a lower level of virulence compared to that of parental isolate. A low level genetic variability in the populations of both single zoospore progenies was also demonstrated using the sequence‐related amplified polymorphism (SRAP) technique. Cluster analysis separated the SZIs from both zoosporangia, Ps411‐1 and Ps411‐2, into four and three SRAP groups, respectively. No close correlation among SRAP and virulence could be established among SZIs. The results of this study suggest that virulence variability may be regarded as part of the total genetic changes among the zoospore progenies derived from single‐zoosporangia. The pathogenic variability during asexual reproduction may play a role in changing the virulence structure of P. sojae.