Variation in virulence, host specificity, hyphal growth rate, and size of the aplerotic zone in successive single-zoospore and single-oospore progenies of Aphanomyces euteiches

Abstract

Variation in virulence, host specificity, hyphal growth rate, and size of the aplerotic zone were studied in Aphanomyces euteiches using successive single-zoospore progeny (asexual reproduction) and single-oospore progeny from selling (sexual reproduction). No variation in virulence or host specificity to pea or bean was found in generations of single-zoospore or single-oospore isolates derived from four field isolates. In each isolate, hyphal growth rate and size of the aplerotic zone were relatively stable traits among six generations of single-spore progeny; the phenotypes of progeny remained similar to parental phenotypes. The traits were found to be the same within, but different between, two pea and two bean pathotype isolates. Stable randomly amplified polymorphic DNA (RAPD) markers associated with different isolates were selected in successive generations of zoospore and selfed oospore progeny and could be used to distinguish the four isolates. Results suggest that the genes involved in determining the studied traits are homozygous in the isolates used and that new forms of the pathogen differing from parental strains cannot be obtained by the generation of successive zoospore or selfed oospore populations. Therefore, other than variability generated by mutation, major variability in A. euteiches most likely originates by outcrossing.