Single-spore isolate variation: the effect on varietal designation in Phytophthora megasperma
Eight single zoospore isolates (SSI) were collected from each of 35 parent isolates of Phytophthora megasperma from 14 hosts. Morphological characters, hyphal growth patterns, and growth rate at several temperatures were measured and compared to assess variation of characters among the SSI of each parent. Oogonia, usually with paragynous antheridia, were formed by 94% of the SSI. The number of oogonia formed and the proportion of paragynous to amphigynous antheridia varied among SSI. Some SSI differed significantly from their parent in all characters measured. Oogonium (and oospore) diameter and sporangium shape were relatively consistent, but sporangium size varied widely. Parent isolates could not easily be classified as P. megasperma var. megasperma or P. megasperma var. sojae by oogonium diameter. Of the 35 parent isolates, 43% produced some SSI with oogonia averaging < 45 μm and some with oogonia averaging > 45 μm, the spore diameter used for varietal separation. Basing varieties on oogonium diameter is not justified because of such differences among SSI of a single parent and between wild-type isolates. The results supported a revised description of P. megasperma.