The Influence of Temperature and Nutrition on Formation of Sexual Structures by Phytophthora cinnamomi

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Abstract
The influence of temperature on the abundance, developmental sequence, morphology and size of P. cinnamomi sexual structures was studied in a cross between isolates of A1 and A2 mating types and in stimulation by avocado-root extract of homothallic oospore production in single cultures of an A2 isolate. Gametangia were formed from 9-33.degree. C, and oospores from 12-30.degree. C. The optimum temperatures for oospore production were 15-21.degree. C in the cross and 18-24.degree. C for homothallic production in root extract. Temperature did not significantly influence size of sexual structures, but did affect the rate of development and abundance. Nutrition greatly influenced the sexual stage: significant differences in size and abundance of sexual structures were observed using the same cross on different media. Oogonia and oospores produced homothallically in root extract and those from the cross on a synthetic medium were larger than those from the cross on 4 natural media. Comparisons of sexual-structure measurements for taxonomic purposes should be made between structures produced on the same medium.