Stimulation of Sexual Reproduction in the A2Mating Type ofPhytophthora cinnamomiby a Substance in Avocado Roots

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Abstract
Oospore formation was induced in unmated isolates of the A2 mating type of P. cinnamomi by a substance in an extract or diffusate of avocado (P. americana) roots. Thus, this heterothallic fungus functioned as a homothallic organism when provided with a specific organic (hormone-like) stimulus. The sexual stage also was initiated by the root extract in A2 mating types of P. capsici and P. drechsleri, but not in A2 types of several other Phytophthora spp. A1 isolates of P. cinnamomi did not produce oospores in the avocado root extract. Similar stimulation was not found in extracts from roots of several other hosts of the pathogen. The stimulatory substance apparently is organic in nature, is nonvolatile, heat stable, water soluble, and can be extracted by several organic solvents.