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Background

Biology/pathology. The biology of P. infesrans is typical of all oomycetes, but the pathology resembles that of a downy mildew (a specialized group of oomycetes) rather than that of most other Phy-~ opophthora spp. The oomycetes appear to have closer affinities with algae and higher plants than with ascomycetes and basidiornycetes (4, 2 1). They are diploid and coenocytic, lack chitin in the cell watls, and produce biflagellate zoo-spores. For P. infestans there are two mating types, Al and A2. Sexual struc-tures (antheridia and oogonia) are induced only in the presence orthe opposite mating type, and genetic fusion results in oospores (Fig. 3). Oospores are probably survival structures, whereas infections of foliage (Fig. 4) or tubers (Fig. 5) are initiated by asexual sporangia (Fig. 6) andlor zoospores. Individuals in asex-ual populations most commonly survive as mycelium in infected tubers. Hosts include …