Oospore Morphology and Germination in the Phytophthora Palmivora Complex from Cacao

SUMMARY

Oospore morphology and germination were studied using 17 isolates representing three morphological forms (MF) of Phytophthora palmivora from cacao. Oospore production was most abundant on carrot agar. The shapes of gametangia and oospores produced in crosses between different MF isolates were more variable than those produced when the mated isolates were of the same MF. Mating between the MFs produced oospores with varying capacities for germination. Germination of oospores increased with increasing age. Pretreatment for 1 wk at 12 and at 30 C stimulated germination two-fold over oospores pretreated at 21 C. F₁ progeny from various matings within and between MFs showed differences in colonial morphology, production and morphology of sporangia, and mating type. However, the lengths of the sporangial stalks of F₁ progeny from germinated oospores derived from either crosses within or between MFs were all similar to the parental types. Two modes of oospore germination were defined; in one mode, which has not been previously reported, oospores germinated within 24 h by producing many germ tubes, each with a germ-sporangium deciduous with a long stalk. In the other mode, more typical of P. palmivora, one germ tube formed a single, papillate, nondeciduous sporangium.