CALCIUM-DEPENDENT, GENUS-SPECIFIC, AUTOAGGREGATION OF ZOOSPORES OF PHYTOPATHOGENIC FUNGI

Dense populations of zoospores of Phytophthora palmivora, Pythium cate nulatum, and Pythium dissotocum formed multicell dumps, or autoaggrega tes. Autoaggregation was the result of active taxis and was shown to b e density-dependent, calcium-requiring, and influenced by pH. In addit ion, autoaggregation appeared to be species-specific, since aggregates of Ph. palmivora did not attract zoospores of three Pythium species a nd aggregates of Py. catenulatum did not attract Ph. palmivora zoospor es. Aggregation centers generated a calcium ion gradient and induced c hemotropic growth of germ tubes emerging from zoospore cysts. Autoaggr egation also functions to enhance zoospore accumulation at plant loot surfaces, thereby increasing inoculum potential for infection. In the absence of roots, autoaggregation may enhance zoospore population surv ival. (C) 1995 Academic Press, Inc.