GERMINATION TRIGGERS OF ZOOSPORE CYSTS OF APHANOMYCES-EUTEICHES AND PHYTOPHTHORA-PARASITICA

Zoospore cysts of four isolates of Aphanomyces euteiches with differen t host-specific pathogenicity germinated in response to peptone, pea r oot extract, CaCl2 or gum arabic but, in tests on one isolate, did not respond to several individual sugars, amino acids or polysaccharides. A. euteiches showed differential responses to substances applied at d ifferent times in the encystment process. Gum arabic triggered germina tion only when added to motile zoospores, which it caused to encyst. C aCl2 triggered germination when added to motile zoospores or during vo rtex-encystment of zoospores, but seldom immediately (< 2 min) after e ncystment. Peptone and pea root extract triggered germination when add ed to motile zoospores, during vortex-encystment or immediately after encystment, but not ca 20 min after encystment. In comparative tests, Phytophthora parasitica was less stage-specific than A. euteiches, bec ause it germinated in response to gum arabic applied to motile zoospor es, during encystment and immediately after encystment. Zoospore taxis , encystment and germination of A. euteiches and P. parasitica cysts w ere recorded on pea roots by video microscopy. The speed of germinatio n and pattern of sporeling development on roots were most closely matc hed by treatment of motile spores with gum arabic in pure culture. The behaviour of A. euteiches conformed to a previous model for Pythium a nd Phytophthora spp., in which zoospores encyst by recognition of root surface components (simulated by gum arabic), germinate autonomously by a calcium-mediated process (simulated by calcium treatment during v ortexing), and the germ-tube produces an assimilative hypha (simulated by peptone or pea root extract).