Zoospore cysts of four isolates of Aphanomyces euteiches
with different host-specific pathogenicity germinated in
response to peptone,
pea root 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 different times in the encystment process. Gum
arabic triggered germination only when added to motile zoospores,
which it caused to encyst. CaCl2 triggered germination when
added to motile zoospores or during vortex-encystment of zoospores,
but seldom immediately (<2 min) after encystment. Peptone
and pea root extract triggered germination when added 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,
because it germinated in response to gum arabic applied to motile
zoospores, during encystment and immediately after encystment.
Zoospore taxis, encystment and germination of A.
euteiches and P. parasitica cysts were recorded on pea
roots by video
microscopy. The speed of germination and pattern of sporeling
development on roots were most closely matched by treatment of
motile spores with gum arabic in pure culture. The behaviour of
A. euteiches conformed to a previous model for Pythium
and 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 vortexing), and the germ-tube produces an
assimilative hypha (simulated by peptone or pea root extract).