Introduction to Fungi, Third Edition

SAPROLEGNIALES
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Fig 5.13 (a d) Dictyuchus sterile. (a) Zoosporangium showing cysts within the sporangium and the release of principal zoospores
through separate pores in the sporangium wall. Note the network of auxiliary cyst walls. (b) Principal zoospores. (c) Principal cyst.
(d) Germination of principal cysts by means of germ tubes. (e i) Pythiopsis cymosa. (e) Zoosporangium. (f,g) Auxiliary zoospores.
(h) Auxiliary cyst. (i) Auxiliary cyst germinating by means of a germ tube. Principal zoospores have not been described.
(a c,e,f) to same scale; (g i) to same scale.
Dictyuchus. Some species produce sporangia only
rarely and the genus Aplanes has been erected for
these forms. However, in very clean cultural
conditions, all have been shown to behave as
Achlya, and they are currently accommodated
within that genus (Dick, 2001a). Two species of
Saprolegniaceae are not known to form sporangia
at all. They are common in soil, and have
been placed in a separate genus, Aplanopsis.
Another genus, Geolegnia, forms sporangia containing
thick-walled aplanospores which never
produce a flagellate stage. The final classification
of these small genera of Saprolegniaceae will
have to await the results of comparisons of
suitable DNA sequences (see M. A. Spencer et al.,
2002).