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SUMMARY
The host range of Olpidiopsis incrassata was tested on 32 members of the Saprolegniaceae. It was parasitic on all members of the genus Saprolegnia, except for S. parasitica, and was parasitic on Isoachlya unispora and I. monolifera. It was not able to parasitize any member of the genus Achlya, Protoachlya paradoxa, Dictyuchus sp., Thraustotheca clavata, Aphanomyces laevis, A. scaber, or S. parasitica.
Presence or absence of light does not affect either the growth of host or parasite.
A change in temperature does not affect susceptibility of a host to O. incrassata. A temperature of 6° C retards the growth of the parasite and reduces greatly the number of resting bodies that are formed.
It is difficult to infect a host at a temperature of 30° C. O. incrassata grown at 30° will not infect another host unless put at a lower temperature. Degeneration of the parasite will occur at 30° if left at that temperature for 24 hours. When the parasite is established at a more reasonable temperature and then placed at 30°, degeneration will not occur.
O. incrassata appears unable to infect any saprolegniaceous host after zoosporangia or oogonia have been initiated. The indications are that there is some enzyme or other chemical that is a precursor to the reproductive stages of the host that is also necessary for infection by O. incrassata.