Water Relations of Water Molds

As a plant pathologist my definition of the water molds necessarily includes zoosporic fungi ranging all the way from truly aquatic, nonmycelial forms to completely terrestrial forms with well-developed mycelium (92). In other words, rather than confining the water molds to those Chytridiomycetes, Hypochytridiomycetesand Oomycetes that occur in fresh water, terrestrial and marine members of these classes are included. In fact, most of what we know about the water relations of water molds comes from research on their salt tolerance or parasitism of terrestrial plants. Notable in this regard are Phytophthora infestans and the downy mildews, for rudimentary knowledge of their water relations has facilitated disease forecasting and control [e.g. see references in (21, 84)]. The aerial environment oscillates between ex­ tremes of water status and, fortunately, the water requirements of P. infes­ tans and the downy mildews are satisfied only part of the time. Most of the substrates on which zoosporic fungi grow have a water status that is very much higher and more constant than the humidities of ambient air would' suggest, and for even closely related plant pathogens that inhabit soil, such as Phytophthora and Pythium species, the determination of permissive conditions for epidemic increase will be more difficult. Nevertheless, the literature and considerable field experience indicate that very wet soil condi­ tions enhance many Phytophthora and Pythium diseases (e.g. 17, 34, 36, 45, 93, 99; 117), and through more· r.esearch their water requirements may become sufficiently known for practical use in disease control and irrigation management. Even the most terrestrial examples of zoosporic fungi have not lost all aquatic affinities, and the response of zoosporic fungi to small changes in