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an order of fungi of the class Phycomy-cetes. The vegetative body consists of a noncellular mycelium and sometimes has haustoria. The organs of asexual reproduction are separate sporophores bearing zoosporangia or conidia. With sexual reproduction, one oospore is formed in the oogonium after fertilization.
The order Peronosporales embraces four families: Pythiaceae, Phytophthoraceae, Cystopaceae, and Peronosporaceae. The family Pythiaceae has about 100 species—hemiparasitic aquatic or terrestrial fungi that live on dead or living plants and animals. The mycelium lacks haustoria and is marked by almost undifferentiated spore formation. Most common are species of the genus Pythium.
The family Phytophthoraceae is represented by the single genus Phytophthora, which embraces several dozen species. The fungi are all dangerous parasites. They are the causative agents of phytophthoric plant diseases: for example, P. infestans infects Solanaceae, and P. cactorum infects numerous plants. The fungi are also characterized by saprophytic properties. Their mycelia sometimes produce haustoria, and spore formation is only slightly differentiated. The family Cystopaceae contains the single genus Cystopus, which has ten species. The fungi are obligate parasites of flowering plants, causing deformities of affected plant organs. The mycelia have haustoria, and the sporebearing organs are well differentiated. The sporophores are distributed in a compact layer, and the zoosporangia are in chains.
The family Peronosporaceae includes five genera, which differ from one another in the branching of their treelike sporophores. The mycelia have haustoria. Many of these fungi are the causative agents of downy mildew. Plasmopara viticola parasitizes grapes, and Peronospora tabacina parasitizes tobacco.
N. S. NOVOTEL’NOVA