a subclass of lower fungi, or Phycomycetes. The fungi have a well-developed unicellular, multinuclear mycelium and are characterized by oogamy. The spherical oogonium contains several egg cells (in the simplest Oomyceti-dae) or just one (in the more highly organized ones). The an-theridium is cylindrical and forms on the mycelium alongside the oogonium; it releases its multinuclear contents by means of fertilizing processes that penetrate the egg cell. The fertilized egg cells are converted into oospores, which rest in the soil or on plant remains. The oospores sprout in the spring, forming organs of asexual reproduction, most often zoosporangia. At the same time, meiosis occurs in the diploid nucleus of the oospore. Thus, the diploid stage in Oomycetidae is represented only by the mature oospore. In the zoosporangia, uniflagellate or biflagellate zoospores are formed. In some Oomycetidae (higher forms of Peronosporales) the zoosporangia are separate from the sporan-giophores, having been converted into distinctive conidia.
The Oomycetidae are divided into a number of orders, which predominantly embrace aquatic saprophytes. The orders Blasto-cladiales, Monoblepharidales, and Pythiales develop on plant remains. The order Saprolegniales develop on insect remains and the carcasses of other small animals. Many orders of Oomycetidae are parasites: Ancylistales and some Pythiales parasitize algae and worms, Saprolegniales parasitize fish roe, fish fry, and adult fish, and higher Peronosporales parasitize terrestrial plants.
The evolution of Oomycetidae was marked by the transfer from aquatic to terrestrial existence. Harmful species include Phytophthora infestans, which is the causative agent of potato phytophthora rot; Pythium debaryanum, which causes root rot of sugar beet, wire stem of various agricultural crops, sprout rot, and seedling blight of pine and other timber trees; and Plas-ntopara viticola, which is the causative agent of grape mildew.
M. A. LITVINOV