Myxochytridiales

an order of lower fungi of the sub-class Archimycetes (a class according to a former classification system) of the class Phycomycetes. The vegetative body at the beginning of its development consists of an amoeboid—a naked, microscopic, uninucleate lump of protoplasm—which is subsequently covered with a membrane and transformed into one or more zoosporangia.

Most Myxochytridiales are characterized by asexual reproduction; zoospores appear in the zoosporangium as a result of the repeated division of its nuclei. Some of the fungi undergo sexual reproduction by means of the fusion of motile gametes. The zygote, which has a single diploid nucleus, is temporarily motile and equipped with two flagella. On striking a nutrient substrate, the zygote casts off its membrane, as does a zoospore, and loses its flagella; in the form of an amoeboid it then penetrates the cell of the host plant and is converted into a thick-walled resting spore, which eventually germinates into one or more sporangia or immediately into zoospores.

Most Myxochytridiales are intracellular parasites, predominantly of algae, aquatic fungi, and some higher aquatic and terrestrial plants. Synchytrium endobioticum is the causative agent of potato wart, and Olpidium brassicae causes wire stem, a disease of cabbage seedlings.