The Polychytriales ord. nov. contains chitinophilic members of the rhizophlyctoid alliance.

During the past 5 y the Rhizophydiales, Cladochytriales and Lobulomycetales have been segregated from the formerly recognized Chytridiales. Descriptions of new chytridiomycete orders are based on molecular and ultrastructural characters, which have been phylogenetically mutually supportive. The Polychytrium clade has consisted of a few chitinophilic, soil and aquatic chytrids that clustered in phylogenetic hypotheses but have not been placed in a new order. We isolated additional putative members of this clade, sequenced their nucSSU and nucLSU rDNA and examined zoospores of some of the isolates with TEM. Our isolates are in a well supported clade with previous Polychytrium clade members, but zoospore ultrastructural types vary within the clade, with characters that often are conserved within other orders (e.g. flagellar plug, rumposome) being either present or absent. Based on the isolates in culture we describe the Polychytrium clade as the Polychytriales. This order contains Polychytrium, Lacustromyces, Karlingiomyces, two new genera (Arkaya and Neokarlingia) and additional undescribed taxa.