The Trentepohliales (Ulvophyceae, Chlorophyta): an unusual algal order and its novel plant pathogen, Cephaleuros

Most plant pathologists know certain algae can be used as gelling agents in culture media. Pathologists practicing in tropical or subtropical environments also know that some algae damage plants. The five genera in the order Trentepohliales (Chlorophyta) are unique and fascinating. Among other characteristics, they are subaerial, bright orange to red in color, and one genus, Cephaleuros, is a plant pathogen while another, Stomatochroon, is a space parasite. Cephaleuros causes algal spot and includes 17 accepted species. Of these, 13 develop between the cuticle and the epidermis of their hosts and four grow intercellularly. The latter are especially damaging, causing chlorosis and branch dieback. Zoospores and gametes germinate on plant surfaces during the rainy season and probably penetrate through breaks in the host cuticle. Their filamentous growth forms thalli that produce sporangiophores and spherical gametangia the following year. Several species of Cephaleuros have a broad host range and though their damage is usually superficial, it can be economically important on certain crops. Plant stress is the greatest predisposing factor to this algal disease. Management includes providing plants with sufficient moisture and nutrients, modifying cultural and harvesting practices, and planting resistant cultivars when available.