|Red Rust (Cephaleuros virescens)|
Spices: Cephaleuros virescens
Cephaleuros species are filamentous green algae and parasites of higher plants. In Hawai‘i, at least two of horticultural importance are known: Cephaleuros virescens and Cephaleuros parasiticus. Typically harmless, generally causing minor diseases characterized by negligible leaf spots, on certain crops in moist environments these algal diseases can cause economic injury to plant leaves, fruits, and stems. C. virescens is the most frequently reported algal pathogen of higher plants worldwide and has the broadest host range among Cephaleuros species. Frequent rains and warm weather are favorable conditions for these pathogens. For hosts, poor plant nutrition, poor soil drainage, and stagnant air are predisposing factors to infection by the algae.
Symptoms and crop damage can vary greatly depending on the combination of Cephaleuros species, hosts and environments. The most severe disease combination in Hawai‘i is C. parasiticus on guava (Psidium guajava). Spots caused by C. parasiticus on guava leaves are a top-down, intercellular, full-thickness necrosis, destroying both upper and lower epidermal cell layers and all intervening tissues. Then, a bright but relatively sparse orange algal bloom bursts forth from the undersides of leaf lesions, rather than from the upper side of leaves as is usually the case with C. virescens, the more commonly occurring species in the genus. The distinctive and much more common spots caused by C. virescens on many hosts in Hawai‘i appear on the upper leaf surface as raised yet flattened, textured, burnt-orange to brown or rust-colored, circular areas up to about 2 cm in diameter, having indistinct, filamentous margins and fuzzy surface topographies.