Cystotheca mexicana

Cystotheca mexicana is a species of powdery mildew in the family Erysiphaceae. It is found in Mexico on plants in the genus Quercus.

Cystotheca mexicana forms white mycelium on the underside of leaves. Cystotheca mexicana, like most Erysiphaceae, is highly host-specific and only infects species in the genus Quercus. It has only been found on oak hybrids with at least one species endemic to Mexico, including Quercus glaucoides × Quercus microphylla and Q. liebmannii × Q. microphylla. Another species infecting Quercus in North America, Cystotheca lanestris, is found on a variety of oak species, and can be found in north-western Mexico. This species can cause witch's-broom galls. Microscopically, C. lanestris has fibrosin bodies and longer special aerial hyphae than C. mexicana.^[1]

The fungus was formally described in 2023 by Michael Bradshaw, Allen J. Coombes and Carlos Contreras-Paredes. The type specimen was collected in Mexico, the country from which the specific epithet also takes its name.^[1]