British distribution: Scattered records, England, Wales, north to southern Scotland, an introduction.
This is a powdery mildew, parasitic on species of Lycium (Solanaceae).
This is a host-specific powdery mildew on Lycium species, in Britain occasional on the introduced shrub, Duke of Argyll's Teaplant (L. barbarum) and on the rarer introduction, Chinese Teaplant (L. chinense). L. chinense is much confused with broad-leaved variants of L. barbarum, but a recent record (November 2014) made by myself in East Lothian, S.E. Scotland, is indeed on L. chinense. In Europe it is known on other Lycium species.
It is the only species in the genus Arthrocladiella, whch differs from other powdery mildews in the abundant, short, repeatedly dichotomously branched appendages ('hairs') covering the fruit-bodies (cleistothecia). Cleistothecia contain numerous asci, each 2(-4)-spored. The conidia of the anamorph (asexual state) are hyaline, shortly cylindrical to slightly barrel-shaped, 20–36 × 10–18 µm, produced in chains, and lack fibrosin bodies. The fungus produces extensive, white patches on the surfaces of leaves, pedicels and calyces of the host, but causes little distortion. Cleistothecia apparently are not common.
Descriptions and diagrams of the sexual and asexual stages are provided by Uwe Braun (1987, 1995) and Braun & Cook (2012). This appears to be the only powdery mildew known on Lycium species in Europe, but the rather tough leaves of the host, with thick cuticles, might well be susceptible to the generalist powdery mildew, Golovinomyces orontii, and microscopic confirmation of identifications is recommended.
For an explanatory overview of the powdery mildews, go to the profile of the Erysiphales.