British distribution: A few scattered records, mainly from London and in southern Britain, an introduction, necessarily confined to localities where the host is planted.
This is a powdery mildew, parasitic on species of Plane (Platanus) (family Platanaceae), frequently causing leaves and buds to become severely distorted.
This is a host-specific powdery mildew on Plane (Platanus species and hybrids), in Britain predominantly on London Plane (P. x hispanicus), which is much planted as a street tree, especially in London. The fungus produces extensive, white patches on the surfaces of leaves, often coating them, and causing distortion of leaves and buds, as shown above. Presently still rare, it can be expected to spread, but it is unlikely to cause significant permanent damage, and it is most unlikely to spread to any other tree species.
It is a member of the genus Erysiphe section Microsphaera, characterised by the cleistothecia bearing stiff, radiating hairs (appendages), which terminate in repeated, short, dichotomous branching. Cleistothecia contain several asci, each 3-5-spored. The conidia of the anamorph (asexual state) are hyaline, ovoid to barrel-shaped, 25–50 × 12–22.5 µm (Braun & Cook, 2012), produced singly at the ends of conidiophores. They lack fibrosin bodies.
Descriptions and diagrams of the sexual (teleomorphic) and asexual (anamorphic) stages are provided by Uwe Braun (1987, 1995) (as Microsphaera platani) and by Braun & Cook (2012). This appears to be the only powdery mildew known on Platanus species in Britain, though a member of the Phyllactinia guttata complex has also been reported (somewhere) on Platanus (Braun & Cook, op. cit.). A record of Golovinomyces sordidus in the FRDBI can be safely dismissed as some sort of blunder (confusion of "Platanus" and "plantain"?). However, 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 always recommended. G. orontii produces conidia in chains and has cleistothecia (when formed) with short, flexuous, unbranched appendages.
For an explanatory overview of the powdery mildews and the terminology used here, go to the profile of the Erysiphales.