British distribution: Widespread and common.
This is a common powdery mildew, parasitic on Sonchus (sow-thistles), including S. arvensis (sspp. arvensis and uliginosus), S asper, S. oleraceus and S. palustris.
It is part of the G. cichoracearum complex and, until recently, records on Sonchus were generally assigned to that species. Recent molecular work and careful micromorphological studies have shown that G. sonchicola is best regarded as a separate species (Braun & Cook, 2012).
G. sonchicola forms irregular patches on leaves and stems and not infrequently coats entire leaves and seriously reduces the vigour of the host plant. Smooth Sow-thistle (S. oleraceus) seems particularly susceptible and in warm, dry periods of late summer, the fungus is often conspicuous on this host.
Identification of powdery mildews on composites (members of the plant family Asteraceae) is not simple. Many have host-specific powdery mildews, but G. cichoracearum should normally be considered as an additional possibility. However, as now understood, G. cichoracearum seems unknown on Sonchus.
Another species on numerous hosts in the Asteraceae, but apparently also unrecorded on Sonchus, is the generalist, Podosphaera xanthii, which, when only in its conidial state, is not easily distinguished. Species of the genus Podosphaera possess conspicuous, refractive particles, termed "fibrosin bodies" within the conidia, whereas these are unknown in Erysiphe and Golovinomyces, but this requires examination of fresh material with a good microscope at high power.
At present, it seems that G. sonchicola is the only powdery mildew on Sonchus, but, generally, powdery mildews on members of the Asteraceae cannot and must not be identified by their host alone.
For an explanatory overview of the powdery mildews, go to the profile of the Erysiphales.