A powdery mildew found on non-native sorrels, including the small yellow-flowered Oxalis exilis and O corniculata 

Leveillula oxalidicola is listed as the powdery mildew affecting the native Oxalis acetosella; also possibly Golovinomyces orontii (ref. Woods et al, 2024) 

host

Photograph the mildew on the host plant; ensure you have noted the host plant species in your comments

non-native Oxalis

rarely recorded