Frosty Mildew

You notice white angular patches on the undersides of Oregon oak leaves, with yellow mottling above. Microstroma album, a fungus common on oak, generally doesn't warrant treatment in landscape settings. The incidence is naturally low in the Willamette Valley. Adequate cultural care keeps trees healthy despite this minor cosmetic issue.

Unlike most fungal diseases, powdery mildew does not need standing water to infect. It thrives in warm days followed by cool nights with high humidity. Improve air circulation by pruning for openness and spacing plants adequately. Resistant cultivars are your best long-term solution. For current infections, horticultural oils and biological fungicides can slow spread, but severely infected foliage will not recover.