Downy Mildew

Downy mildew causes purplish-red to dark-brown irregular spots on rose leaves and Rubus species, often with a grayish spore mass on the leaf undersides if humidity stays high. Affected leaves may yellow but retain green islands along the veins; major leaf veins often restrict fungal spread, creating an angular spot pattern. You might confuse it with black spot or rust in early stages, or even with nutrient deficiency. Severe infections cause defoliation, and on young canes you may see purple streaks or cracking that kills tips and slows cane development.

This fungus-like pathogen (Peronospora sparsa) favors humidity above 85% and temperatures of 60-70°F, thriving in the maritime conditions of Western Washington. The key to management is reducing humidity through improved air circulation and sanitation. Space plants widely, prune to open up the canopy, rake and remove fallen leaves and old flowers, and scout regularly for symptoms on new growth and leaf undersides. Never take cuttings from infected stock plants. Keep nitrogen fertilizer moderate, as excess nitrogen fuels disease development. The disease can be systemic in roses, so once established, it requires consistent management.