Downy Mildew
Multiple obligate oomycetes (Peronosporaceae); species are highly host-specific
49 host plants · Oomycete
Last updated
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 maritime conditions with cool springs and wet growing seasons. 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.
What Should I Do?
- Sanitation and low humidity are key factors.
- Lower humidity in the greenhouse by raising temperatures and venting at key times of day.
- Space plants for better air flow and reduced humidity.
Quick Reference
Management
Young, expanding leaves and shoots are most susceptible. Mature, hardened-off foliage is generally resistant. Infection occurs within 8-12 hours of leaf wetness at optimal temperatures. Spring new growth flush and fall regrowth are the primary risk windows in maritime PNW. source: PNW Plant Disease Management Handbook
Infection favored by cool temperatures of 58-72°F (optimum 64°F) with humidity >85% and leaf wetness from rain, dew, or overhead irrigation. Sporulation requires 4+ hours of near-saturated humidity at night followed by rapid morning drying. Below 40°F and above 95°F sporulation stops. Excess nitrogen fertilization favors disease development. Source: PNW Plant Disease Management Handbook; Horst (1983) Compendium of Rose Diseases.
Cultural Controls
- Sanitation and low humidity are key factors.
- Lower humidity in the greenhouse by raising temperatures and venting at key times of day.
- Space plants for better air flow and reduced humidity.
- Rake leaves and prune out old flowers and stems.
- Burn or bury them or send them to a landfill.
- Take cuttings from plants with no history of the disease.
Downy mildew on roses and Rubus is favored by the cool, humid spring and fall weather typical of the Puget Sound lowlands. Peronospora sparsa may be systemic in rose stock, so purchasing disease-free plants is critical. Impatiens downy mildew (Plasmopara obducens) has caused significant nursery losses in western Washington since its first detection in 2011. Grape downy mildew (Plasmopara viticola) is a concern in western Washington vineyards where maritime moisture prolongs leaf wetness.