Downy Leaf Spot
Pseudomicrostroma juglandis (syn. Microstroma juglandis)
6 host plants
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
Downy leaf spot creates tan or brown spots on foliage with white downy growth underneath. You notice spotting increase during cool, moist weather. The fungus spreads via spores in wet conditions. Remove infected leaves and improve air circulation.
Leaf spot and blotch diseases are primarily cosmetic on established plants, though repeated years of heavy infection can weaken them. Rake up and remove fallen leaves to reduce the spore load going into the next season. Avoid overhead watering that splashes spores from the ground onto lower leaves. If the problem is severe, protective fungicide sprays during spring leaf emergence can help, but sanitation does more than chemistry for most leaf spots.
Quick Reference
Management
Spring and fall during cool wet periods (March-May, September-November). Young foliage most susceptible; secondary peaks as conditions cool.
Cool wet conditions (50-65°F) with 10+ hours continuous leaf wetness. Rain and high humidity favor dispersal. Pseudomicrostroma spp. prefer maritime climates.
Cultural Controls
- Remove and destroy infected leaves. Leaves may be composted if completely decayed before spring. Make use of any practice that encourages decomposition of fallen leaves prior to spring bud break, such as mowing or flailing. Apply urea to leaves after leaf fall in autumn to enhance decomposition of fallen leaves.
- Rake and destroy leaves in fall. Grow other, more resistant species of Crataegus.