Downy Leaf Spot (White Leaf Spot) on Nut Trees: Sanitation Management

Downy leaf spot (white leaf spot), caused by Microstroma juglandis, attacks walnut, hickory, pecan, and butternut. Small (1/8–1/4"), white, furry patches on leaf undersides; pale green spots above. Fused patches look like white powder. Progresses to yellow/brown upper spots, early leaf drop (August), and "witches' broom" tufts of stunted, yellow leaves on branch tips. Fungicides impractical for homeowners (canopy height); sanitation is the only viable control.

Management: Sanitation

• Winter/early spring (dormant): Remove and destroy ALL leaves, shucks, nuts—infected or healthy. Overwintering debris = primary inoculum.
• Prune dormant: Remove damaged limbs, witches' brooms.
• Tree vigor: Fertilize and water adequately—healthy trees tolerate infection without decline.

Perspective

Downy spot won't kill the tree, but it weakens it for worse diseases. The grower who cleans the orchard floor every winter keeps the fungus minimal. The one who leaves the leaves guarantees a white spring.