Field Identification
Not a true fungus—oomycete pathogens that cause yellow angular leaf spots (often vein-limited) and fuzzy gray-purple sporulation on the undersides in humid conditions. Cucurbits, grapes, lettuce, basil, and others each have specialized species.
Morning inspection shows purplish mold under leaves while tops look blotchy yellow. Spores spread on cool wet nights; epidemics follow prolonged leaf wetness.
How to Deal With It
Organic Control Methods
Choose resistant cultivars, space plants for drying, water at soil line in morning, and use high tunnels with sides that vent.
Compost teas and Bacillus-based products are used with variable results; strong soil and foliar biology supports plant vigor.
Remove infected leaves early in outbreaks, avoid overhead irrigation, and destroy crop debris that harbors oospores where the pathogen overwinters that way.
Row covers can delay exposure on direct-seeded crops; vertical trellising improves air movement.
Copper, sulfur, or biological fungicides per crop label; potassium bicarbonate solutions sometimes suppress mild infections. Reapply before rain events per local organic rules.
Let Nature Handle It
Natural Enemies
- Hyperparasitic Fungi
- Competitive Microbes
Threat Map