Walnut Anthracnose in Central Washington: Late Summer Defoliation

Jim's walnuts in hot dry climate (irrigated daily) develop dark leaf spots in late July → larger black areas, stunted growth.

Diagnosis: Walnut Anthracnose

Caused by Gnomonia leptostyla. Circular brown lesions on leaves (start underside), 5 mm max. Progresses to yellowing, marginal browning, early leaflet drop. Wet seasons → defoliation by Aug. Also infects petioles, twigs, fruit (shriveled nuts). Rarely kills landscape trees but consecutive early defoliation weakens them.

Management

• Rake/remove infected leaves & fruit in fall (primary overwintering site).
• Spring root-zone nitrogen fertilizer enhances growth, reduces severity.
• No control needed most years.