Walnut Blight

Walnut blight shows up in spring as water-soaked lesions on new leaves, catkins, and developing shoots, turning dark and sunken on nuts if infections are severe. You'll notice black lesions on catkins at budbreak and throughout bloom, and if conditions stay wet and warm, the infection spreads to young nuts. The bacterium Xanthomonas arboricola pv. juglandis overwinters in infected buds and twig lesions, so the disease announces itself early when rain and temperatures above 60°F coincide with new growth. Severe infections cause premature nut drop and render any surviving nuts unmarketable.

The key to managing walnut blight in Western Washington is understanding that spring weather controls the disease: warm, wet conditions during bloom and early nut development create the window when infection happens. Plant late-leafing cultivars when possible to delay growth past the most vulnerable period, apply copper bactericides at budbreak and during bloom if blight has been a problem, and maintain good canopy air circulation through pruning. Remove and destroy infected material to reduce overwintering inoculum, though in a wet spring no amount of sanitation alone will stop the disease if conditions align. Prevention through cultivar selection and timing of applications is far more effective than trying to stop infection once it starts.