Anthracnose Blotch

Look for circular to irregular reddish-brown or grayish-brown spots on walnut leaves, typically appearing in spring and early summer when rainfall and humidity are high. Spots range from 1/16 to 3/4 inch across and develop on both leaflets and petioles. Twigs and nuts may also show sunken lesions with dark edges. Severe infections can cause defoliation and premature walnut drop, weakening nut production significantly. This fungal disease is widespread in walnut orchards and landscape plantings throughout the PNW.

The disease overwinters in infected leaves, nuts, and twig cankers, then releases spores during spring rains. High humidity and temperatures between 60-75°F favor infection. The best management is sanitation: rake and destroy fallen leaves and nuts in fall, prune out infected twigs when practical, and select resistant cultivars such as Carpathian or Chandler where available. Avoid highly susceptible varieties like Adams. In wet years, copper-based fungicides or propiconazole applied at bud break and continuing through early summer at 2-3 week intervals can reduce disease pressure on valuable trees.