Attached to this letter are fungicide efficacy and treatment timing tables for alternaria, Botryosphaeria, and Botrytis. Please post or keep in your files for ready reference. Walnut blight bacteria overwinter in buds and infect emerging shoots and developing nuts in presence of moisture, rain or dew. Walnut cultivars that leaf out and bloom early in spring when rainfall is probable are most susceptible. Here is a list of walnut cultivars showing leafing date and blight sensitivity in the Southern San Joaquin Valley.
Walnut blight infects developing leaves and nuts.
Additional data is needed before this strategy can be recommended. Blight treatments are designed to protect developing walnut tissue. Consequently, treatments need to be applied frequently to protect continuously developing tissue. Treat 7-14 day intervals. Additional treatments will be required for all sensitive cultivars if rainy weather persists.