Problem Identifier Unit Screen//v.w/ RR Template

Problem Identifier Unit Screen//v.w/ RR Template Walnut Blight

Problem
Buds turn dark brown to black, and die. There are brown spots on the leaves and dead, sunken lesions on the shoots. Some of the leaves may be deformed. Black, sunken, hard areas develop on the nuts. A shiny black fluid may exude from these lesions. Infected nuts have stained shells or shriveled kernels. Nut yield may be reduced.

Analysis
This plant disease is caused by a bacterium (Xanthomonas campestris pv. juglandis) that is common on walnuts. The bacteria spend the winter in diseased buds, twig lesions, and old infected nuts attached to the tree. A thick, shiny fluid containing millions of bacteria exudes from the infected plant parts in the spring. Spring rains splash the bacteria to the buds, shoots, flowers, and developing nuts, starting new infections. Bacterial infection reduces nut set and can continue to spread to healthy nuts and foliage throughout the summer during periods of wet weather. If the nuts are infected before their shells harden (when the nuts are three-quarters grown), the bacteria may spread into and decay the kernels. Because the wet, rainy conditions of spring favor the rapid spread of walnut blight, early-blooming walnut varieties are most susceptible to this disease.

Solution
Next spring, spray with basic copper sulfate when catkins (flower spikes resembling cats' tails) start to shed pollen, then spray again when the small nutlets start to appear. If the weather remains wet, spray once more after 2 weeks. Streptomycin is also effective. Plant late-blooming varieties such as 'Vina' and 'Hartley'.

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