Black walnut blight, caused by the bacterium Xanthomonas juglandis, is a pernicious disease that afflicts several varieties of black walnut, especially those grown in rainier climates of northern California and southern Oregon. Plant scientists at the University of California, Davis, and Oregon State University report that infections of this blight can be prevented effectively by timely applications of pesticides containing copper.
Black walnut blight strikes a tree's green tissue during periods of severe, prolonged rain. Black lesions may appear on catkins, the slender spikes of the male flowers. Black lesions also develop on shoots, and irregular spots appear on the blades. Slightly sunken lesions develop at the flower end of young nuts. As the nuts mature, black, slimy spots develop on their sides. The bacteria penetrate the husk, shell and sometimes the edible meat.
Black walnut blight appears not to be spread by rain and wind to adjoining trees. Infections are usually limited to a tree or orchard. The Howe cultivar of walnut is resistant to blight, according to horticulturalists at Oregon State University.
The Bordeaux mixture can be used to treat Black walnut blight. This is not a brand name; it is a mixture of copper sulphate and hydrated lime first used more than a century ago in the vineyards of the Bordeaux region of France. Your local agricultural extension service can tell you how to make it.
The University of California, Davis, recommends a Bordeaux mixture of 4 lbs. metallic copper and 5 lbs. calcium hydroxide in 100 gallons of water per acre. Copper sulfate is 50-percent copper; use 8 lbs. per acre. Hydrated copper sulfate is 25-percent copper; use 16 lbs. per acre.
Commercial pesticides containing copper can also protect against black walnut blight.
Apply 4 lbs. per acre of wettable powders containing 50-percent metallic copper. Some liquids require less than 4 lbs. copper per acre. Check the labels to see if the product may be sprayed on walnuts trees used for organic nut production.
Some bacteria resist treatment to copper. However they do not cause as much disease as bacteria that are sensitive to copper.
When to Treat
In years without unusual rain, apply when 30 to 40 percent of the female flowers unfold, appearing like hands in prayer--called the prayer stage. Apply again a week to 10 days later to treat the female flowers that were not open during the first application.
During heavy rainfall, apply when 30 to 40 percent of the male catkins emerge. This is usually a week to 10 days before the female flowers emerge.
Be alert to oncoming heavy rainfall, and watch the weather forecast to determine when additional treatments might be needed.
Check the label for R.E.I. (Restricted Entry Interval). This is the number of hours from treatment until you can enter the area safely without wearing protective clothing. P.H.I (Preharvest Interval) is the number of days from treating your walnuts until you can harvest nuts safely.