Depending on variety, walnut blight can take a heavy toll on walnut production, particularly when initial inoculum is high and spring weather is warm and wet.

However, with the federal EPA granting Manzate ((flowable or dry flowable formulations) a Section 3 registration last year, walnut growers throughout California now have a reliable option for controlling the disease. For the previous two decades, growers in the state could use this and other ethylene bis-dithio-carbamates (EBDCs) products to treat for walnut blight only in selected counties under a Section 18 (emergency exemption) registration. Applying for Section 18 registration required submitting extensive environmental, health and safety data each year.

 

 

The walnut blight bacterium (Xanthomonas arboricola pv juglandis) over-winters in dormant buds primarily under the outer bud scales or cataphylls. When buds break in the spring, cataphylls open and young shoots extend past them. Rain drops spread the disease by splashing bacteria onto any green tissue, infecting them. Early leafing varieties are most susceptible to walnut blight, which tends to be more severe in northern California.

The disease appears as black lesions on green tissue. As bacteria spread inside the walnut, they grow toward the center of the nut early in the season, destroying the developing kernel. Infections later in the season develop randomly, causing so-called side blight. Depending on when this occurs, side blight may or may damage the kernel.

Success in controlling walnut blight requires application of protective sprays to early shoots, flowers, and developing nuts, reports Rick Buchner, University of California Cooperative Extension farm advisor for Tehama County. In orchards with histories of walnut blight damage, protective treatments at seven to 10-day intervals during prolonged wet springs are necessary for adequate disease control. In areas or years with less intensive rainfall, spray intervals can be stretched, and weather forecasts can help with spray timing, he adds.

“Blight treatments work by protecting walnuts from infection,” Buchner says. “They won’t control the disease if applied after infection has already occurred. Copper, tank-mixed with Manzate for each treatment, is currently the best available choice for walnut blight management.”

Four manzate products are available for use in California – two flowable formulations, Manzate Max and Manzate Flowable, and two dry flowable  formulations, Manzate Pro-Stick and Penncozeb 75 DF. Each have subtle differences in respirator requirement and treatment interval. If you select one of these products, check with your PCA or Ag Commissioner for label requirements.

 

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 From 2010 through 2102, Buchner and his colleagues studied the amount of bacterial inoculum in dormant buds, spray programs and the percent of walnut blight damage for 30 orchards in Butte and Tehama counties. Seven Chandler, six Howard, two Hartley, one Tulare, and one Vina orchard all had very good walnut blight control, he notes.