With reduced-crop problems surfacing for California walnut growers this season and a host of theories as to why the crop in some varieties is off as much as 70 percent, one reason that can be discounted is walnut blight.

Richard Buchner, Tehama County farm advisor, says walnut blight trials in his county show good control this year.

That’s really no surprise, what with the dry weather and excellent control strategy developed by a team of University of California farm advisors and specialists.

Walnut blight is spread by spring rains and can reduce nut quality and production. Northern California walnut growers are typically the hardest hit because they get more spring rains.

Preventive copper sprays have long been the first line of defense against walnut blight for the northern producers. However, an investigation by a team of UC ANR researchers several years ago showed that the sudden decrease in disease control was caused by resistance to the copper pesticides.

Research by two Northern California UCCE farm advisors demonstrated that adding Manex to the copper treatments provides control levels 50 percent better than copper alone.

The research team has successfully obtained annual registration of Manex, but new data must be provided each year to the Department of Pesticide Regulation in order to continue the temporary registration.

As with any disease prevention program, timing is critical. Conventional wisdom suggested that copper worked as a protectant and needed to be on flowers and developing nuts to prevent infection. As a result, first sprays were often applied at 1 percent to 2 percent bloom, with re-treatment every 7 to 10 days, depending upon weather conditions.

Growers recognized the value of early treatments, but it was not clear how early sprays should start.

Several years ago, walnut blight research technology significantly improved with the installation of rainfall simulators in the Tehama and Butte County research plots. The ability to manage rainfall and leaf wetness made it possible to conduct more controlled experiments and look more accurately at walnut blight bacteria.

Experiments were designed to investigate spray timing versus walnut blight control under simulated plus natural rainfall. The results clearly showed the value of sprays applied seven days after what was described as terminal bud break.

Using that timing, about 50 percent of the buds would have started to open. Under severe disease pressure, associated with frequent rains after bud break, additional sprays would be necessary to protect developing walnuts, but the early sprays clearly had value in the walnut blight disease control program.

Steve Lindow at UC Berkeley began sampling walnut buds and shoots to look more closely at the distribution of walnut blight bacteria within buds.

By carefully dissecting buds, Lindow discovered that almost all of the walnut blight bacteria can be found in the outer bud scales (cataphylls), and that the inner flowers are relatively bacteria-free. The next questions were, how do walnut blight bacteria get from the outer bud scales to the inner flower parts, and could that process be interfered with?

Using the tagged buds, blight treatments were applied at different stages of leafing.

Sprays applied closest to the prayer stage (as the first leaves unfold) of bud development had the largest effect on reducing populations of walnut blight bacteria. Sprays applied even a few days after the prayer stage achieved much lower control of walnut blight bacteria.

Rainfall and/or leaf wetness are likely moving bacteria from the outer bud scales toward developing walnuts. Sprays applied at the prayer stage are decreasing bacterial populations and preventing flower and nut infection.

A reasonable spray strategy would be to apply the first blight spray when 30 percent to 40 percent of the buds reach prayer stage and apply a second spray 7-10 days later. The second spray would cover the remaining bud break. Any additional sprays can be timed using weather predictions.

The XanthoCast spray prediction model is available to help with spray decisions.