By Richard P. Buchner – UC Farm Advisor, Tehama County and Steve E. Lindow – Professor of Plant Pathology, UC Berkeley
Our most current information indicates the section 18 for ethylene bisdithiocarbamate materials (Manzate flowable or Manzate Pro-stick) for walnut blight control are on track with issuance expected about March 1. We have not seen the actual 2012 section 18, but expect similar wording and restriction as in previous years.
Copper tank mixed with Manzate flowable or Pro-stick is currently the most effective spray choice. Good quality copper products are all effective for controlling walnut blight. Follow label rates because metallic rates and copper availability vary depending upon product. Full coverage at full material rates is recommended.
Walnut blight (Xanthomonas arboricola pv juglandis) bacteria overwinter in the outer bud scales or cataphylls. Within the dormant bud, the inner leaf tissue and flowers are pathogen free. As the shoot grows through the infected outer bud scales, bacterial have the opportunity to move and infect developing leaves, shoots and flowers. Infection occurs when rainfall and/or wet conditions transport blight bacteria to developing tissue. The probability of infection depends upon how much pathogen exists on individual buds and environmental conditions favoring bacterial spread and infection. First walnut blight sprays are timed to coincide with early shoot emergence. This places a protective layer of bactericide on leaf tissue. If bacteria are splashed from the out bud scales to developing shoots and flowers, the bactericide barrier prevents infection and subsequent blight lesions.
Since all walnut shoots do not emerge at the same time, the first protective spray is applied when 40 percent of the shoots are elongating and before leaves expand. This is usually referred to as the “prayer” stage since the unfolded leaves resemble hands held in prayer. A second spray is applied about seven to 10 days later and is designed to protect the remaining opening buds. Additional spray decisions are based upon measurements of infected buds, disease history, weather conditions and variety.
Many walnut growers wonder if they need to blight spray Chandler or other late leafing varieties. We have measured over 50 percent crop damage on Chandler walnuts when overwintering bud populations were high and spring weather favored disease. Conversely, we have measured little to no blight on Chandler walnut with low to zero bud population levels even when wet spring weather favored disease. Late leafing walnut varieties have less opportunity time to build high walnut blight populations in dormant buds. As a result, initial inoculum levels are low. This does not eliminate, but reduces the probability of disease incidence. A good late leafing strategy would be to apply the first two applications with the intention of maintaining low inoculum levels.
Bud pathogen information, disease history and weather conditions can be used to improve spray decision accuracy. The California Seed and Plant Lab, 7877 Pleasant Grove Road, Elverta, Calif., 95626 (916-665-1581) is available to evaluate dormant walnut buds for blight bacteria. The sampling technique is available at cetehama.ucdavis.edu. Click on orchard crop, click on walnuts and scroll down to “sampling dormant walnut buds.”