Adapted from the article “Walnut Blight Management” by Richard P. Buchner, UCCE Farm Advisor, Tehama County, Steve E. Lindow, Professor of Plant Pathology, UC Berkeley, in the February 2012 Sacramento Valley Walnut News, and “2013 Walnut Blight Management” by Richard P. Buchner, UCCE Farm Advisor, Tehama Co., in the March 2013 Sacramento Valley Walnut News.
How Walnut Blight Infects Tissue. Walnut Blight (Xanthomonas arboricola pv juglandis) bacteria over winter in the outer bud scales. 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, heavy dew or otherwise 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.
Spray Timing. 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 outer 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% of the shoots are elongating, before leaves expand. This is referred to as the “prayer” stage since the unfolded leaves resemble praying hands. A second spray is applied about 7 to 10 days later to protect the remaining opening buds. Additional spray decisions are based upon measurements of infected buds, disease history, weather conditions and variety. See “Walnut Blight Bud Sampling” for more details on measuring infected buds.
Many growers wonder if they need to blight spray Chandler or other late leafing varieties. We have measured over 50% 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. 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.
What to Spray
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. Remember, it takes at least two years of a very good spray program to drive the disease back down. For comparison of efficacy of different sprays, see the annually updated “Efficacy and Timing of Fungicides, Bactericides, and Biologicals”.
Common Mistakes with Walnut Blight Control:
- First spray timing too late.
- Blight population increased resulting in high disease pressure.
- Material rates too low.
- Poor spray coverage both by air and ground.
- Using a weak material in high blight potential orchards.
- Not tank mixing with Manzate.
- Dense tree canopies.