Updated March 2018.
Written by Richard Buchner, UCCE Orchards Advisor Emeritus & Jim Adaskaveg, Plant Pathology Specialist, UC Riverside
Walnut blight is the result of infection by the bacterium Xanthomonas arboricola pv juglandis (Xaj).All green tissue is susceptible to infection, including buds, flowers and leaves but nut infections are the most serious, usually resulting in economic damage. Early in the spring, infected walnuts develop a dark sunken lesion at the blossom end (“end blight”) killing the developing kernel. As walnuts mature, blight lesions can develop elsewhere on the husk surface (“side blight”). Husk lesions begin as small water-soaked spots that later darken, enlarge, sink and often crack. Infections that do not invade the kernel may increase the possibility of secondary insect or disease damage. Drought years do not favor infection but years with extensive spring rainfall can be disastrous if management action is not taken.
How Walnut Blight Infects Tissue. Walnut blight 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 with water 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. A successful walnut blight control program focuses on protecting developing shoots and flowers and decreasing Xaj bacteria over-wintering in dormant buds. 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 has been the most effective spray choice until spring 2018, which saw the new registration of Kasumin 2L. 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.
- Half sprays from every-other-row application
- Not tank mixing with a Mancozeb formulation.
- Dense tree canopies.