By John Wood, Certis USA Regional Manager
Walnut blight (Xanthomonas arboricola pv. juglandis) can be disastrous and cause substantial crop loss. It doesn’t have to. Three things are needed for disease to attack: pathogens, hosts and favorable weather.
While we can’t control the weather, we have some control of the hosts and can monitor pathogens by using bud population levels to help determine what protocols we need to follow.
Inoculum in dormant buds can help predict disease severity. Bud population sampling can measure whether walnut blight populations are growing or are being adequately suppressed. This is especially important in orchards with histories of heavy blight infestations.
Your 2019 walnut blight control plan should include checking blight inoculum levels. Extension specialists recommend these straightforward steps.
- Sample buds in December, January, February and March through early April. Sample them when they start to open. Earlier sampling will give you more time to implement a successful disease control plan.
- Select 100 dormant walnut spurs with fat terminal buds. Cut a 3” length. Those you can reach from the ground are a good choice because bacteria sprinkle downward through the canopy.
- Walk the entire area and collect random samples. One or two buds per tree should adequately spread the samples. Your experience with disease uniformity can guide you on how many samples to collect on an orchard-by-orchard basis.
- Save the spurs in a paper bag and store in a cool, dry place. Labs typically select a subsample and leave some remaining buds as a backup sample.
- Mail or deliver the buds to your lab. Your Extension advisor or consultant can help you interpret the results.
Provide top-level control
Should you need help controlling walnut blight, Kocide® 2000 and Kocide® 3000-O copper fungicides/bactericides, and ManKocide, a pre-mix formulation of Mancozeb and Kocide are highly effective. Use of these products will deliver efficacious protection against walnut blight. ManKocide is currently registered in California on several tree and fruit crops with a label extension for use on walnuts pending final approval in the first quarter of 2019. ManKocide’s unique formulation helps penetrate bacterial cell membranes, which allows the copper to enter the cell and denature enzymes and other metabolically important proteins, ultimately killing the bacteria that make up the inoculum. It also helps control copper-resistant bacteria.
Start Kocide 2000, Kocide 3000-O or ManKocide protective sprays when pistillate flower emergence begins, generally when 20 percent to 40 percent of female flowers are visible, i.e. bud break, commonly known as the prayer stage of elongation. If weather favors disease and when there are high-disease-pressure conditions, spraying at bud break or catkin emergence may be beneficial. Make additional applications during bloom and early nutlet stage. Remember that a seven- to 10-day interval is required when rain is frequent or there are extended periods of high moisture.
Thank you for your feedback and for helping make 2018 a wonderful year. We’ll visit again January 17. Until then.