By John Wood, Certis USA Regional Manager
As year-end nears, it is a good time to evaluate last year’s disease history of your orchard. This should be the starting point of your walnut blight (Xanthomonas arboricola pv. juglandis) control plan for 2019. Luckily, you don’t spray for walnut blight during dormant or delayed dormant periods. However, do maintain proper sanitation protocols, such as removing mummy nuts on trees and the ground and keep ground cover in check. Monitor for scale and European red mite eggs, too.
If you observed black lesions on catkins, nuts, shoots or leaves last year, you likely have walnut blight. All green tissue is susceptible to walnut blight. The inoculum overwinters in dormant buds. Rain spreads the bacterium and aids infection. Generally, infections occur in early season, especially in early leafing varieties, but wet weather can extend infections through May and June.
If needed, your Extension specialist can help you determine your variety’s susceptibility to walnut blight.
Provide maximum control
Two of the more effective products that help control walnut blight are Kocide® 2000 and Kocide® 3000-O, both copper fungicides/bactericides. Used in combination with mancozeb, Kocide 2000 or Kocide 3000-O deliver efficacious protection against walnut blight. The mancozeb 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.
Start Kocide 2000 or Kocide 3000-O protective sprays early 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 is favorable for 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. Keep in mind, a seven- to 10-day interval is required when rain is frequent or there are extended periods of high moisture.
Applied correctly, you’ll find this approach is effective. Next time, I’ll cover a few tips to wrap up your walnut blight management plan. Until then.