By Deborah J. Meier, Ph.D., Technical Development, Kocide® LLC
Q. Walnuts are in their dormant period – is there anything I need to do now for disease control?
The dormant season is a good time to evaluate your orchard disease history from 2017 to begin your walnut blight management plan for 2018. However you don’t need to do any spray applications for walnut blight during dormant or delayed dormant periods. In winter, implement good sanitation practices (i.e. removing mummy nuts on trees and/or on the ground, keeping weeds and ground covers in check) and monitor for scale pests and European red mite eggs.
Q. How do I determine if I have walnut blight in my orchard?
Last year did you notice black lesions on your catkins, nuts, shoots or leaves? If so, this is likely walnut blight (Xanthomonas arboricola pv. Juglandis). Walnut blight can infect any green tissue, and inoculum overwinters in dormant buds. The bacterium is spread by rain. Infections can occur in early season. If the weather is wet, infections can occur in May and June.
Review your orchard history and walnut variety. Consult with your state extension or UC pathologists to provide information on your variety’s susceptibility to walnut blight (early leafing varieties and varieties like Vinca are more susceptible). Carefully monitor the weather and growth stages of your orchard to determine timing and rates for maximum effectiveness.
Q. What products can I use to control walnut blight, and how do I maximize control?
To date the most effective registered products for walnut blight are copper products. Kocide® 3000 and Kocide® 2000 have been tested as the industry standards for years in university trials. Due to the lack of rotational chemistry, some walnut blight strains have become copper tolerant. A combination of copper (Kocide 3000 or Kocide 2000) and mancozeb is the best choice for walnut blight management. Mancozeb helps penetrate cell membranes that in turn helps the copper protect the tree against the walnut blight bacteria. Additionally, using a combination of two actives with multi-site modes of action can reduce disease inoculum and incidence.
**To maximize effectiveness, initiate Kocide 3000 or Kocide 2000 plus mancozeb protective sprays early when pistillate flower emergence occurs (20 to 40% female flowers are visible or when emerging shoots have reached the “prayer stage” of elongation). If in-season weather conditions are favorable for disease and in high-pressure situations, an earlier spray at bud-break or catkin emergence may provide benefit. Use full orchard sprays and apply product as a protectant. If previous year blight levels were high and moist conditions prevail, higher use rates may be necessary to achieve optimum control. Additional applications should be made during bloom and early nutlet stage. A 7- to 10-day interval is needed when frequent rainfall or extended periods of moisture occur. In wet weather using a weather forecasting system like Xanthocast, developed by Dr. James Adaskaveg, can help identify the best timing intervals for sprays.
**Adapted from University of California IPM Walnut Pest Management Guidelines