Spray timing and materials for walnut blight (Xanthomonas campestris pv. juglandis, Xanthomonas arboricola pv. juglandis) control in Northern California USA.

Depending on weather conditions, pathogen population and walnut variety, walnut blight bacteria Xanthomonas arboricola pv. juglandis (Xanthomonas campestris pv.juglandis) can cause significant crop loss. Damage in Northern California, USA can exceed 50% on untreated trees. Copper formulations tank mixed with Manex, Manzate or Dithane (ethelyene-bis-dithocarbamate) represent the best available control materials. Manex is the formulation registered for use in Northern California. First sprays applied after bud break when new leaves are just beginning to unfold "prayer stage" confer the greatest decrease in pathogen populations. In addition, buds that open first are more likely to set a crop and represent the most important shoots and flowers to protect. Walnut blight bacteria over-winter on dormant buds primarily under the outer bud scales or cataphylls. When buds break in the spring, cataphylls open and young shoots extend past them. During wet weather conditions bacteria are splashed from the cataphylls and potentially infect new shoots and developing walnuts. A good quality penetrating adjuvant added to the Copper/Manex mix will wet cataphylls and encourage the Copper/Manex solution to penetrate between bud scales killing over-wintering bacteria. California walnut blight research reports are available at http://walnutresearch.ucdavis.edu.