Walnut Blight

In walnut blight, one to several black lesions may appear on catkins. Infected nuts develop black, slightly sunken lesions at the flower end (end blight) when young; more lesions will develop on the sides of the nut as it matures (side blight). Shoots develop black lesions, and leaves show irregular lesions on blade.

All green tissue is susceptible to walnut blight. Economically significant damage occurs when the developing nut is infected. The bacterium that causes walnut blight overwinters primarily in dormant buds. Rain is important for spreading bacteria and aiding infection. Early-leafing varieties are most severely affected, and the disease tends to be more severe in Northern California.

Management of this disease depends on the application of protective sprays to buds, flowers, and developing nuts. In orchards with histories of walnut blight damage, protective treatments at 7- to 10-day intervals during prolonged wet springs are necessary for adequate disease control. In areas or years with less intensive rainfall, spray intervals can be stretched, and weather forecasts can help with spray timing. Full coverage sprays are recommended and important to resistance management.

Infection of Xanthomonas arboricola pv juglandis bacterium depends upon environmental conditions, the amount of the pathogen in individual buds (inoculum), and in the amount of walnut blight cankers present on some walnut varieties. Blight treatments are timed to coincide with early shoot emergence, which places a protective layer of bactericide on emerging green tissue. In most years, the first bactericide application can be delayed and should be applied when 30 to 40% of the buds reach the "prayer" stage (when terminal leaves of pistillate flower buds first unfold and appear like hands in a prayer position). A second spray should be done 7 to 10 days later to effectively treat the pistillate flowers that weren't sufficiently open during the initial application. Additional treatments can be timed using inoculum, disease history, variety, and weather forecasts. A spray prediction model (XanthoCast) is available to help determine the need for additional treatment.

Estimates of inoculum levels can be done by collecting 50 to 100 buds per orchard block and having them evaluated in a microbiology lab or by using the disease levels in the previous growing season. A disease rating scale can be used annually (e.g., each June: Low disease risk: less than 50 total blighted nuts per tree; moderate disease risk: 50 to 150 blighted nuts per tree; and high disease risk: 150 blighted nuts per tree or more). In orchards with varieties where catkins emerge before the pistillate flowers (e.g. Chandler), if there is disease incidence in the previous season and forecasted rainfall during bloom, consider bactericide application when 30 to 40% of the catkins emerge (note: this is usually 7 to 10 days before pistillate flowers emerge).