How to Manage Pests

UC Pest Management Guidelines


Walnut Blight

Pathogen: Xanthomonas campestris pv. juglandis

(Reviewed 12/07, updated 11/10, corrected 11/10)

In this Guideline:


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 sensitive to walnut blight infections. Economic 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.

In years with high rainfall during catkin flowering, treatments may be applied when 30 to 40% of the catkins emerge (Note: this is usually 7 to 10 days before pistillate flowers emerge.) In most years, the first 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 weather predictions. A spray prediction model (XanthoCast) is available at to help determine the need for additional treatment.

Common name   R.E.I.+ P.H.I.+
(trade name) Amount/Acre (hours) (days)

Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to efficacy and environmental impact Not all registered pesticides are listed. Always read label of product being used.
  8-5-100 Label rates see label
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Adding 0.5 gal summer oil emulsion can reduce phytotoxicity. The objective is to apply 4 lb metallic copper and 5 lb of calcium hydroxide in 100 gal water/acre. If using basic copper sulfate, which is 50% copper, apply 8 lb/acre. For hydrated copper sulfate, which is 25% copper, use 16 lb/acre. Not all copper compounds are approved for use in organic production; be sure to check individual products. For information on making Bordeaux mixtures, see UC IPM Pest Note: Bordeaux Mixture, ANR Publication 7481.
B. FIXED COPPER# Label rates see label
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Resistance to copper is common in Sacramento Valley orchards and has been found in a few San Joaquin Valley orchards. A Section 18 registration may be available for some areas of the Sacramento and San Joaquin valleys (check with your county agricultural commissioner) allowing the addition of Manzate (flowable or Pro-Stick formulations) to copper. This combination improves control. If Manzate is used, it must be added to every copper treatment for the greatest benefit. The use of surfactants does not increase the efficacy of copper materials. Wettable powders with 50% metallic copper: rates equivalent to 4 lb metallic copper/acre are effective. Dry flowable formulations with less than 50% metallic copper or liquid formulations (Copper-Count N, Champ, etc.): use label rates. Some liquid formulations of copper require less than 4 lb metallic copper/acre. When used at recommended label rates, these formulations provide the same control as that of wettable powders with 4 lb metallic copper. Not all copper compounds are approved for use in organic production; be sure to check individual products.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
# Acceptable for use on organically grown crops.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471


  • J. E. Adaskaveg, Plant
  • Pathology, UC Riverside
  • R. P. Buchner, UC Cooperative Extension, Tehama County
  • G. T. Browne, USDA Crops Pathology and Genetics, UC Davis
  • W. D. Gubler, Plant Pathology, UC Davis
Acknowledgement for contributions to Diseases:
  • B. L. Teviotdale, Kearney Agricultural Center, Parlier

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