How to Manage Pests

UC Pest Management Guidelines


Walnut Blight

Pathogen: Xanthomonas campestris pv. juglandis

(Reviewed 6/17, updated 6/17)

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.

Comments on the Disease

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).

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide’s properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being listed.
  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.
  (Kocide 3000, Badge X2 and others) 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. The use of surfactants does not increase the efficacy of copper materials. Wettable powders with 50% metallic copper: rates equivalent to 4 lb or higher metallic copper/acre are effective. Dry flowable formulations with less than 50% metallic copper (Kocide 3000, Badge X2, ChampION++, Cuprofix Ultra, Nordox) or liquid formulations (Copper-Count N, Champ, etc.): use label rates. If resistance is known to occur in the orchard, use higher label rates. Some liquid formulations of copper require less than 4 lb metallic copper/acre. When used at recommended label rates, some of 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.
  (Dithane F-45, Manzate Max) 58 fl oz 24 75
  (Manzate Prostick) 2.4 lb 24 75
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: This product must be tank mixed with a fixed copper product that is registered for use on walnuts. Do not apply more than 10 applications (18 lb a.i.) per season.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. 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 (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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, Davis, CA
W. D. Gubler, Plant Pathology, UC Davis
T. J. Michailides, Kearney Agricultural Research and Extension Center, Parlier
J. K. Hasey, UC Cooperative Extension, Sutter, Yuba, and Colusa counties
E. J. Fichtner, UC Cooperative Extension, Tulare County
S. J. Seybold, Entomology, UC Davis (thousand cankers disease)
R. M. Bostock, Plant Pathology, UC Davis (thousand cankers disease)

Acknowledgement for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Research and Extension Center, Parlier (Emeritus)

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