W.H. Olson, R.P. Buchner, J.E. Adaskaveg, S.E. Lindow
Early leafing walnut cultivars such as Ashley and Vina can sustain substantial yield losses due to walnut blight in many Northern California orchards. The causal bacterium Xanthomonas campestris pv. juglandis (Pierce) dye, survives from one year to the next mainly in and on buds, and in twig lesions. Vegetative and fruitful buds, serve as an important reservoir of inoculum that will infest leaves and nutlets. The bacterium is spread by moisture, primarily rainfall. In Butte County the average rainfall for the spring months is 14,8 cm. Consequently walnut blight control is a major challenge to Butte County Walnut Growers.

Copper based compounds have been the only means of control for more than 40 years. Previous research showed that treatments containing 4,5 kg of metallic copper per hectare provided the most economic control. Spring treatments applied every seven days at this rate to protect the developing nutlets, provided better control than treatments applied at twice the rate every 14 days. These treatments, however, have little effect on inoculum populations located within buds. Treatments applied after rainfall showed little benefit. As a result of this research growers may apply up to 10 treatments each spring.

Until recently failures in walnut blight control have been related to a lack of protection during environmental conditions conductive to infection and disease development. The extensive use of copper in Northern California orchards is now attributed to the development of copper resistant populations of the walnut blight organism, first recognized in 1990.

Data indicates that copper resistant strains of the walnut blight pathogen are not killed by standard copper applications under field conditions. Because of the development of copper resistant populations of the bacterial pathogen evaluations of non-copper based reducing disease incidence include copper hydroxide-Manex, copper hydroxid-Ferric Chloride mixtures, and zinc based compounds.

Since inoculum build-up starts in the buds the year prior to the year in which nutlets are infected dormant treatments to eradicate the inoculum from the buds are also being investigated.

Olson, W.H., Buchner, R.P., Adaskaveg, J.E. and Lindow, S.E. (1997). WALNUT BLIGHT CONTROL IN CALIFORNIA. Acta Hortic. 442, 361-366
DOI: 10.17660/ActaHortic.1997.442.57
Xanthomonas campestris pv juglandis, copper resistance, chemical control, Manex

Acta Horticulturae