VIEW ARTICLE

Ecology and Epidemiology

Ecology of Xanthomonas campestris pv. juglandis on Persian (English) Walnuts. E. N. Mulrean, Former graduate student, Department of Plant Pathology, University of California, Berkeley 94720; M. N. Schroth, professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 72:434-438. Accepted for publication 13 July 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-434.

Isolations made from apparently healthy dormant and developing buds and catkins of seven Persian (English) walnut varieties showed that these structures were frequently infested with the walnut blight bacterium, Xanthomonas campestris pv. juglandis. Buds were infested significantly more often than catkins, indicating that dormant buds represent the major overwintering site of the pathogen. Epiphytic populations of X. c. pv. juglandis on buds ranged up to 5.9 × 10⁵ colony-forming units (cfu) per bud, with internal populations up to 1.4 × 10⁶ cfu. Ninety percent of the dormant buds with epiphytic populations of the pathogen were also internally infested. Infected foliage, with epiphytic populations of up to 10⁹ cfu per leaf, served as a source of secondary inoculum for late season infection of nuts and infestation of developing buds and catkins. No evidence was obtained to substantiate the role of blighted branch cankers in the overwinter survival of X. c. pv. juglandis. The number of epiphytically infested buds was reduced by dormant season applications of Kocide 101 and an experimental compound. However, the addition of dormant season sprays to conventional spray programs did not significantly improve walnut blight control.