Evaluation of Relative Resistance of Some Walnut Commercial Cultivars and Promising Genotypes to Shallow Bark Canker (Brenneria nigrifluens)

Shallow bark canker caused by Brenneria nigrifluens is an important walnut disease which causes reduction in yelid and wood quality, tree decline and death. In this research, relative resistance of four local walnut genotypes KZ3, 88-1, H2-12, H2-1, one improved cv. Jamal, and eight commercial introduced cultivars; Lara, Serr, Pedro, Vina, Shinova, Chandler, RDM and Hartley (susceptible control) to brack canker and its relationship with the inoculated organs were evaluated. Disease samples were collected from Kohgiloyeh and Boyrahmad province and causal bacterial strains isolated and identified using common phenotypic tests. A mixed suspension from four selected strains was used as inoculum. Plant materials were grafted in winter 2012 and inoculated in spring 2015 in orchard conditions and canker length was measured 18 months later in winter 2016. The canker causal agent was identified as Brenneria nigrifluens. Canker severity was different among walnut genotypes and cultivars, Hartley and Shinova with average canker length at four centimeters and 1.75 cm were rated as the most susceptible and resistant, respectively. No cultivar or genotype was completely resistant. Both shoot and trunk were susceptible, but trunk with mean canker length of 2.91 cm was more susceptible than shoot with 1.90 cm mean canker length. A significant positive correlation was found between organ diameter and canker length (r = 0.56**). Walnut cultivars and genotypes demonstrated different levels of resistance to shallow bark canker. Considering adverse effects of drought on canker severity and prevailing drought conditions in Iran, identification and growing walnut cultivars with relative resistance to this disease can reduce economic losses.