Evaluating Walnut (Juglans spp.) for resistance to walnut blight and comparisons between artificial inoculation assays and field studies

The host resistance of walnut (Juglans spp.) against Xanthomonas arboricola pv. juglandis (Xaj) was evaluated for 18 walnut genotypes in 2016 and 2017 at Chengdu, Sichuan, China, using three assays: one under natural conditions (field investigations) and two under controlled conditions (detached leaflet assay and whole plant assay). Field investigations were used as a benchmark for comparing the other two assays. Five genotypes, ‘JS 91’, ‘JS 92’, ‘JS 86’, ‘Qingxiang’ and ‘JS 71’, displayed partial resistance in all assays, suggesting that they may be good candidates for further evaluation. The rankings for resistance among the 18 genotypes showed significant correlations between field investigations and both the detached leaflet assay and whole plant assay. However, low determination coefficients indicated that assays under controlled conditions should be considered as complementary methods and not as a substitute for field analysis when evaluating resistance to walnut blight.