Population features of Xanthomonas arboricola pv. juglandis and epidemiology of walnut blight in Romagna (Italy)

Bacterial blight of walnut caused by Xanthomonas arboricola pv. juglandis (Xaj) is an emerging disease, which has the potential to severely affect walnut orchards (Mulrean and Schroth, 1981). Symptoms are visible on all aerial parts of the host plant and particularly on leaves and nuts; the disease develops more rapidly during spring, causing spots on leaves and immature fruits, followed by the formation of small cankers on leaf petioles and twigs. Affected fruits fall down throughout the growing season, with a peak from mid-May to mid-June. Primary inoculum is released early in spring, from small overwintering cankers present on twigs. Primary inoculum is spread by wind-driven rain droplets and by pollen. Our study confirms the spread by pollen, but catkins seems to become infected during their spring development from bacteria oozing out from small twig cankers. Female flowers are not contaminated before pollination, and become infected during pollination and/or during spring rain. The source of primary inoculum appears to be the small overwintering cankers developing near the fruiting buds. The population structure of a broad collection of Xaj isolates, obtained from affected orchards in Romagna, confirms the presence of different genetic groups, identified by rep-PCR (using the REP, BOX and ERIC primers) and by multilocus sequence typing (MLST) and multilocus variable number analysis of tandem repeats (MLVA). Copper resistance was studied on a wide collection of over 150 Xaj strains isolated in Romagna walnut orchards during 2007-2009: 83% of the collection strains proved to be tolerant to copper, whereas 36% proved to be highly resistant. Control strategies are difficult to implement and are based on the timely effective use of copper compounds with an emphasis on spring treatments. The Walnut Blight Forecast Model “Xanthocast”, developed in California (Adaskaveg et al., 2004), is under evaluation in European walnut orchards. In order to avoid the development of copper resistance, the use of possible resistance inducers is under evaluation, coupled with a reduced use of copper. Glucohumates (active humic and fulvic acids, obtained from leonardite and gluconic acid) were able to control lesion development on walnuts in vitro and reduce disease incidence in field experiments.