Recent avances on the epidemiology and control of bacterial blight of walnut, incited by Xanthomonas arboricola pv. juglandis

The bacterial blight of walnut caused by Xanthomonas arboricola pv. juglandis (Xaj) is an emerging disease, which can severely affect walnut orchards. Symptoms are visible on all aerial parts of the host, particularly on leaves and nuts. The disease develops more rapidly in spring, causing spots on the 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 within mid-May and mid-June. The primary source of the inoculum is available very early in spring, since evasion is likely to occur from small overwintering cankers present on twigs as soon as the new vegetation busts. The primary inoculum is spread by wind-driven rain droplets and pollen. Our study confirms the spread by pollen, but catkins seems to become infected during their spring development from bacteria oozing from small twig cankers. Female flowers are not contaminated before pollination, and become infected during pollination and/or during spring rain. Copper resistance has been studied on a collection of over 150 strains isolated in Romagna during 2007- 2009. A high percentage (83%) of the collection proved tolerant to copper, whereas 36% was highly resistant. Control strategies are difficult to implement and are based on the timely and effective use of copper compounds with emphasis on spring treatments. In order to bypass copper resistance the use of alternative molecules as resistance inducers is under evaluation, coupled with a reduced use of copper.