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.