BIOCONTROL OF THE BACTERIAL BLIGHT OF WALNUT: IS THERE A CHANCE TO REDUCE COPPER INPUTS INTO WALNUT GROVES?

The bacterial blight is a re-emerging disease, severely affecting
the productivity of walnut groves. Disease symptoms are observed
on all aerial parts of the host plant, but the most damaging are the
necrotic spots developing on fruits, leading to massive fruit drops,
mainly before endocarp hardening. The causal agent is Xanthomonas
arboricola pv. juglandis (Xaj), a gram-negative rod with a worldwide
distribution. The population structure of Xaj includes several morphotypes,
at least five sequence types, and other features confirming
its genomic heterogeneity. Copper resistance is widespread and very
effective among Xaj populations: therefore, disease management
based on copper sprays is not particularly effective, even in cases
of 10-14 treatments during the growing season. The recent, severe
outbreaks reported in Italy are due to two concomitant events: i) the
rapid increase of walnut acreage, especially in the northern part of
the Country and ii) the development of Xaj populations showing
high resistance to copper. Since most of the cultivated walnut varieties
are either highly susceptible or susceptible to the disease, we
tried to implement control strategies based on the use of a single antagonist
or the field application of microbial consortia. Additionally,
innovative agrochemicals, with a reduced copper content, have also
been used. Results showed that: i) microbial biocontrol agents were
able to significantly reduce the disease in the field; ii) innovative
agrochemicals may reduce the bacterial blight, but might enhance
phytotoxicity; iii) a significant reduction of copper inputs is possible,
coupled with an effective disease control in walnut groves.