The bacterial canker of kiwifruit is the most destructive disease
of cultivated Actinidia spp. The causal agent is the Gram negative bacterium Pseudomonas syringae pv. actinidiae (Psa).
The pathogen grows inside the trunk, vines and leaves of its host
plants, causing wilting and formation of cankers, with subsequent
death of the plant. During the past three years, samples have been
collected from various parts of asymptomatic Actinidia spp. within
infected kiwi orchards and many endophytic bacteria have been
isolated. Among them, a few isolates were identified and proved
to be able to strongly inhibit Psa namely an isolate of Pantoea agglomerans,
two pseudomonads belonging to the fluorescens/putida
group, and one novel Pseudomonas sp., which proved to be highly
effective in inhibiting, in vitro, several important phytopathogenic
bacteria. This last isolate was chosen for further studies. Concentrated
supernatant of its liquid culture in LB medium could
inhibit Psa, indicating that the active compound produced by the
antagonist is excreted from the cell into the environment. The
nature of that biotoxin is not proteic, and its size is smaller than
3 kDa, as a result of its partial purification. Moreover, a bank of
mutants deficient in their antagonistic activity has been constructed
by triparental mating, in order to search and identify genes
involved in antagonism against Psa. Fifty six prospective mutants
have been already obtained: twenty two of them were subject to
Southern blotting to verify a single transposon insertion. Gene
identification is under way
