Endophytes and epiphytes from Actinidia spp. as potential agents for the biocontrol of the bacterial canker of kiwifruit.

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 two commonly grown host species, Actinidia deliciosa (green fleshed) and Actinidia chinensis (yellow fleshed) show different susceptibility to the disease: the yellow kiwifruit is far more susceptible than the green one. The pathogen has significant epiphytic and endophytic phases: it penetrates the host through natural openings and thorough wounds. The control of the disease is difficult, due to the lack of effective chemicals that might be applied in the orchards. Copper compounds alone did not yield a satisfactory control of the disease. For endophyte and epiphyte isolation, orchards were chosen where the disease had the highest incidence and severity. Inside orchard plots, where most kiwi plants were dead, occasional symptomless plants were observed, cut and taken to the lab. Epiphytes were isolated and purified from washings of the leaves and endophytes were obtained from the washings of woody disks, taken at different levels along the trunk. A selection of ca. 60 isolates was chosen and subjected to preliminary genetic characterisation with rep-PCR, using the BOX primer. The selected endophytes and epiphytes were assayed in vitro for their ability to inhibit a set of phytopathogenic Gram-negative and Gram-positive bacteria, among them Psa. The results of the experiments showed that at least a dozen of endophytes and epiphytes were able to strongly inhibit both Psa and other important phytopathogenic bacteria.