EVALUATION OF ANTIBIOTICS AND PLANT EXTRACTS FOR CONTROL OF STREPTOMYCIN-RESISTANT ERWINIA AMYLOVORA

Erwinia amylovora resistant to streptomycin is widespread in British Columbia (BC), Canada so there is an important need for products that are effective against resistant isolates as well as for alternating with the recently registered Pantoea agglomerans biocontrols. In vitro laboratory tests and field trials following standard protocols have been conducted annually at the Pacific Agri-Food Research Centre, Summerland, BC to identify promising materials for several years. Antibiotics were tested against 12 streptomycin-resistant and 15 sensitive isolates in laboratory tests. Plant extracts especially fractions of sea buckthorn juice were also included in these in vitro tests. Promising materials from these tests were applied to potted 'Gala' and 'Jonagold' apple trees inoculated with E. amylovora. Results with these materials showed that all three antibiotics were effective against streptomycin-resistant isolates and produced inhibition zones similar in size to those isolates sensitive to streptomycin. The aqueous phase of sea buckthorn juice produced relatively large zones of inhibition on both streptomycin-resistant and sensitive isolates. Screenhouse trials confirmed that the antibiotics were effective for the control of fire blight if the correct rate and formulation were used. Results with sea buckthorn juice were erratic although in one screenhouse trial it was as effective as streptomycin. Further research is needed on sea buckthorn juice to identify its active ingredient and purify it for use in commercial orchards. Oxolinic acid, oxytetracycline hydrochloride and gentamicin are three antibiotics that could replace streptomycin if necessary and be alternated with P. agglomerans for improvement of biocontrol efficacy.