The virulence of six strains of Erwinia amylovora used in combination for screening fire blight resistance of pear seedlings and advanced selections from the Harrow pear breeding program was evaluated by inoculating a standardized suspension (108 cfu/ml) of the six strains individually and in combination into actively growing shoot tips and measuring the lengths of the diseased shoots six weeks later. Three cultivars provided a range of resistance to fire blight: `Bartlett' was susceptible, HW-605 (`Seckel' × NJ-6) was moderately resistant, while `Kieffer' was resistant. On `Bartlett', one strain was consistently more virulent than the combination, while on HW-605, two strains were consistently more virulent than the combination. One strain was consistently less virulent than the combination on both `Bartlett' and HW-605. No strain was consistently more or less virulent than the combination when inoculated into `Kieffer'. Lesion lengths were greater in the susceptible cultivar `Bartlett' than in either HW-605 or `Kieffer'. These results suggest that a combination of strains of E. amylovora is appropriate for screening for fire blight resistance in pear genotypes.
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