Importance of pear-tissue injury to infection by Erwinia amylovora and control with streptomycin

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Abstract
Greenhouse, orchard, and laboratory experiments have shown the significance of pear shoot and fruit injury to infection by Erwinia amylovora. Bartlett trees injured by sand blasting and then spray-inoculated in the greenhouse were infected predominantly through leaf petioles. Blight development was more severe when inoculated trees were incubated in the moist chamber than on the greenhouse bench and when inoculated within 24 h after injury than after longer time intervals. Streptomycin provided satisfactory blight control of injured trees when applied within 6 h after inoculation, while uninjured trees were protected for 4 days. Excellent control was obtained when streptomycin was applied within 24 h after injury and inoculation, when preceded by a protective streptomycin spray. All injured Bartlett fruit on orchard trees dipped in E. amylovora inoculum at time of wounding blighted, while 70% of those bruised 2 days before or after inoculation became infected, regardless of streptomycin application. Dip-inoculated, streptomycin-treated fruit bruised with breakage of the skin showed 5 times more blighted fruit than when the skin remained unbroken. Orchard observations on four pear cultivars under severe blight conditions revealed that 62% of the trees exhibited both shoot and fruit blight, 8% only shoot blight, and 17% only fruit blight. Moonglow cultivar was the most resistant to fire blight and showed the lowest percent infected fruit. In the laboratory, punctured and inoculated fruit of all cultivars blighted but uninjured DeVoe fruit did not blight regardless of inoculum concentration.

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