Temperature Declines During Storms and Irrigation May Contribute to Fire Blight Infection of Pear Fruit

During summer of 2002, fire blight caused by Erwinia amylovora appeared in immature pears (Pyrus communis) in five Ulster County pear orchards in southeastern New York State. Foliar symptoms of fire blight were absent or infrequent in many trees that had infected fruit, suggesting that fruit infections occurred after shoots had stopped growing and were no longer susceptible to infection. In one orchard where overhead irrigation was initiated on 23 July when air temperatures were 96°F, incidence of fruit infection in September ranged from 10 to 76% with a mean of 36% of all fruit showing fire blight symptoms. Laboratory experiments showed that non-wounded fruit preheated to 95°F and then submerged in inoculum suspensions at 70°F or 40°F developed fire blight infections whereas similar fruit preheated at 70°F and immersed in 70°F-inoculum did not develop infections. Overhead irrigation initiated during the heat of the day can disperse inoculum at the same time that rapid cooling from irrigation water may cause E. amylovora to be drawn into immature pear fruit. The same combination of water-disseminated inoculum and rapid cooling can occur during summer thunderstorms, and storm events may explain the unusual incidence of fire blight on immature pears that was observed in 2002.