Fire blight has been the most devastating disease of commercial pear production in the eastern United States. This disease will continue to threaten the industry because highly susceptible rootstocks and cultivars are continuously being planted and the recent development of streptomycin-resistant strains of the pathogen found in the east.
This disease should not be confused with the fire blight or leaf spot diseases of pears. Leaf blight and fruit spot are caused by the fungus Fabraea maculata, which infects the leaves, fruit, and shoots of pear and quince and the leaves of apple trees. The disease can build up rapidly, even in orchards where it has not been a problem. If conditions favor the disease and it is not controlled, pear trees can become defoliated in a few weeks.
The pear leaf spot fungus, Mycosphaerella pyri, infects the leaves of pear, quince, and occasionally apple trees. Numerous leaf spots can produce defoliation. Fortunately, this does not occur often before fall, except in nurseries.
Pear scab is an economically important disease throughout the world and can cause serious losses on susceptible cultivars. Pear scab occurs on leaves, shoots, and fruits. Symptoms are very similar to apple scab. A major difference, however, is the frequent appearance of pear scab on twigs, where it can overwinter and sporulate to initiate new infections in the spring.
Stony pit of pear is presumed to be caused by a destructive virus, but the virus has not been isolated. Affected fruit are unsightly and unmarketable. This disease is sometimes referred to as "dimpling" because of the symptoms observed on fruit.