“The majority of the samples submitted to the plant clinic identified fire blight on pear trees and ornamental pear trees,” Miller said. Ornamental pears are popular trees in the Midwest, but they are also susceptible to fire blight. Bradford callery pear is fairly resistant, but it is not immune. Aristocrat is more susceptible. Chanticleer is reported to have good resistance; Redspire is susceptible.
Fire blight is caused by a bacterium that infects in warm, humid conditions, which have been prevalent in northern Illinois. The primary mode of entry into the plant is via flowers, so the critical infection period is during bloom. Infection can also occur via wounds, especially after wind or hail storms. The bacterium moves systemically in plants to shoot tips. Blighted leaves and blossoms near twig tips appear first. Leaves may wilt and turn brown or black.
Stem cankers develop as sunken, cracked areas on stems. The pathogen may live over winter in these cankers. The bacterium oozes from these cankers in the spring, attracting insects. The insects spread the bacteria to blossoms, fruit, or other plant parts. Water and pruning tools may also spread the bacteria.
“When severe stem death occurs, what most people want to know is what can be done now,” Miller said.
Unfortunately, there is no effective management option for infected trees. The U of I plant clinic recommends pruning out infected wood in the dormant season, if you can wait. If not, prune in an extended dry period and disinfect pruning tools after every cut. The bacterium may have extended down the stem ahead of the canker, so wood should be removed 8-10 inches below the edge of the visible canker. Chemical options are limited for home growers because the timing of sprays is so critical. Commercial growers apply copper products in the dormant season and streptomycin at four to five-day intervals throughout bloom.
Fertilization and watering will not fix the problem; it will promote lush growth, which is more susceptible to infection by the fire blight bacterium.
If you suspect your tree has fire blight or have questions about this disease, call the Master Gardeners at your local Extension office, 815-758-8194, for more information.