We've gotten many calls and samples concerning fire blight this season, and the same goes for our plant clinic on campus in Champaign-Urbana according to their recent report.
The majority of the samples submitted to the plant clinic identified fire blight on pear trees and ornamental pear trees. Ornamental pears are popular trees in the Midwest, but they are also susceptible to fire blight. 'Bradford' callery pear is fairly resistant to this disease, 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, Erwinia amylovora. The fire blight bacterium infects in warm (>60 degree F), humid conditions which have been prevalent this late spring here 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. Typical shepherd's crooks at stem tips are evident as seen in the attached image from the plant clinic.
Stem cankers develop as sunken, cracked areas on stems. The bacterial 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 may also spread the bacteria. We also contribute to spread via pruning tools.
When severe stem death occurs, what most people want to know is what can be done now. 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. Unfortunately this means 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 4-5 day intervals throughout bloom. Don't try to fix this problem with fertilization and watering. You 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 for more information or send a sample to the U of I Plant Clinic. Questions can also be posted on our Northwest Illinois Extension Horticulture Facebook Page: https://www.facebook.com/northwestillinoishorticulture