The landscape problem I have seen and heard about the most in the last two or three weeks has been fire blight on flowering pear trees.
It is one of the most devastating and difficult-to-control diseases we get. It also affects crabapple, pyracantha, hawthorn, quince, loquat, spirea, red-tip photinia (what’s left of them) and cotoneaster. It basically affects any plant in the Rosaceae family. Similar stem dieback that is mistakenly identified as fire blight occurs on other plants.
The first symptom of fire blight occurs in early spring when temperatures are above 60 degrees and the weather is rainy or humid. Infected flowers turn black and die. The disease moves down the branch, resulting in death of young twigs. Twig tips appear as if they were scorched by fire or damaged by frost and may be randomly distributed throughout the tree. Twigs become blackened as the disease progresses downward toward larger stems, and affected leaves tend to cling to the branches. Twig tips normally develop a “shepherd’s crook” which is very useful in disease diagnosis. Stem lesions develop a sunken appearance with small cracks at the margins.
Primary infection begins in the spring with the infection of blossoms or shoots, providing a source of bacteria in “holdover cankers” on infected plants. During the wet weather, bacteria ooze from these cankers and attract insects that spread the bacteria to other susceptible plants or plant parts. The bacteria are commonly carried to the blossoms, fruit, shoots, and leaves by flying or crawling insects, including honey bees. Overhead watering also can be an important method of spread for smaller plants. Secondary spread can occur by insects, birds and people using contaminated pruning tools.
At this point for flowering pears or any other plant with the disease, the only thing to do is prune the disease portion out of the tree. Once the bacteria are in the tree, no chemicals can remove and eliminate them.
Prune as soon as possible, cutting 4-6 inches below the symptoms and burn or throw away the cuttings. While pruning, it would also be advisable to disinfect your tools after each cut in rubbing alcohol or use a 10 percent Clorox solution.
I realize pruning is not practical for most homeowners who have fairly large flowering pear trees. You would have to hire someone to do it for you.
I usually don’t recommend preventative treatment for ornamentals such as pyracantha, hawthorn, quince, photinia, and cotoneaster in the home landscape for fire blight because the severity varies from year to year because of the weather.
Next year, susceptible plants can be treated with streptomycin or copper hydroxide every five days during the bloom period to prevent infection.
For fruit trees, some varieties are more resistant to fire blight than others. Refer to our Extension brochures on fruit-tree selection to see the ones we recommend as more resistant than others.
For example, a Bartlett pear is a favorite among fruit growers, but we don’t recommend it because of its susceptibility to fire blight.