Fire blight is known for its damaging effect on pear and apple trees, both fruiting and ornamental. At first thought the disease is not too concerning for local home gardeners because we donít typically grow apple and pear trees in coastal Carolina; however, there are more than 75 species of trees and shrubs that are vulnerable to the disease.
The susceptible plants are in the Rosaceae (rose) family. They include ornamentals such as crabapple, pyracantha, hawthorn, loquat, photinia, quince, roses, raspberry, blackberry and spirea which we do grow in our yards.
The disease is caused by a bacterium, Erwinia amylovora. It kills blossoms, shoots, limbs and even entire shrubs and trees. The infected parts of the plant become dark brown to black and shrunken-looking. They appear as if they have been scorched by fireóhence the name fire blight.
For those readers who like and grow Bradford pear trees, take comfort because the trees are considered resistant to fire blight, although under the right conditions they can be affected.
Early spring brings optimal conditions for infection. The bacteria come out of dormancy when temperatures are 60 ñ 80 degrees and conditions are rainy or humid.
Open flowers are the most common infection site. Blossom blight is typically the first symptom, though the disease can also enter a tree or shrub at a point where it has been injured. Infected flowers turn black and die. However, by then but the bacteria have already entered the plant and are on the move to twigs and branches.
Bees and other insects spread the bacteria from diseased to healthy flowers. Splashing rain can move the bacteria to new entry pointsópossibly a site of injury caused by hail or wind.
Once the bacteria get into the flowers they move on within the tree or shrub. It is then necessary to prune out the diseased plant parts later in the year.
Infected shoots often droop in the form of a shepherdís crook. The leaves become black but remain attached to the plant. These blighted shoots remain on trees and shrubs throughout the growing season and into winter. The clinging dead leaves are an important indicator that the tree or shrub is infected with fire blight bacteria.
The disease moves from flower blight to stem blight and on down a branch. Branch infections are apparent when bark is pulled away. The wood is streaked with reddish brown. Farther along the branch where infection is newer, the wood is speckled with reddish brown. If the diseased wood is not removed the bacteria can spread to the trunk, then into roots, thereby killing the tree or shrub.
Dormant fire blight bacteria overwinter in black slightly sunken crater-like cankers in diseased branches and twigs. In the spring when the weather is warm and wet, a sticky bacteria-laden substance often oozes from cankers and infected plant parts. This indicates that the tree or shrub was previously infected.
To control fire blight diseased tissue must be cut out and disposed ofónot recycled or put into a compost pile. Cuts should be made 8 ñ 12 inches below the infected area. If the diseased material is not cut out, fungus can enter blighted shoots and cause rot problems in addition. Infected tissue should not be cut during then early growing season. Wait until late summer or fall when growth has slowed to make cuts. This will help prevent reinfection next year. Disinfect pruning tools between cuts in one part bleach and nine parts water.
Avoid heavy applications of nitrogen and severe winter pruning. Both can stimulate excessive vegetative growth which is more susceptible to the infection come spring. Remove suckers year round. Prune in dry weather.
There is no cure for fire blight. Sprays can be used to help control the disease before and during bloom. Keep in mind that timing is critical and multiple applications are needed. A copper fungicide and streptomycin are two options. Before using pesticides read the entire label and then follow all instructions.
For more information about fire blight google Clemson hgic 2208 Fire Blight of Fruit Trees.
Reach Debbie Menchek, a Clemson Master Gardener, at firstname.lastname@example.org.