In my last article, I wrote about powdery mildew, a common fungal disease affecting the younger tissue on many crape myrtles this summer. Another prevalent disease this year is fire blight on pears, apples and other related species. This disease is easy to spot in the landscape.
The disease affects many plants in the Rosaceae family. Other susceptible plants include flowering quince, cotoneaster, hawthorn, loquat, crabapple, photinia, plum, cherry, pyracantha, rose and spirea.
Fire blight, caused by the bacterium Erwinia amylovora, was named for the common symptoms. The tips of the branches have a scorched appearance and are curved into a distinctive “shepherd’s crook.” The first symptoms, infected flowers that turn black and die, occur in early spring when the weather is cool and humid. The disease then works its way down the branch resulting in the death of young twigs, which then blacken and curl. Leaves on these branches remain attached, giving the plant a scorched appearance.
The disease often enters the plant through natural openings, especially flowers and wounds, in the early spring. Once established in the tree, fire blight quickly moves through the current season’s growth into older growth. It can be spread via rain, wind and pruning tools.
The bacterium can survive the winter in sunken cankers on infected branches. In spring, the bacteria ooze out of the cankers and attract bees and other insects, which then move the disease to nearby plants. The bacteria spread rapidly through the plant tissue in warm temperatures (65 degrees or higher) and humid weather.
Because there is no cure for fire blight, disease prevention is very important. One of the most effective controls is to plant varieties that are resistant to, or tolerant of, fire blight. Although these methods are not 100-percent effective, they help reduce disease severity.
For a list of select cultivars that have improved resistance to fire blight, take a look at the University of Georgia publication, “Fireblight: Symptoms, Causes, and Treatment” found at: extension.uga.edu/publications/detail.cfm?number=C871.
Disease management is difficult on established trees. Reduce spread by removing and destroying all infected plant parts. Prune each limb 12 to18 inches below the infected tissue at the tips.
Disinfect pruning tools between cuts by using either a 10-percent bleach solution (1 part household bleach to 9 parts water) or 70-percent isopropyl alcohol (rubbing alcohol). Also, avoid excess nitrogen fertilization during the summer. Tender, succulent growth fuels the spread of the disease.
Chemical control is not always effective and needs to be applied as a preventative during spring bloom. In years when warm, humid, wet weather coincides with flowering and leaf emergence, spray plants with a fungicide containing basic copper sulfate (Kocide) or an antibiotic (Agrimycin) to reduce infection. Application of Agrimycin needs to begin at the start of blooming and continue every three to four days during the bloom period. Application of Kocide should begin at bloom and continue every seven days during bloom.
Fire blight is a tough problem to control once plants are infected. As with many disease problems, prevention is the key. Contact your local County Extension office for help with this and other landscape and gardening issues.
Bibb County Master Gardeners will be at the Mulberry Market at Tattnall Square Park from 3:30-6:60 p.m. Wednesday to help with your gardening questions.
Contact county Extension agent Karol Kelly at firstname.lastname@example.org.