Recent reports bring to mind that warm and wet spring weather is perfect for development of the bacterial disease known as fireblight in susceptible hosts. Fireblight is caused by Erwinia amylovora, a bacterium that only affects plants in the rose family (Rosaceae). Some common hosts include apple, crabapple, cotoneaster, hawthorn, mountain ash, pyracantha, and pear.
Development of fireblight usually begins in the spring as bacterial cells ooze in a yellow-amber liquid from existing cankers on infected plants. These cells are spread to susceptible flowers, leaves, fruit, and stems by insects, wind, splashing rain, or pruning equipment. Insects are especially attracted to this sweet, sticky, bacterial ooze; bees and other pollinating insects commonly pick up bacteria while visiting the cankers and subsequently deposit the cells on developing flowers. Erwinia bacteria penetrate tissue through wounds and natural openings such as stomates and nectaries. Twigs and branches infected with the pathogen die rapidly and appear scorched, hence the name “fireblight.” Tender shoots tend to droop and bend as they die, developing a symptom commonly associated with the disease called a shepherd’s crook. As the disease advances, cankers form at the base of infected branches, and highly susceptible plants may die.
Conditions optimal for fireblight development are extended periods of warm (greater than 65 to 75 F), wet weather during spring. To manage the disease, improve plant vigor, but avoid heavy spring fertilization that would promote succulent growth. In late summer (after bacterial oozing no longer occurs), prune all diseased wood at least 6 to 8 inches below the infection, surface sterilizing tools between cuts. Remove water sprouts, and dispose of any infected plant material which may harbor the pathogen.
For chemical control, several bactericides are labeled; many are applied during bloom and might be an option for next spring in plant material with a history of this disease. Check the label for host, timing, and rates: copper (ammonium complex, Badge, basic sulfate, cuprous oxide, hydroxide, pentahydrate, salts), fosetyl-Al, Junction, phosphorous acids and salts, oxytetracycline, or streptomycin sulfate.
Finally, consider replacing highly susceptible plant material with material that is more resistant to this disease. Highly susceptible callery pears include Aristocrat, Autumn Blaze, Capital, Fauriei, and Redspire; moderately susceptible cultivars include Cleveland Select, Earlyred, and Whitehouse. Although Bradford is reported to be moderately resistant in certain locations, it will develop fireblight under conditions optimal for disease development.
Note: Refer to label for host and application timing and rates. In some instances trade names are used to avoid using long and complicated chemical names. No endorsement of named products is intended, nor is criticism implied of similar products that are not mentioned.
