Fire blight blackens leaves and branch tips of infected trees as if they were scorched by fire, and can kill large fruit trees. It is an important disease of apple, pear, and relatives, including ornamental species. When conditions are right, it can cause tremendous damage in a short time.
Host
Over 200 species in the family Rosaceae are hosts . Most seriously affected are Malus and Pyrus spp. (apple and pear) in orchards and ornamental plantings. We’ll focus on a very common urban amenity tree, Callery pear, Pyrus calleryana (also called Bradford pear, but Bradford is one of many similar varieties).
Pyrus calleryana was sent to the USA from China in 1918 by a USDA plant explorer, the inimitable Frank N. Meyer. Unfortunately, Meyer himself never returned home, having drowned in the Yangtze River that year. I highly recommend the fascinating and authoritative telling of the complete story by Theresa Culley of Harvard’s Arnold Arboretum .
Pathogen
The pathogen is the bacterium Erwinia amylovora. The specific epithet, “amylovora”, means “apple devourer” in Latin (mixed with Greek). It could just as well have been named “pyrovora”, as it can devour pear trees also.
Environment
Temperature and moisture in Spring are critically important for this disease. Warm, wet weather during flowering favors the disease. Fire blight can cause tremendous damage in some years when conditions are favorable, and very little in other years.
Pathogen growth is slow between 12 and 18 C. It increases rapidly with temperature up to the optimum at about 26-28 C . At the optimum, it can multiply from 10,000 bacteria to 1 million in 24 hr.
Infected tissue must be moist to produce the bacterial ooze that serves as inoculum. This can occur with a trace of rain or even high humidity. Rain splash can increase dispersal of bacteria to new infection sites.
Disease Cycle
Infection and blight occur mostly in Spring during bloom. The bacteria are exuded in drops of ooze that attract insects. Insects, birds, and rain splash then spread the bacteria to flowers, young leaves, and succulent shoot tips. These parts are infected and become further sources of inoculum. Ooze from an infected flower can contain as much as a billion bacteria per drop . The bacteria can enter through flower parts, small wounds, stomata, hydathodes, you name it. They overwinter in cankers and bud scars. The pathogen can move internally down shoots, and this can continue through the growing season.
Signs and Symptoms
Flowers at first appear water-soaked, then die and shrivel when dry. Leaves become black and necrotic, first along the midrib or in blotches, then curl and shrivel. Twigs wilt from tip downward, then turn black. Shoot tips are often curled, a symptom called “shepherd’s crook”. Twigs and branches may develop dark, sunken cankers; as they enlarge and girdle the branches, the branch eventually dies (branch dieback). Basal cankers, originating by infection of basal shoots or wounds, may girdle and kill entire trees .
A sign of the disease is the bacterial ooze emanating from lenticels and small wounds on infected parts.
Distribution and Damage
Management
In fruit orchards, there are management tactics that can reduce damage. Most are not practical for amenity plantings.
- Sanitation pruning during winter dormancy, removing branches with cankers
- Sanitize tools between cuts when pruning for any reason.
- Avoid excessive nitrogen fertilization that can cause increased succulence in the spring, which means more susceptible tissue.
- Some varieties are more resistant than others.
- Copper-containing sprays, bactericides, and antibiotics.