The fire blight pathogen, Erwinia amylovora, only attacks plants in the rose family. More than 130 species in 40 genera world wide are susceptible. In the Midwest, some of the most susceptible plants are: apple, crabapple, pear, mountain ash, and cotoneaster.
Symptoms
Major symptoms are blossom blight, fruit blight, shoot blight, and canker. On pear, the killed twigs/branches are black-looking while the killed tissue on other rose family plants will be brown to chocolate brown in color. Often, but not always, the tip of the twig/branch curls back on itself thus forming a "shepherds crook" appearance. First symptoms usually start with the blossom appearing water soaked then drooping and browning. The disease continues growing down through the twig, killing as it moves systemically. Under the favorable weather conditions, the entire above-ground portions of the plant can be killed by the bacterium. Cankers form at the base of the dead areas on the woody plant tissue. Bacterial ooze may leak from cracks along canker margins or through lenticils (pores in the bark).
Life Cycle
The bacterium overwinters along margins of the canker. In the spring when conditions are warm (65-86 F) and wet, the bacteria multiply and ooze out the cankers. Insects, especially flies and bees, are attracted to the ooze and spread the bacteria onto flowers and other tissues. The ooze may also be spread by splashing rain and wind. Besides being carried to flower blossoms by the pollinators, the bacterium can enter through wounds and natural openings into leaves and bark.
Management
Over-stimulated rosaceous plants (due to heavy pruning and/or high nitrogen rates) are at high risk for fire blight infections. In an infected planting, and while trees are dormant, remove all cankered wood, pruning 6 to 12inches below the last visible sign of the canker (avoid leaving a stump, remove the affected branch all the way to the trunk or nearest side branch that is at least 6 inches below the canker). Canker removal will eliminate a large part of the primary inoculum for the following year. Pruners do not need to be disinfected between cuts unless pruning is done during the growing season. If pruning is done during the growing season, 6-12 inches of healthy tissue should be removed along with the cankered areas. All pruning during the growing season must be carried out in hot & dry conditions.
Because young, succulent shoots are very susceptible to infection, avoid using high rates of nitrogen fertilizer, which promotes rapid shoot elongation. It is best to use a balanced fertilizer and to consider split applications of nitrogen (half before growth starts and half after petal fall). Copper compounds and antibiotics are available to help manage this disease in commercial settings such as nurseries and orchards. Copper compounds may be sprayed during green tip to prevent bark and bud colonization, while antibiotics are used during bloom to protect flowers. When establishing new plantings of rosaceous plants, look for resistance to fire blight as the most effective disease-control practice.