Fire blight is destructive to apple and quince trees and is the most serious pear disease in the eastern United States. In recent years significant losses to the apple industry have resulted due to this disease. Caused by the bacterium Erwinia amylovora, the disease can attack some 75 species of plants in the rose family. Fire blight also occurs frequently on pyracantha, spirea, hawthorn, and mountain ash. In fruit trees, the disease can kill blossoms, fruit, shoots, limbs, and tree trunks. Certain varieties of apple are more susceptible than others. Susceptible varieties include Gala, Ginger Gold, Braeburn, Jonathan, Rome, Yellow Transparent, and Idared.

Symptoms

Fire blight kills fruit-bearing spurs, branches and entire trees. It is important to distinguish fire blight from other disorders by recognizing the presence of bacterial ooze characterized by droplets of red-brown, sticky liquid that seeps from the surfaces of infected tissue.

The disease gains entry to the tree through two main points, blossoms and new shoots, and often appears first in spring as blossom, fruit spur, and new shoot blight. Infected blossoms wilt rapidly and turn light to dark brown. Bacteria may move through the pedicel to the fruit spur and out into the leaves. The leaf midrib and main veins soon darken after infection. The leaves wilt, turning brown on apple and quince and dark brown to black on pear. The blighted leaves remain attached for much, if not all, of the growing season.

Fire blight's two main symptoms are shoot blight and cankers on limbs. Shoot blight begins with the infection of the young, succulent growing tip. These shoots turn brown to black. It may occur any time during the season while the shoots are still growing and when environmental conditions are most favorable for the disease. The leaves wilt rapidly, turn dark, and remain attached as in the case of spur blight.

A characteristic symptom of shoot blight is the bending of terminal growth into the shape of a shepherd's crook. Pearly or amber-colored droplets of bacterial ooze are often present on diseased blossoms, fruit, and leaf stems, on succulent shoot stems, and on the exterior of infected fruits. Inside these droplets are millions of bacteria, which may cause new infections.

Fire blight bacteria can move from blighted spurs and shoots into larger limbs and tree trunks. Infected branches may be girdled, resulting in loss of the entire branch. Suckers at the base of trees are often invaded and may blight back to the trunk or rootstock, causing the loss of the entire tree in one season. This is true of susceptible pears, especially Bartlett, Bosc, and Clapp's Favorite, and certain clonal apple rootstocks, especially M26 and M9.

Cankers, slightly sunken areas of various sizes surrounded by irregular cracks, occur on small to large limbs, trunks, and even roots. They often begin at the bases of blighted spurs, shoots, and suckers. Active blight cankers are characterized by an amber or brown exudate on their surfaces or on the bark below. The bacteria may also invade fruit, which becomes water soaked. Droplets of bacterial ooze appear on the surface. Later the fruit becomes leathery, turns brown on apples and black on pears and quinces, shrivels, and usually remains attached to the fruit spur.

Disease Cycle

Bacteria overwinter in the margins of cankers on branches and trunks. Once temperatures become warm in the spring, bacteria begin to multiply. The bacteria are spread to blossoms primarily by rain with some transmission by flies and ants. Blossom-to-blossom transmission is carried out mainly by bees and other insects that visit the flowers. If the temperature is 62 degrees F or above and there is rain, new blossom infections can occur. At 75 degrees F, blossom blight and shoot blight will be evident in 4 to 5 days. Bacterial ooze appears on the new infections soon after the symptoms, providing additional sources of bacteria for new infections. In early to midsummer, during prolonged periods of muggy weather, blighted shoots and spurs, infected fruit, and new branch cankers all may have droplets of ooze on them.

The bacteria usually enter the leaves through natural openings such as stomates. Wounds are also important entry points to leaves, shoots, and fruit. Insects may transfer fire blight bacteria directly into susceptible tissues. Wounds from hail and wind-driven rain often lead to a severe outbreak of fire blight. Any fresh wound can serve as an entry point.

Disease Management

Temperatures just before and during bloom will determine if fire blight becomes serious in early spring. Daily temperatures must average 65 degrees F or above during pink through petal fall for bacterial populations to grow enough to cause severe disease. The disease also occurs later in the season when bacteria enter late-opening blossoms or growing tips of new shoots. Where this disease was present the previous year, the following management program is suggested:

• Prune out all cankers in limbs 1 inch or more in diameter. Cut apple limbs at least 8 inches below external evidence of the canker and cut pear limbs at least 12 inches below. Pruning tools do not need to be disinfected when temperatures are below 45 degrees F.
  
• Where the disease was severe the previous year, apply a dilute Bordeaux spray plus miscible superior oil at silver tip.
  
• Blossom infections can occur when the blossoms are wet and the temperature is 62 degrees or more. Apply streptomycin when these conditions are expected within 24 hours. Repeat sprays as needed through late bloom. (Streptomycin formulations are much more effective when applied during slow drying conditions, such as at night.)
  
• To detect the first appearance of fire blight, inspect trees at 5- to 7-day intervals beginning at petal fall. When the disease is seen, prune or break out all new infections. Again, remove shoots well below the last signs of browning (12-18 inches). Remember to disinfect pruning tools between cuts with a 10 percent Clorox solution or 70 percent alcohol, as contaminated tools can spread the disease.
  
• Applying streptomycin sprays within 24 hours after hail to prevent new infections is a good practice.

Proper fertilization practices can help in reducing the potential for fire blight. Excessively vigorous trees due to high nitrogen applications can be more prone to fire blight. Cultivar selection can also help reduce the incidence. Many of the scab-resistant cultivars are resistance to fire blight. Cultivars such as Gala, Ginger Gold, Braeburn, Jonathan, and Rome Beauty are susceptible to fire blight. Resistant pear cultivars include Magness, Moonglow, and Potomac.