Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State, Bugwood.org
Fire Blight Symptoms
Caused by the bacterium Erwinia amylovora, the disease can attack some 75 species of plants of 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, Idared Jonathan, Rome Beauty, and Yellow Transparent.
Fire blight kills fruit-bearing spurs, branches, and entire trees. It can be distinguished from other disorders by droplets of red-brown, sticky, liquid, bacterial ooze 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 the spring as blossom, fruit spur, and new shoot blight. Infected blossoms wilt rapidly and turn light to dark brown. Bacteria might move through the pedicel to the fruit spur and out into the leaves, where they follow the midrib and main veins, which soon darken. 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. Some remain even after normal leaf fall.
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, which turns brown to black. This infection might occur at 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 can cause new infections.
Fire blight bacteria can move from blighted spurs and shoots into larger limbs and tree trunks. Infected branches can be girdled, resulting in loss of the entire branch. Suckers at the base of trees often are invaded and can 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 M.26 and M.9.
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 also can 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.
Fire Blight Disease Cycle
Fire blight overwinters in cankers on branches and trunks. When the temperature reaches about 65°F, bacteria begin to multiply and appear on the outsides of the cankers as clear amber ooze.
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. The spread of the bacteria from flower to flower by bees is rapid. Insects also transmit bacteria to growing shoots. If the temperature is 65°F or higher and relative humidity is at 60 percent or more, or if there is rain, new infections can occur. At 75°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 might have droplets of ooze on them.
The bacteria usually enter the flowers through natural openings such as stomates. Wounds also are important entry points to leaves, shoots, and fruit. Aphids, leafhoppers, lygus bugs, and other insects with piercing mouth parts can 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.
Temperatures just before and during bloom will determine if fire blight becomes serious in early spring. Where this disease was present the previous year, we suggest the following management program:
- Prune out all cankers from 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°F.
- Where the disease was severe the previous year, apply a dilute Bordeaux spray plus miscible superior oil at silver tip. This spray is not warranted if only occasional infections occurred.
- When daily temperatures average 65°F or higher during pink through petal fall, make at least two applications of a streptomycin formulation. Apply the first streptomycin spray anytime after first blossoms open, when daily temperatures are above 65°F or are expected to be so within 24 hours. Repeat sprays at 5- to 7-day intervals through late bloom. A minimum of two applications is necessary to provide control. (Streptomycin formulations are much more effective when applied during slow drying conditions, such as at night.)
- When average daily temperatures fail to reach 65°F during pink through petal fall, delay the streptomycin application until the disease first appears. To detect the first appearance of fire blight, inspect trees at 5- to 7-day intervals beginning at petal fall. When the disease appears, prune out all new infections. Again, remove shoots 8 to 12 inches below the last signs of browning. Remember to disinfect pruning tools between cuts with a bleach solution or alcohol since contaminated tools can spread the disease.
We do not recommend cutting out blighted shoots after terminal growth has stopped. When growth stops, the spread of fire blight should also stop. The most important thing to do to control fire blight during the summer is to control sucking insects like aphids and leafhoppers. Applying streptomycin sprays within 24 hours after hail to prevent new infections is also a good practice.
Proper fertilization practices can help reduce the potential for fire blight. Trees that are excessively vigorous due to high nitrogen applications can be more prone to fire blight. Variety selection also can help reduce the incidence of this disease. Many of the scab-resistant varieties are resistant to fire blight (see Table 4.1 ). Varieties such as Jonathan and Rome Beauty are more susceptible to fire blight. Resistant pear varieties include Magness and Moonglow.