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Fire Blight

Erwinia amylovora

Fire blight, caused by Erwinia amylovora, occurs sporadically in most apple orchards in the southeastern United States. While it is a minor problem in many orchards each year, serious epidemics generally occur every 5 to 10 years, causing extensive damage to scaffold limbs and tree death. It is likely that the disease is going to become more common in the Southeast as growers replace older orchards of Delicious with more susceptible cultivars that are propagated on M.26 and other rootstocks that impart greater susceptibility.

Symptoms

Blossom blight is usually the first symptom observed in the spring. Flowers first appear water-soaked; then they wilt, shrivel, and turn brown or black. The bacterium moves down the peduncle into the fruit spur or branch, often girdling the branch and causing it to die (Fig 1). Young fruit can become infected and, during warm, humid weather, droplets of bacteria can ooze from the peduncle and fruit. These fruit turn black, shrivel and often remain attached to the tree.

Symptoms on succulent shoots and watersprouts are similar to those on flowers. First symptoms often appear in leaves as dark streaks in the midvein and larger leaf veins (Fig 2). Tips of shoots frequently droop to form a shepherd's crook (Fig 3) and drops of ooze can often be seen along the infected shoots (Fig 4). Shoots also may be killed by infections that occur some distance back from the terminal. These infections girdle the twig, killing all tissues beyond the point of infection. Foliage often remains at­tached to dead twigs and shoots (Fig 5). Extensive infections can result in loss of major scaffold limbs as well as death of the tree (Fig 6). When the weather becomes unfavorable for disease development, cankers form at the base of the diseased tissue (Fig 10). Cankers vary in size, are slightly sunken, and are sometimes surrounded by cracks in the bark.. Trees propagated on highly susceptible rootstocks such as M.26 may be killed outright by the disease (Fig 7). Necrotic streaks are often visible beneath the bark and run from affected shoots to the root stock (Fig 8). This phase of the disease, known as rootstock blight, occurs when the bacterium moves downward in the vascular tissue to the susceptible rootstock, where it kills the root system. Trees affected with rootstock blight often begin showing symptoms and collapse in the late summer.

Fig 1 - Girdled branch

Fig 1. Girdled branch

Fig 2 - Dark streaks on midvein

Fig 2. Dark streaks on midvein

Fig 3 - Crooked shoot

Fig 3. Crooked shoot

Fig 4 - Oozing shoot

Fig 4. Oozing shoot

Fig 5 - Dead foliage

Fig 5. Dead foliage

Fig 6 - Dead limb

Fig 6. Dead limb

Fig 7 - Dead trees

Fig 7. Dead trees

Fig 8 - Necrotic streaks under bark

Fig 8. Necrotic streaks under bark

Disease Cycle

Fire blight bacteria overwinter in cankers in the tree and, following warm weather in the spring, bacteria ooze from the cankers providing the primary inoculum. Most cankers that produce bacteria are found on limbs less than 1.5 inches in diameter. Splashing rain and insects spread bacteria to open blossoms. These bacteria can infect the blossoms on days with average temperatures > 60F and rainfall (> 0.01 inch) or heavy dew. Bacteria can be spread from diseased flowers to healthy ones by bees. Throughout the summer, secondary infections can occur on leaves and shoots as long as there is succulent new growth and the weather is favorable. There seems to be little relationship between disease one year and the next. Injuries caused by hail can be entry points for the fire blight bacterium. If hail injury is followed by weather suitable for fire blight, the risk of fire blight infections is great.

Control

The first step in fire blight con­trol is orchard management. Avoid planting highly susceptible cultivars (www.caf.wvu.edu/kearneysville/tables/fbsus.html) that are propagated on susceptible rootstocks (www.caf.wvu.edu/kearneysville/tables/fbrootsus.html). High applications of nitrogen fertilizers and heavy winter pruning can lead to excessive succulent growth in spring that is very susceptible to fire blight. Maintaining a good balance between N-P-K and sufficient calcium will prevent excessive vegetative growth. Planting less vigorous rootstocks will also reduce excessive vegetative growth. Controlling vegetative growth is especially important on susceptible cultivars such as Jonathan, Granny Smith, and Gala.

All fire blight affected tissues, including cankers, need to be cut out, removed from the orchard, and burned. Cuts should be made 8 to 12 inches below the diseased tissue. Dip pruning tools in 10% household bleach or Lysol between cuts to prevent spreading the bacterium. Bleach will cause tools to rust unless they are washed and oiled at the end of each day. Fire blighted shoots will be invaded by fungi that cause the rot diseases bitter rot, black rot, and bot rot, and if not removed can greatly increase the possibility that rots will become a problem in the orchard.

A copper spray in the dormant/green tip stage, and streptomycin sprays during bloom, are needed to manage fire blight on susceptible cultivars. Sprays containing copper should be avoided after the 1/2 inch green tip stage to reduce the likelihood of fruit russet. Copper sprays will help reduce the inoculum of the fire blight bacterium, but will not provide adequate control alone. Begin streptomycin sprays at first bloom (5-10% open blooms). Streptomycin sprays protect only the open blossoms, consequently additional applications are needed every 3 to 5 days during the bloom period. Often the most severe fire blight infections in the Southeast are a result of infections that occur on the “rat-tail” bloom. The Maryblyt model (http://www.caf.wvu.edu/kearneysville/maryblytfaq.html) has worked well in the Southeast for timing streptomycin sprays. It has proven especially useful for predicting infections during the “rat-tail” bloom period. It is important to minimize the number of streptomycin sprays used in order to reduce the likelihood of resistance developing. If hail or damaging wind storms occur, streptomycin sprays should be made as soon as possible to minimize the number of new infections.

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Format updated March 29, 2011