Christopher J. Starbuck
Division of Plant Sciences
Michele R. Warmund
Division of Plant Sciences
Fire blight is a bacterial disease affecting apple, crabapple, pear, hawthorne, pyracantha (firethorn) and related species. The bacteria commonly live over winter in cankers (old diseased areas) in the tree, from which they ooze in early spring. The bacteria usually are spread from the cankers by insects and by wind-blown rain. Sometimes careless pruning practices also may spread the bacteria.
The arrow points to fire blight canker on a large limb. Note the sticky exudate or ooze below canker.
Symptoms and plant parts attacked
The blossoms on susceptible trees usually are the first plant parts to become infected in the spring. This is partly because of their attractiveness to insects, which carry the bacteria from the cankers. But it also is due to the nectaries of the flowers, which have an abundance of nutrients to support rapid growth of the pathogen. Infected blossoms turn brown and die, usually with the flower parts remaining attached.
Shoot or terminal blight
The succulent new shoot growth also is susceptible to bacterial infection in the spring and early summer. When infected, the tender tips wilt, die and assume a characteristic "shepherd's crook" appearance. These infections often kill a foot or more of the terminal growth. Apple leaves and shoots turn rusty brown in color; in pears, these parts turn black. In either case the dead leaves remain attached to the dried shoots.
Tender shoots infected with fire blight have the typical "shepherd's crook" appearance. Leaves remain on the shoot.
Spur infection of fire blight kills leaves and shoots.
Fire blight cankers of various sizes can develop on twigs, limbs, and even the trunks of trees. The bacteria can colonize an area where the bark has been wounded (pruning cuts, hail damage, mechanical damage, etc.), or move systemically through the tissues of the plant from infected blossoms or shoots. In either case, the infected area will become discolored and superficially sunken. Cankered areas do not penetrate deeply into the wood. A sticky ooze containing billions of bacterial cells often develops in the region of these cankers in the spring and early summer, especially during wet weather. Because of the high sugar content of this ooze, infected limbs and cankered areas often develop a sooty black overgrowth of fungi.
One of the best ways to avoid fire blight is to not plant varieties that are susceptible. The most susceptible apple varieties, essentially in the order given, are Gala, Jonathan, Yellow Transparent, Lodi, York and Wealthy. Apple varieties such as Red Delicious, Golden Delicious, Dayton, Liberty and Jonafree usually are only slightly affected and generally do not require special treatment for fire blight.
Most pear varieties are susceptible but the following varieties show some resistance: Kieffer, Garber, Seckel, Tyson, Lincoln, Dutchess, Starking Delicious, Moonglow, Magness and Maxine. Some ornamental pear cultivars that are highly susceptible to fire blight include Aristocrat, Autumn Blaze,Capital, Fauriei, and Redspire. Bradford pear trees are moderately resistant to the disease. Several species of cotoneaster such as Cotoneaster adpressus, apiculatua, dielsiana, faveolatus, franchetti, integerrimus, nitens, and zabelii are resistant to fire blight. Pyracantha coccinea Sensation, P. koidzumii Santa Cruz Prostrata, San Jose hybrid and Shawnee hybrid firethorn also show resistance to this bacterial disease. Most cultivars of crabapple sold in nurseries are resistant to fire blight. An updated list of disease-resistant crabapple cultivars is available on the Disease resistant and tolerant plant varieties, http://soilplantlab.missouri.edu/plant/diseases/resistant.aspx page of the Plant Diagnostic Clinic's website. Unfortunately, little choice exists among the susceptible hawthorne species. Most cultivars of crabapple sold in nurseries are resistant to fire blight.
Eliminate fire blight infections by pruning out diseased branches. Dormant pruning to remove overwintering infections helps reduce inoculum for the next season. Remove pruned material from the orchard. Beginning about one week after petal fall, monitor the orchard to locate blighted limbs for removal. On young trees, and those on dwarfing rootstocks, early strikes in the tops of the trees often provide inoculum for later infections of shoots and sprouts on lower limbs near the trunk, which may result in tree loss. Give these early strikes in the tops of trees a high priority for removal. Always cut an infected branch at least 8 to 12 inches below the visible injury or canker. A greater distance below infections may be required on major branches, scaffolds, or trunks in May or June, when blight bacteria are moving rapidly. Ideally, an infected shoot or branch should be removed at its point of attachment, without damaging the branch collar. The appearance of new infections below a pruning cut indicates that the cuts were not made far enough below the infection and the bacteria had already spread past the cutting point.
To avoid spreading bacteria during pruning, dip or spray the pruning tool before each cut with a 10 percent solution of bleach (one part bleach to nine parts water). Dry and oil tools after use to prevent rust.
Control with pesticide
Streptomycin is a bactericide used to control fire blight. Various products are labeled for application to apples, pears and ornamentals. Sprays are generally applied when wet weather occurs during bloom and temperatures are 60 to 75 degrees F. A minimum of two applications are necessary to provide control. Consult the label for specific application instructions.
Fire blight has been observed to be most severe on vigorously growing trees. For this reason, the amount of fertilizer (mostly nitrogen) applied needs careful attention. Allow the tree to show when it needs fertilizer rather than making routine applications each year. Bearing apple trees, for example, should not produce more than 10 to 12 inches of new terminal growth in any given season; pear trees, about 6 to 8 inches. With ornamental species like pyracantha that respond to even low levels of nitrogen, withhold all nitrogen fertilizer until the plant definitely shows lack of vigor. Keep in mind that fertilizers applied to lawns also will be available to the tree roots.
Fire blight runs in cycles
That is, the disease may appear for several years in various degrees of severity, then show little or no damage for a few years. Fire blight is most severe in springs when soil moisture is high, when bud and shoot development is rapid, when temperatures of 60 to 75 degrees F persist, and when frequent rains occur (especially wind-blown). These conditions promote rapid development of succulent shoots and blossoms that favor both the rapid multiplication and subsequent dissemination of the bacteria. During seasons like this, homeowners with valuable susceptible ornamental and fruit plantings need to be especially diligent in their fire blight control programs.