Fire blight is a common bacterial disease affecting apples, pears and over 75 other host plants, mostly in a section of the Rose family. Apples, pears, cotoneaster and mountain ash are affected most severely. The blight damages susceptible hosts by killing flowers and twigs and by girdling large branches and trunks. Late in the summer trees may look as if they had been scorched by fire, thus the name “fire blight.”
Recognizing Fire Blight
The first symptoms of fire blight are usually on the flowers, which appear water-soaked, then rapidly shrivel and turn brown. The bacteria quickly spread to leaves on the same or nearby twigs. Leaves killed by fire blight turn brown (apple) or black (pear), shrivel and hang downward forming a gooseneck. They usually remain attached to the twig all summer and sometimes through the winter. Tender terminal twigs and water sprouts (suckers) are also easily infected, becoming shriveled, darkened and hardened. The tip of the twig bends over, forming a characteristic “shepherd’s crook.”
Infections of branches and twigs result in canker formation. New cankers are light brown; as they age the bark cracks and the canker surface becomes darker and sunken. If the cankers are not removed, they may eventually encircle the affected branches, girdling them and causing all parts above them to die.
Young fruit may also become infected. Diseased fruit is firm and leathery, eventually shriveling and turning brown to black, and usually remaining firmly attached. Affected parts of the plant (blossoms, spurs, fruit, twigs, branches and trunk) often produce droplets of clear, milky or amber-colored exudates. These droplets contain millions of bacteria which can initiate new infections. Control measures are aimed at reducing the amount and transmissions of inoculum as well as susceptible tissue.
Fire blight bacteria overwinter at the margins of cankers formed during the previous season, mostly on large branches, rarely on twigs less than 1/2-inch diameter. Cankers with smooth, indistinct margins are more likely to provide overwintering sources than are those with rough, cracked, well-defined margins.
In the spring the bacteria in the “holdover” cankers become active, multiply and spread into adjoining healthy bark. During humid or wet weather, water is absorbed by these bacterial masses, which swell and exude through natural openings of the tissue. The bacterial ooze appears about the time of the “pink stage” (just as pink shows in the buds) and may continue until mid-summer. Rain, mist and insects such as bees and aphids will spread the bacteria in these droplets to blossoms, leaves and succulent shoots. When the ooze dries, it forms aerial strands which can be spread by wind and infect new tissues. The bacteria enter the host through wounds or natural openings, such as stomates. Moist plants are much more susceptible to invasion, as are succulent tissues such as newly formed leaves and twigs.
After penetration, infection may spread into all parts of a susceptible host. Progress of the disease may cease at any point and usually only very young or highly susceptible tissue undergoes complete blighting within a single growing season. Epidemics can occur, but remain largely unpredictable, generally occurring with high relative humidity and temperatures ranging from 65-85 degrees F.
Resistance. There are no known varieties completely resistant to fire blight, but some are less susceptible than others. In areas where fire blight has become a problem, resistant varieties should be used. Susceptible and resistant varieties should not be interplanted, as the susceptible plants can harbor the bacteria and cause resistant ones to show more disease. Planting sites with low humidity, good air movement and adequate sun for rapid drying of leaves will help lessen infection. See list of varieties and their degree of resistance or susceptibility.
Cultural Management & Sanitation
Because fire blight is much more severe on succulent tissue, cultural practices that favor moderate rather than lush growth are recommended. Avoid using heavy amounts of nitrogen fertilizers. Avoid excess irrigation and aim sprinkler heads low to reduce the relative humidity. Watersprouts should be removed to prevent the entrance of bacteria into their succulent tissues.
In late fall, winter or early spring, remove and burn all cankered limbs. Where possible, make the cuts at least 12-18 inches back along healthy wood. This insures removal of all tissues that may contain the bacteria. Where larger limbs are cut and a red discoloration is noted in the region just beneath the bark, a lower cut should be made.
Remove and promptly burn or discard all infected material taking care to avoid touching remaining trees with infected tissue.
During the growing season, cut off and destroy any infected twigs, but avoid heavy pruning as it may overstimulate tender and susceptible new growth. To aid accurate removal of infected portions during the dormant period, mark them in late summer.
During all pruning, all cutting blades should be disinfected after each cut to prevent the spread of the blight bacteria. Either denatured alcohol or a 10% household bleach solution (one part bleach, nine parts water) may be used. Dip the tools for at least two seconds and make fresh disinfectant every three to four hours. Because of the corrosive nature of bleach, tools should be rinsed and dried thoroughly after use.
Once fire blight becomes evident in an area, susceptible trees should be protected from infection by a spray program. Recommended is streptomycin. Begin spraying at start of the blooming period and continue on a three-to four-day schedule as long as flowers develop, then five to seven days until fruit set is visible.
Application of chemicals only protects the trees from new infections, so sanitation measures should be practiced as well, along with use of resistant varieties and proper cultural practices.
- Resistant: Beacon, Duchess, Haralred, Haralson, Heyer 12, Sweet 16
- Slightly Susceptible: Goodland , Mantet, Hazen, Honeygold, McIntosh, Red Prairie Spy, Wealthy
- Susceptible: Wolf River, Yellow Transparent
- Resistant: Centennial, Chestnut, Dolgo
- Susceptible: Whitney
ORNAMENTAL FLOWERING CRABAPPLES
- Resistant: Prairiefire , Radiant, Thunderchild
- Susceptible: Louisa, Royalty, Selkirk, Spring Snow
No PEAR variety is highly resistant to fire blight.