Fire Blight Cooperative Extension Service College of Agriculture and Guide A-230 Home Economics

Natalie P. Goldberg, Extension Plant Pathologist

This publication is scheduled to be updated and reissued 2/04.

The characteristic symptom of fire blight is DIAGNOSIS AT A GLANCE that affected plant parts (most notably the branch terminals) appear to have been scorched by fire Causal agent: Erwinia amylovora, a bacterium. (fig 1). Fire blight can infect blossoms, fruit, stems, leaves, and woody branches. During Hosts: Plants in the rosaceae family, periods of high humidity, infected tissue may notably apple, pear, pyracantha, produce a characteristic ooze. cotoneaster, and photinia. In spring when the bacterium becomes active, wind-blown rain or insects spread it from oozing Key symptom: Branch tips turn black as if cankers to blossoms. The bacterium enters the scorched by fire. plant through blossoms, wounds, and natural openings (such as stomata, hydathodes, lenticels, Sign: Watery ooze produced from and nectaries). The infection moves from infected plant tissue. blossoms to young fruit and then to adjacent Conditions for leaves. Infected tissue turns brown to black, disease: • Rainy, humid weather in spring. shrivels and droops, but remains attached to the • Day temperatures between 75° tree. Secondary infections on young shoots are and 85°F. common following injury caused by hail, wind- • Night temperatures above 55°F. blown dust, or insects. One of the earliest symptoms of the disease Management • Prune out infected tissue. occurs on flowers and is “blossom blight.” The • Avoid heavy nitrogen fertiliza- blossoms appear discolored (water soaked, gray- tion. green color progressing to black). The necrotic • In commercial orchards, spray (dead) blossoms may or may not remain attached with bactericide. to the tree. On apple, it is common for the • Good insect management. affected blossoms to cling to the spurs. On fruit, the symptoms depend on the stage Fire blight, caused by the bacterium Erwinia at which the fruit was infected. When infected amylovora, is one of the oldest known bacterial early, the fruits remain small, become discolored, diseases of plants. It affects only plants in the shrivel, and remain attached to the tree. Fruits rose family (Rosaceae). Within this plant family, infected later in development do not shrivel as the bacterium infects at least 39 different genera. much or become as discolored as immature fruits. In New Mexico, the disease is most common on Fruits infected following injury by hail or insects apple, pear, crabapple, pyracantha, photinia, and develop red, brown, or black lesions. Infected cotoneaster. Other hosts include rose, quince, fruits may exude large amounts of bacterial ooze. hawthorn, loquat, almond, apricot, plum, cherry, Young, succulent shoots infected with fire chokecherry, mountain ash, raspberry, blackberry, blight often develop a characteristic “shepherd’s- and strawberry. crook” symptom—the bending of the shoot tip to approximately 180° (fig. 2). The infected tissue The primary inoculum is reduced by pruning turns gray-green, and eventually black. overwintering cankers during the dormant season. Symptoms on mature woody tissue are some- An application of a copper bactericide at bud break what difficult to discern from other types of helps to reduce additional inoculum, which may injury, such as those caused by low- or high- be produced in cankers missed during pruning. temperature injury. Symptoms include sunken Removing active infection sites as they appear is and discolored areas (cankers), cracks, and important in reducing secondary inoculum. Active bark splitting or peeling. When the bacterium infections must be removed at least 12 to 15 inches is active, the inner tissue (under infected bark below the margin of visible infection because the areas) is water-soaked with reddish streaks. This bacterium moves inside the plant ahead of visible reddening can help to distinguish fire blight symptom development. Pruning tools must be cankers from freeze injury. disinfected between cuts to reduce the spread of Environmental conditions favorable for fire the bacterium on tools. Disinfect tools by dipping blight are rainy or humid weather with daytime in either a 10% bleach solution or 70% alcohol temperatures in the range of 75° to 85°F, especially (rubbing alcohol). Burn or otherwise dispose of when night temperatures stay above 55°F. Hot, (away from susceptible plants) all prunings. Do dry weather (over 90°F) slows or stops disease not compost diseased plant tissue, as compost piles development, but does not cure the disease. generally do not reach high enough temperatures In areas where fire blight is established, use a to kill the bacterium. Additionally, prune all three-pronged approach to disease management. unwanted suckers from trunks, roots, and branches This management program is designed to reduce as these fast-growing shoots are highly susceptible losses associated with the disease by reducing to the disease. the amount of inoculum in the area, reducing Fire blight is more severe on fast-growing, the susceptibility of the plants with horticultural succulent tissue. Therefore, avoid heavy practices, and preventing new infections during applications of nitrogen fertilizers. Additionally, favorable periods. applying required nitrogen in fall or spring as foliar

Fig. 1 Fire blight causes affected Fig. 2 Young, succulent shoots plant part to appear scorched. infected with fire blight often develop “Shepherd’s crook”.

Guide A-230 • Page 2 sprays after flowering helps to reduce the incidence program. Controlling insects can help reduce the of the disease. spread of disease on vegetative shoots. Insects In commercial apple orchards, properly reported to spread the disease include ants, timed bactericide applications are an important aphids, flies, wasps, fruit-tree bark beetles, honey component of a management program. bees, tarnished plant bugs, and pear psyllids. Bactericides used during bloom can be highly Control of sucking insects, such as aphids, is effective against the blossom blight phase of especially important. These insects not only carry the disease. Reducing infection at this time also the bacterium from one location to another, but reduces the amount of disease that develops on create feeding wounds, which serve as sites for vegetative shoots later in the season. Bordeaux entry of the bacterium into the plant. Although mixture (lime and copper sulfate), other copper controlling insects can reduce the spread of the compounds, and the antibiotic streptomycin are disease, take care not to harm pollinators such registered materials for the control of fire blight as honey bees. Therefore, insecticides should on apples. Copper compounds are relatively be used only on vegetative growth, not during inexpensive and can be effective, but great care pollination. must be used as they can cause russetting on fruit, When considering new plantings, select which lowers their fresh market value. Although disease-tolerant cultivars and rootstocks. Avoid more expensive than copper compounds, M9 and M26 apple rootstocks, which are highly antibiotics rarely damage fruit. Bacteria can susceptible to fire blight. Apple cultivars to avoid easily develop resistance to repeated use of include Twenty Ounce, Rhode Island Greening, chemicals, so it is important to rotate the type Yellow Transparent, Jonathan, Idared, Rome of materials. These materials are best used as Beauty, Lodi. Most popular pear varieties are preventative sprays because they have limited susceptible to fire blight, as are most cultivars of curative or systemic properties. General spray crabapples. recommendations include two or three sprays Research on resistance and tolerance on some during the bloom period. Routine sprays during ornamentals has been conducted at the University the summer have not been shown to be cost- of California. Results indicate that Pyracantha effective, although sprays following hail storms angustifolia is very susceptible, but P. coccinea have proven to help reduce secondary infections and P. crenulata are somewhat tolerant. There is in orchards with a history of fire blight. To also quite a difference in susceptibility among assure proper pesticide use, be sure to read and species of Cotoneaster. For example, C. salici- understand the product label and follow all label folia is susceptible, C. dammeri, C. pannosa, instructions to the letter. and C. horizontalis are somewhat tolerant, and Controlling insects can also be an important two species, C. adpressa and C. microphylla, are component of an overall fire blight management nearly resistant.

Guide A-230 • Page 3 New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

To find more resources for your home, family, or business, visit the College of Agriculture and Home Economics on the World Wide Web at http://aces.nmsu.edu.

February 1999 Las Cruces, NM 5C

Guide A-230 • Page 4