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Fire Blight of Apples and Crabapples

Source: NDSU Extension Service Horticulturist

Fireblight is a serious disease of apples, crabapples and pears. Blossoms, blossom spurs, and branch tips are infected, turning brown and dying rapidly. The tips of branches often curl to form what resembles a shepherd's crook, or the curved end of a walking cane. The dead leaves remain brown to almost black and may remain on the branch all winter. The disease is caused by a bacterium that lives in the conducting or vascular tissues of the plant. The bacterium progresses down the diseased shoots and spurs, and sometimes reaches a larger branch where a dark or discolored area is formed. This discolored area is called a canker. The bacterium lives over the winter in young cankers with smooth, indefinite margins. In the spring, bacteria ooze from the canker, forming a sticky, thick liquid that insects feed on. These insects transmit the bacteria to blossoms, where new infections start. Later, the disease may spread from infected blossoms and old cankers to the shoots. Rain, wind, and hail help spread the disease.

Successful fireblight control requires a combination of actions. For the first step, overwintering cankers should be pruned away by cutting at least six to ten inches below the edge of the canker. Smaller diseased branches should be pruned about ten inches below the edge of the diseased area. Pruning is best done in late fall after leaf drop, when the diseased shoots can be readily seen since they retain their leaves. Or, pruning can be done in late February to early March. Pruning tools should be sterilized between each cut, Lysol diluted 3/4 cup per gallon of water, or household bleach diluted 1 cups in each gallon of water. Household bleach works very well, but corrodes tools, which must be carefully washed and oiled after use to prevent rusting. For the second step, apples or pears can be sprayed with streptomycin at blossom time. Use streptomycin at 50-100 parts per million every 3-5 days during blossoming. It can also be used every 14 days after blossoming, but streptomycin is not as effective for control of shoot blight as it is for control of blossom blight. After a hail storm, streptomycin should be sprayed immediately to reduce infection in the hail-induced wounds. Uptake of streptomycin is improved if applied during the evening. Streptomycin should not be applied within 50 days of harvest for apples or within 30 days of harvest for pears. It is not registered for use on crabapple, cotoneaster, or mountain ash.

Fireblight is most severe on succulent shoots. Avoid overfertilizing trees, and do not fertilize in late spring or early summer.

Additional information on this topic is included in the Extension bulletin PP-454, "Diseases of Apples and Other Pome Fruits," which is available at your county office of the NDSU Extension Service.

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