Spring Gardening News Distributed 02/22/08
Fire blight disease can be extremely damaging to some varieties of apples, pears and mayhaws. Other plants susceptible to fire blight include crabapple, hawthorn, cotoneaster, quince, pyracantha, loquat and blackberry.
The disease frequently starts in flowers, and the bacteria will often move down the flower petiole and infect twigs. Infected flowers will turn black. Fire blight also can cause blackening of leaves at the tip of new shoots. The infected tip usually bends into a shepherd’s crook. The bacteria can move down the stem to larger limbs and cause infection.
Fire blight is caused by bacteria that can be spread by insects, hard driving rains and hail, according to LSU AgCenter horticulturist Dr. John Pyzner. He says plants are most susceptible while blooming and when fast, tender growth is occurring in the spring.
Fire blight sometimes runs in cycles. That is, the disease may appear for several years in various degrees of severity and then show little or no damage for a few years.
Bacteria causing fire blight frequently overwinter in cankers at the base of killed twig. Removal of these twigs by pruning can reduce the severity of fire blight the following year. Pyzner says removing infected twigs in late winter is best, although damaged twigs and cankers are sometimes difficult to see. The horticulturist discourages pruning infected shoots during wet periods because of the likelihood of spreading bacteria into otherwise healthy parts of the plant.
Make all pruning cuts at least 8 to 12 inches below damaged wood because the bacteria may be present in stems beyond the area showing symptoms. Sterilize the shears between each cut by dipping them into a 10 percent bleach solution to reduce the chances of bacteria being spread.
Vigorously growing apple and pear trees are more susceptible to fire blight. Bearing pear trees should not produce more than 6 to 8 inches of new terminal growth in a season or 10 to 12 inches for bearing apple trees. Reducing nitrogen fertilizer levels will reduce the growth of the trees.
The best way to prevent fire blight is to plant resistant varieties. The pear varieties Orient, Maxine, Moonglow and Ayers have good resistance. Bartlett and Le Conte pears have poor resistance. Apples with good resistance are Red Delicious, Arkansas Black and Winesap. Fuji, Gala and Mollies Delicious apples have poor resistance.
Timely use of chemicals can help control fire blight, but Pyzner says this might not be practical for homeowners except on small or dwarf fruit trees. Spray during bloom and other susceptible times during spring with fixed copper (Kocide) or streptomycin according to manufacturer’s directions.
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On the Internet: LSU AgCenter: www.lsuagcenter.com
Contact: John Pyzner (318) 644-5865 or Jpyzner@agcenter.lsu.edu
Editor: Mark Claesgens (225) 578-2939 or mclaesgens@agcenter.lsu.edu