Pear tree blight is also referred to as fire blight. It is a bacterial disease that affects pears, apples and quinces. The blight can destroy leaves, branches and limbs, and if left untreated can kill the tree. Prompt diagnosis and treatment of the disease can help to minimize the risk to trees and crops.
The Erwinia amylovora bacterium causes fire blight. The disease symptoms usually appear in spring, when new growth is beginning. Watery bacterial ooze will run from the infection site, which appears as a canker on the bark.
The bacterium affects the flowers next, causing them to wilt and turn black. The infection can then move into the branches from the blossoms which will grow malformed and will eventually turn black and die, giving the plant a scorched appearance, hence the name, fire blight. Stems and branches infected with fire blight typically have pink streaks below the bark.
Fire blight bacteria over-winter in cankers in the bark, which may go unnoticed, until it is too late. The infection spreads as days warm up in spring and moist conditions begin to prevail. The oozing exuded by the cankers contains bacteria that can be spread by rain, wind, mechanical transfer, such as in poor pruning practices or animals or insects, especially honey bees.
Depending on the vigor of the tree, infection and resulting damage may only affect some of the blossoms and branches. Proactive pruning may help to reduce the risk of continued infection. If left unchecked, the illness can continue to infect more of the tree and may jump from one tree to another, threatening to become epidemic in the orchard.
Its important to select from blight tolerant cultivars when preparing to plant pear trees. Bartlett, Clapp's Favorite, Bosc, Aristocrat and most Red and Asian varieties of pears are highly susceptible to fire blight infection. Bradford, Capitol, Red Spire, Kieffer, Starks Delicious, Orient, and Dawn are less prone to infection as is the Shinko variety of Asian pear. Magness and Moonglow pears are relatively resistant to the disease.
When an infection does occur, eliminate it by pruning out diseased branches. Ensure that the pruning is sufficiently far from the infection site to guarantee that the infection has been eradicated. If the trunk or major limbs are infected, scrape the bark down to the cambium layer. Be sure to disinfect tools after each cut to prevent spreading the disease. Chemical control is also an option. Copper-based fungicides can be applied after blossoms have appeared to reduce the effects of the disease. This will not eradicate it, however, and will not stop infections in the wood.