Fire blight is destructive to fruit trees

Published on Wednesday, 7 June 2017 18:07 - Written by GREG GRANT, Texas A&M AgriLife Extension Service

It’s been a hard year on pears, especially ornamental cultivars like the omnipresent Bradford. First, the winter was so mild that, like most peach trees, they didn’t receive enough chilling hours to bloom properly. Then, fire blight left clusters of black-leafed, dead branches throughout the trees. Bradford and other Pyrus calleryana pear selections are considered resistant to fire blight. But as you can see, they certainly aren’t immune.

Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of fruit trees and related plants in the rose family. Pear and quince are extremely susceptible. Apple, crabapple and pyracantha also are frequently damaged. Fire blight is less common on hawthorn, spirea, contoneaster, photinia and loquat. The disease can destroy limbs and even entire shrubs or trees.

In spring, branch and trunk canker symptoms appear as soon as trees begin active growth. The first sign is a watery, light tan bacterial ooze that exudes from cankers on branches, twigs or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. Most cankers are small and inconspicuous, thus infections might not be noticed until later in the spring when flowers, shoots or young fruit shrivel and blacken. Infected flowers and flower stems wilt and turn black or dark brown. Fire blight infections might be localized, affecting only the flowers, or they might extend into the twigs and branches, causing small shoots to wilt and form a characteristic crook at the end of each infected shoot.

Fire blight bacteria overwinter in cankers on twigs, branches or trunks of host trees. In spring, a small percentage of the cankers become active as bacteria multiply and ooze from branch or twig surfaces. Splashing rain or insects transmit the bacteria to nearby blossoms or succulent growing shoots, so rainy springs make the problem worse. Once blossoms are contaminated with the bacteria, honey bees become efficient carriers of the pathogen.

Tree vigor has a major influence on the extent of fire blight damage. Once established, the distance the pathogen moves relates directly to the susceptibility of the tree and rate of tree growth. Vigorously growing shoots are the most severely affected; therefore, conditions such as high soil fertility and abundant soil moisture increase the severity of damage to trees. In general, trees are more susceptible when young and suffer less damage as they age. The bottom line: Don’t prune, fertilize or water your pear trees. All exacerbate the problem.

Successful removal of fire blight infections is done in summer or winter when the bacteria is no longer spreading through the tree. At these times cleaning pruning shears is unnecessary. Rapidly advancing infections on very susceptible trees should be removed as soon as they appear in spring. In these cases, dip pruners in 10-percent bleach between cuts. To locate the correct cutting site, find the lower edge of the visible infection in the branch, trace that infected branch back to its point of attachment, and cut at the next branch juncture down without harming the branch collar.

If a fire blight infection occurs on a trunk or major limb, the wood often can be saved by scraping off the bark down to the cambium layer in infected areas. When scraping, look for long, narrow infections that extend beyond the canker margins. Remove all discolored tissue plus 6 to 8 inches more beyond the infection. This procedure is best done when trees are dormant. Don’t apply any dressing to the wound. If the limb has been girdled, the whole limb must be removed.

Copper products are the only materials available to homeowners for fire blight

I’m fully aware that this is more than you ever wanted to know about fire blight and I’m fully aware that most of you aren’t capable of pruning all those dead branches out of your Bradford pears. So what do I recommend you do? Nothing, really, unless your pear is so disfigured that you need to remove it entirely.

Actually Bradford and other callery pears are no longer recommended for landscaping because of their breaking branches when they get older and the mass potential for thorny, invasive seedlings now running amuck through the eastern countryside.

Resistant edible hard pears like Orient will go on living with or without a bit of fire blight, while soft pears like Bartlett always die from fire blight in East Texas.

If you have other diseased plants that you would like diagnosed, you can submit samples to the Texas A&M Disease Diagnostic Laboratory. You can find information, instructions, and submission forms at http://plantclinic.tamu.edu/forms/.

Greg Grant is the Smith County Horticulturist for the Texas A&M AgriLife Extension Service. You can follow him on Facebook at “Greg Grant Gardens,” read his “Greg’s Ramblings” blog at arborgate.com or read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). For more information on local educational programming, go to smith.agrilife.org.