Garden Detective

Garden Detective: Fire blight torches pear, apple trees

This is an example of fire blight. It is caused by a bacterium that affects pome fruit trees.
This is an example of fire blight. It is caused by a bacterium that affects pome fruit trees.

Experts tackle readers’ garden questions.

Q: I have fire blight on my pear tree and a little on my young apple tree. The pear tree’s blight is pretty bad and started two years ago. It just keeps getting worse. I’ve been told there is nothing I can do about it but cut off diseased branches. Is there anything that can be done?

Betty Miller, Sacramento

Master gardener Rachel Tooker: Removing the infected branches is the most important thing you can do to help stop the spread of fire blight.

Fire blight is caused by a bacterium (Erwinia amylovora) that affects pome fruit trees, such as pear and apple, and related plants. Pear and quince are particularly susceptible. Loquat, pyracantha and crabapple also can be infected.

The disease gets its name from the blackening and shriveling of affected shoots, flowers and young fruit, which look like they have been singed by fire. The first evidence of a fire blight infection is a watery, tan bacterial ooze that seeps out of branch, twig and/or trunk cankers as soon as tree growth resumes in the spring. The ooze turns dark after exposure to air, leaving streaks on branches and trunks. The cankers are often small and hard to spot, so infections are not usually noticed until later in the spring when flowers, shoots and/or young fruit shrivel and blacken. Dead, blackened leaves and fruit cling to branches throughout the season, giving the tree a scorched appearance.

Open flowers are the most common infection sites and are susceptible until petal fall. Tender young leaves and shoots injured by wind, hail or insect punctures also can be easily invaded by the bacteria. Daytime temperatures in the 75- to 85-degree range, combined with rain or humid weather and nighttime temperatures above 55 degrees, are ideal for infection. Dry conditions slow further new infections. However, once infected, the plant will harbor the pathogen indefinitely.

To manage fire blight, trees should be regularly monitored, especially during the heavy infection months of May and June. Avoid heavy pruning and excess nitrogen fertilization, which promote abundant growth. Also, avoid irrigating the trees during bloom.

Slow the spread of fire blight by fully removing infected branches, ideally in summer or winter when the bacteria is no longer spreading throughout the tree. For very susceptible trees, such as pear and apple, the infected branches should be removed as soon as they appear in the spring. To avoid spreading the disease, maintain rigorous sanitation practices by dipping shears in a 10-percent bleach solution after each cut. Do not dispose of infected cuttings in a compost pile.

When removing infected branches, the location of the cut is critical. Cuts should be made far enough below the visible injury or canker to remove all the infected tissue. This can mean cutting at least 8 to 12 inches below the canker. A telltale dark ring in the branch will indicate if the cut is not deep enough. To locate the correct cutting site, follow the infected branch to its point of attachment and cut at the next branch juncture down. This will remove the infected branch and the branch to which it is attached.

If the infection has traveled into a trunk or major limb, the wood often can be saved, but needs careful management.

To help prevent the disease from reoccurring, spray a weak (0.5 percent) Bordeaux mixture or other copper product several times (every four to five days) during bloom season and when daytime temperatures are above 60 degrees. Spraying can help prevent new infections, but will not eliminate any infections that are already in the wood. The spray must be applied to open blossoms and may cause russeting or scarring on the fruit surface.

For more information on fire blight, including how to manage infected trees, check out the University of California, Integrated Pest Management Program, Pest Note 7414, “Fire Blight,” available free online at

Rachel Tooker is a UC Cooperative Extension master gardener for Sacramento County.

Garden questions?

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