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This destructive bacteria can turn the leaves of an apple tree an ugly brown color

Fire blight is a bacteria that can turn colorful plants such as apple trees a deep, ugly brown color.
Fire blight is a bacteria that can turn colorful plants such as apple trees a deep, ugly brown color.

Q. Why are the blossoms and leaves on my apple tree turning brown?

Deb R., Arroyo Grande

A: Your tree may be infected with fire blight.

Fire blight is a common and frequently destructive bacterial disease that affects pome fruit trees and other related plants. Pear and quince trees are highly susceptible. Apples, crabapples and Pyracantha species can also be susceptible to damage. Fire blight infections may destroy limbs and even entire shrubs or trees.

Fire blight is caused by a bacterium, Erwinia amylovora, that overwinters in cankers on twigs, branches or trunks of host trees. Warm, daytime temperatures between 75 and 85 degrees interspersed with intermittent rain or hail create ideal conditions for bacteria to thrive.

Symptoms first appear in spring as trees begin to grow. A watery, tan liquid oozes out of infected areas. The ooze darkens after exposure to air, leaving streaks on branches and trunks. Cankers may be inconspicuous and go unnoticed until later in the spring when flowers, shoots and young fruit shrivel and turn black.

Vigorously growing shoots are the most severely affected; conditions such as high soil fertility and abundant water increase the severity of damage.

Management begins by first selecting varieties of plants that are less prone to damage. For instance, Fuji, Gala, Golden Delicious, Jonathan, Mutsu, Pink Lady and Yellow Newtown are susceptible to fire blight; Empire, Pristine and William’s Pride apples are considered more resistant.

Once infections have taken hold, it is necessary to prune out diseased branches. Cut infected branches at least 8 to 12 inches below the visible injury or canker. A greater distance below infections may be required on major branches, scaffolds or trunks in May or June, when fire blight bacteria are moving rapidly.

To avoid spreading bacteria during the pruning process, dip or spray pruning tools with a 10 percent solution of bleach (one part bleach to nine parts water) before each cut. Dry and oil the tools after use to prevent rust.

For more information about fire blight, visit these websites:

▪ http://ipm.ucanr.edu/DISEASE/DATABASE/fireblight.html

▪ http://ipm.ucanr.edu/PDF/PESTNOTES/pnfireblight.pdf

▪ https://ucanr.edu/sites/placernevadasmallfarms/files/112366.pdf

Linda Lewis Griffith is a UCCE Master Gardener.

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In San Luis Obispo call 805-781-5939, Arroyo Grande, 805-473-7190 and Templeton, 805-434-4105. Visit us at http://ucanr.org/sites/mgslo/ or email us at anrmgslo@ucanr.edu. Follow us on Instagram at slo_mgs and like us on Facebook. Informative garden workshops are held the third Saturday of every month, 10 a.m. to noon at 2156 Sierra Way, San Luis Obispo. Garden docents are available after the workshop until 1 p.m. To request a tour of the garden, call 805-781-5939.