- Author: Linda Lewis Griffith
- Editor: Noni Todd
By Linda Lewis Griffith UCCE Master Gardener
“Why are the blossoms and leaves on my apple tree turning brown? Deb R. Arroyo Grande
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. Pears 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. Splashing rain or insects transmit pathogens to nearby blossoms or succulent new shoots.
Symptoms first appear in spring as trees begin to grow. A watery, light 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 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, Johnathan, Mutsu, Pink Lady and Yellow Newtown are susceptible to fire blight; Empire, Pristine and Williams Pride 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 tools after use to prevent rust.
For more information about fire blight, visit these websites: