As spring approaches and fruit trees begin to bloom, growers are being urged to watch for fire blight, a bacterial disease that can severely affect apples, pears, crabapples, and quince in California. Ornamental trees such as pyracantha (firethorn), hawthorn, cotoneaster, mountain ash, and toyon are also susceptible.
Fire blight is caused by the bacterium Erwinia amylovora. The disease survives in infected trees over winter and becomes active when nighttime temperatures exceed 55 degrees Fahrenheit and daytime temperatures range between 75 and 85 degrees. Rain or high humidity during this period can further encourage the pathogen’s activity.
Early signs of fire blight include light tan liquid oozing from branches or trunks. Infected blossoms turn black on pear trees and brown on apple trees. Leaves may appear dry or scorched but remain attached to the tree instead of falling off. Young shoots often wilt and curve downward into a shape resembling a shepherd’s crook.
Managing fire blight requires regular observation and immediate action. Recommended tools for removal include pruning shears, a curved saw, and a hand lopper—all kept sharp for clean cuts. Fire blight bacteria spread through healthy wood in narrow paths about one-and-a-half inches wide but can extend two to three feet beyond the main infection site.
To limit spread, experts recommend pruning back to healthy tissue with no visible discoloration—then removing an additional eight inches as a safety margin. “Start inspecting your trees now (January) while the bacteria are dormant. Remove any wounds or cankers which are visible along with any twigs or branches exhibiting signs of past infection.”
As temperatures rise in spring, growers should continue monitoring their trees for new infections and prune affected areas immediately since fire blight can spread rapidly within one to three weeks via pollinators inhabiting blossoms. Young trees are especially vulnerable compared to established ones.
There is currently no chemical treatment available for fire blight; diligent inspection and prompt removal of infected material remain key strategies for control. All pruned branches should be disposed of in green waste bins rather than composted to prevent further contamination.
No pome fruit or Rosaceae family species is completely immune to fire blight; however, some varieties exhibit resistance that limits its spread. When planting new or replacement trees, selecting resistant varieties is advised.
For more detailed guidance on identifying symptoms and managing outbreaks of this disease, consult the University of California Integrated Pest Management resource at https://ipm.ucanr.edu/home-and-landscape/fire-blight/pest-notes/#gsc.tab=0.
