The bacterium erwinia amylovora is the cause of fire blight, a disease that severely harms fruit trees like apples, pears, and quince as well as some species of ash trees. The bacteria will also attack many flowering shrubs including cotoneaster and serviceberry. During winter, the bacteria live in cankers on the affected trees. When spring returns, the bacteria begin to multiply in warm weather and they drip out of the cankers in a watery sap-like liquid which falls on twigs, buds, and flowers. When the sap falls on the buds and blossoms of the host tree, the flowers get infected. The bacteria and the disease are then quickly spread from tree to tree by pollinators. Many varieties of pear trees and the pome family of trees are particularly susceptible to fire blight disease.
Symptoms: Black and grey cankers, some sunken, appear in the winter on the branches. Infected blooms wither and turn dark grey. Shoots may show signs of wilting and curve to seem like a shepherd’s crook. They may also ooze a clear watery substance.
More information: Wahington State University, University of California Statewide Integrated Pest Management Program, Ohio State University Extension
Photo: Treefruit.wsu.edu of Washington State University