November 6, 2015
Your beloved apple tree had blackening blossoms in spring and this fall you see black shoots. The ornamental pear tree on your street has browning shoots. The quince tree in your aunt’s backyard has a light tan ooze on its trunk.
The common denominator is apples, pears and quince are members of the same plant family, rosaceae, or commonly called rose family, all susceptible to a bacterial disease called fire blight. Also frequently damaged are crab apple and pyracantha. Fire blight is less common in some other members of the rose family, including hawthorn, cotoneaster, mountain ash, loquat and other related California natives such as toyon, spirea and serviceberry.
Simply defined, fire blight is a bacterial disease that causes the stems and fruit buds of trees to wither. Stems bent over like a shepherd’s crook are symptomatic of a fire blight infection.
Remember the song “I’ll Be with You in Apple Blossom Time”? It is fire blight’s refrain, as the blossom is the most common infection site. In spring, bees and insects traveling from blossom to blossom help spread the bacteria Erwinia amylovora.
“As climate shifts, we will see warmer springs here in Marin so resistant varieties and fastidious attention to blight removal is the new norm,” horticulturalist John Valenzuela, the volunteer co-manager of the orchard and fruit tree nursery at Indian Valley Organic Farm & Nursery, advises.
Here are four practical steps to take now to protect your trees from looking like they have been scorched:
• Select fruit trees that are more resistant to fire blight. For ornamental pears, select Bradford or Chanticleer. For apples, select Grimes Golden and Snow (Fameuse) and fruiting pears, Anjou, Seckel, Magness and Warren. For quinces, Aromatnaya is classified disease resistant. You can find these trees at your local nursery and at the nursery at Indian Valley.
• Refrain from fertilizing, watering and pruning the trees during bloom time, which encourages new tissue and suckers, both extremely susceptible to fire blight.
• Remove infected wood during the summer. In the case of rapidly advancing infection, fire blight should be removed immediately, even in spring when the disease is active. Diseased wood needs to be disposed of completely. The cut to remove diseased wood should be made at the next branch juncture down from the infected branch. For trunk cankers, peel back bark around cankers to expose infected reddish or blackish tissue and cut it out.
In cases where the disease has spread through large portions of the tree, it may be best to remove the entire tree. This is a disease that can reduce yield by as much as 25 percent as well as threaten to kill infected trees.
• Clean tools with bleach diluted 1:9 with water (and rinse to avoid corrosion) or 70 percent alcohol.
Chemical control of fire blight has proven difficult and produced spotty results. Copper sprays have to be applied every four days during bloom especially when temperatures exceed 60 degrees. Copper sprays should not be applied when bees are present. Copper products can cause russeting of fruit, a brownish, roughened area on the skin of fruit.
The “Apple Blossom” lyrics end with making you mine in apple blossom time, reminding us of love and its regenerative power. Don’t let fire blight cankers overwinter in apple, pear and quince trees. Diagnosing and eliminating disease helps maintain biodiversity and keep our edible landscapes thriving.