By Guy K. Ames, NCAT Horticulture Specialist
With an overabundance of rain in most of the eastern United States and May temperatures warming, conditions are conducive for outbreaks of the “shoot blight” phase of fire blight.
The primary infection site for the causal organism of fire blight, the bacteria Erwinia amylovora, are the tender tissues of the pear and apple flowers. This year, at least in my orchard in Northwest Arkansas, conditions for the blossom phase of fire blight were not favorable for infection. We had lots of sun and only short-lived rains during bloom.
However, that changed as May rolled in. We’ve had lots of blowing rain and long rain events. Temperatures have been in the 55-75 degree F. range—perfect for the bacteria to spread and infect.
The bacteria can’t penetrate lignified (hardened) wood, but it can penetrate the soft, tender tissues of rapidly growing apple and pear shoots. Once inside, the bacteria proliferate and begin to move down the vascular system of these twigs into older wood. A pear or apple variety with a high degree of blight resistance is often able to stop the blight from “running” down the vascular tissues and into older wood. But susceptible varieties like Bartlett pear and Jonathan apple can do little to stop the blight. They can suffer significant loss of wood, sometimes even dying in a single season.
Fighting Fire Blight
Most commonly, the advice to growers is to wait until late summer or dormancy to remove infected wood. However, if you run across shoots that look like these photos and can break off a young shoot about 6-8 inches below the last visible sign of infection, you might prevent a worse infection. Since the possibility exists that pruners could spread the disease, it’s best to break out infected twigs with your hands. If the infection has apparently moved into older wood than you can easily break out with your hands, stop and wait until late summer or dormancy to remove the infected wood.
The ATTRA publications Apples: Organic Production Guide and Pears: Organic Production have much more information about fire blight, including copper and biological treatments for control, as well as proper pruning to inhibit the disease. And watch for the soon-to-be-published Pruning for Organic Management of Fruit Tree Diseases.
About the Author
Guy K. Ames is a Horticulture Specialist with NCAT and its ATTRA Sustainable Agriculture Program. He has over 40 years of experience gardening, farming, and orcharding. Guy has a B.A. in history from Texas A&M, Commerce and an M.S. in horticulture (fruit crops and pest control in fruit crops) from the University of Arkansas, Fayetteville. Guy has operated Ames Orchard & Nursery since 1983.