Good Fruit Grower, Washington Tree Fruit Research Commission // Apr 30, 2019
Mike Willett demonstrates how to use a custom fire blight knife for scraping blight from fruit trees in late March in Wapato, Washington. The scraper was designed to remove fire blight cankers from green wood, allowing the tree to heal. Blight knives were widely used to save trees in Oregon’s Rogue River Valley pear orchards beginning in the early 1900s. (TJ Mullinax/Good Fruit Grower)
Pears were first planted in Oregon’s Rogue River Valley in 1885, and commercial interest grew every year. However, by 1905, pear growers were near desperation.
The root of that desperation: fire blight. In early April 1904, during bloom, official weather records from Ashland, Oregon, recorded nearly a week of maximum temperatures from the high 70s to the high 80s Fahrenheit, followed by almost a quarter-inch of rain. Growers had little understanding of the relationship between periods of bloom, warm weather and rain, but by the next year, trees were dying in large numbers. In the summer of 1905, growers there appealed to the United States Department of Agriculture to assign a plant pathologist in Medford, Oregon, to help them manage fire blight.
Apparently not wanting to move too quickly, in 1909, the USDA stationed P.J. O’Gara, a fruit tree pathologist, in Medford. His main message was sanitation. Before the availability of antibiotics, pear growers had no really useful preventative control tools, forcing them to rely heavily on pruning to remove diseased tissue.
That he was effective, even with this limited tool kit, was evident in 1911 when the USDA tried to relocate O’Gara. To prevent his departure, Rogue River Valley fruit growers petitioned the Jackson County commissioners to fund a county plant pathologist position. That position was created and then filled by O’Gara, who resigned from USDA.
By the time O’Gara eventually moved on, three years later, Congress had passed the Smith-Lever Act, which established the USDA Cooperative Extension Service. Using the funds that had been appropriated to hire O’Gara, Jackson County had its share of the resources necessary to hire C.C. Cate, one of the first county agents in the country, posted to Medford in December 1915.
This illustration, from the May 1, 1994, issue of Good Fruit Grower, was included in the debut of a new column at the time titled “The Practical Grower,” written by Mike Willett, who is still sharing his tree fruit knowledge 25 years later. (Illustration by Good Fruit Grower)
Given that tree surgery was the only control option for fire blight at the time, specialized tools developed. The blight knife pictured above was given to me by one of Cate’s successors, Cliff Cordy, who served as the Oregon State University county extension agent in Medford for 32 years, beginning in 1935. Blight knives like the one Cliff gave me were used to scrape cankers that could not be removed by pruning.
Canker scraping may have been more important in Medford than in other districts because, in the early years, not all orchards were irrigated. Since it was difficult to grow replacement trees under dryland conditions, growers went to extraordinary lengths to save established trees. With the eventual districtwide availability of irrigation water and more effective fire blight control measures, Medford growers eventually spent less time scraping fire blight. However, a good blight knife should still be considered an important part of a fruit grower’s tool kit.
Blight knives were and are generally used to remove cankers that occur where a blighted branch or spur meets a larger branch or the trunk. First, the diseased limb is removed flush with the canker. Then, all the diseased tissue in the canker area, as well as about an inch of healthy tissue beyond the canker, is scraped away. This tissue is scraped off using downward vertical strokes to create a cut area the shape of a pointed oval, for quicker healing.
Recommended steps to use a fire blight knife: 1) Identify blighted branches to determine pruning locations. 2) Remove the diseased limb flush with the canker. 3) Scrape the diseased tissue along with about an inch of healthy tissue surrounding the canker. (Photos by TJ Mullinax/Good Fruit Grower)
Some cankers are probably too large to justify scraping. It is recommended that if a canker extends around more than half of the limb or trunk, the entire limb or tree should be removed.
Cliff gave me the blight knife as a part of my education and, I think, as a symbol of the end of his years in service to growers. He would be fascinated to learn of the current toolbox that research and extension has given growers to manage fire blight.
However, he might not be amazed at the damage that continues to be caused by this disease. And that a blight knife might still come in handy. •
—by Mike Willett
Mike Willett, manager of the Washington Tree Fruit Research Commission, started working with Pacific Northwest tree fruit producers in 1980 as a county agent in Jackson County, Oregon. Thanks to Eden Valley Orchards, Medford, for Cliff Cordy’s self-published 1977 “History of the Rogue Valley Fruit Industry,” which can be found online at bit.ly/2CsAKfY.