Aug 26, 2010
Anthracnose found in Michigan?

This spring, growers across Michigan noticed that a few of their Macintosh trees were dying. Most shrugged it off as winterkill, a common thing in a state known for harsh winters. While some reports called it a blight spreading through all the Mac trees in Michigan and New York, university educators and researchers hesitated to make claims before further testing and evaluation could be done.

The cause isn’t certain yet, but it might be anthracnose canker – a disease that was never considered much of a threat in Michigan.

“Anthracnose is just not supposed to happen in Michigan,” said Denise Donohue, executive director of the Michigan Apple Committee (MAC). “Our winters are widely considered too harsh for it to be much of a threat.”

MAC and Fruit Growers News partnered to hold an educational webinar about anthracnose for growers who aren’t familiar with the disease. The webinar featured Jim Rahe, emeritus professor of plant pathology at Simon Fraser University in British Columbia.

Anthracnose is much more common in the Northwest, where winters are mild and wet. The disease is severe in the high-rainfall apple-growing areas west of the Cascades and British Columbia, according to Oregon State University (OSU). The fungus causes cankers on twigs and branches and can lead to bull’s eye rot in the fruit when it is in storage. Fall rains spread fungal spores from limb cankers to maturing fruit and to young limbs and twigs.

Dan Dietrich, who works on his family farm in Conklin, Mich., tried for a long time to find what was damaging his trees and fruit. He’s convinced it’s anthracnose.

“I kept looking for information on the web and talking to people,” Dietrich said. “Folks in Oregon and British Columbia who are more familiar with this were a big help. It took a few years to get it confirmed in our trees, but that’s what it is here.”

During the presentation, Rahe stressed how important it is to be quick and vigilant when dealing with this infection.

“It is vitally important to get all of the infection out, as just one canker can infect all of the trees in an area,” Rahe said.

Cut out the canker or pull the tree as soon as the canker is found. Don’t wait for pruning.

The infection spreads rapidly and occurs in the fall, but cankers will not appear until spring, according to OSU. Small injuries to the tree may facilitate initial infection, but the fungus can also penetrate uninjured bark, making for a problem that is difficult to manage. The canker grows actively only one year. The fungus, however, continues to thrive for two or three years in the tissue, and produces large numbers of spores.

What should you be looking for? Small, circular, reddish-brown spots on the bark extending to the underlying tissues are the most prominent indication that there is a problem. These cankers are most abundant on smaller branches, but may be on larger limbs or on trunks of young trees, according to OSU. Complete girdling often results if more than one canker is on a limb.

Development of these cankers is arrested during winter, hence the reasoning that this shouldn’t be much of an issue in colder climates. The canker expands most rapidly along the branch in spring. The cankers cease to extend and a well-marked crack forms, delimiting the canker from the surrounding healthy tissue, according to OSU. The surface of the canker becomes shrunken and shriveled as surrounding tissue continues to grow during the summer. The dead tissue gradually disintegrates and falls out.

“We basically pruned it out of our orchards as much as we could,” Dietrich said. “There is no real ‘cure’ for it other than to remove it. Fall sprays can help prevent it, which is something Michigan growers generally don’t do. A spray of post-harvest fungicides can help, but around here, once the apples are off we don’t really worry about the few leaves that are left.”

Both infected orchards and nursery stock can be sources for this disease, according to OSU. Other trees can act as a host if close to an orchard. These include crabapples, most pome and stone fruit, Amelanchier, hawthorn and mountain ash.

According to Donohue and Rahe, besides Macintosh, Gala, Jonagold and Honeycrisp are all susceptible to anthracnose.

Rahe recommends that growers learn to recognize the canker and practice timely removal of the cankers when they are found. Flag the tree and inspect it on a regular basis, he said, and remove the tree if necessary.

If you suspect you have an anthracnose infection, testing is available through Michigan State University Diagnostic Services.

A podcast digital copy of the webinar will be available to download at, and MAC will offer a special program on anthracnose at the Great Lakes Fruit, Vegetable and Farm Market EXPO this December in Grand Rapids, Mich.

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