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A mysterious disease that has been killing the best looking, reddest strains of McIntosh apples in the nicest orchards in the northwest quadrant of Michigan has tentatively been identified, and growers now have some idea what to do about it.

“We’re not quite there yet,” said Dr. George Sundin, the Michigan State University plant pathologist who was called in to evaluate the situation. He’s still looking for ­scientific proof of what’s now a hypothesis.

Three types of organisms—anthracnose, black rot, and Nectria canker—have been associated with the symptomatic trees, but Sundin’s not ready to say they cause it. “These are opportunistic pathogens that attack trees that are already stressed,” he said.

Why these specific trees are stressed is not completely understood. The disease has shown up on McIntosh strains that are very red, like Lindamac and Rubymac, in high-density plantings on dwarfing rootstocks, and it usually appears just when the orchards are getting into high production in their fourth and fifth leaf and after their first big crop.

Another associated stress may be winter injury. In the last two years, cool, wet springs and summers in Michigan have been followed by early, cold winters, and these trees may not have hardened off adequately, Sundin said. Red strains, for reasons unknown, may not harden off as well as conventional McIntosh, particularly when grown on dwarfing rootstocks. The anthracnose and black rot pathogens find cool, moist springs to their liking.

Growers saw the problem intensify this spring, ­following the big apple crop last year.

It may have seemed like an epidemic, Sundin said. Growers on Fruit Ridge near Grand Rapids and in the fruit area around Traverse City began planting these new strains heavily about five years ago, so there are a lot of young orchards. Some of the new strains are sports discovered in Michigan and favored by Michigan growers.

Dr. Nikki Rothwell, the Michigan State University researcher who works with growers in the Traverse City area, said some growers there have planted large blocks of these red McIntosh. It’s quite a scary sight to see a 10-acre block with 25 percent of the trees declining or dead, she said. At least a dozen growers have reported the problem. It’s always on these red McIntosh strains on dwarfing rootstocks.

The disease strikes some branches, causing dieback at the tips first and progressively killing the branch. Growers prune out infected side branches, but if the affected branch is the central leader, the tree is lost, Sundin said. It is not known whether these diseases can act systemically, like fireblight. Pruning wounds do seem to be sites that can be infected.

“We saw a few strikes two years ago and then more last fall,” he said. “This spring, following last year’s big crop, we saw a lot.”

In New York, some growers apparently have reported the same problem, but it’s not nearly as severe as in Michigan. Dr. Dave Rosenberger, the tree fruit pathologist at Cornell University’s Hudson Valley Laboratory, noted that not all McIntosh strains are equally winter hardy, and that young trees, which grow later in the season before hardening off, may be affected. There have not been significant losses of trees in New York, he said.

What should growers do? They need to discourage late-season growth and encourage the tree to harden off, Sundin said. Make sure there’s no excess nitrogen. Growers like to push young trees, to get good growth even as they promote early cropping. This might be one of those stresses.

Prune these trees late in the dormant season, he suggested. Make sure they have hardened as completely as possible.

Sundin is working now to develop a program for Michigan growers. It will likely be a hot topic for winter horticulture meetings that start in December.