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A lack of reduced-risk fungicides makes it difficult to control cherry leaf spot without using traditional chemicals, reports Dr. George Sundin, plant pathologist at Michigan State University, and the best solution could be development of cherry cultivars that are resistant to the disease.

Cherry leaf spot is the most economically devastating disease of tart cherry in the Great Lakes area. With future development of leaf-spot-resistant cultivars, fewer fungicide applications would likely be needed, and the disease could be –controlled more successfully, he said.

Sundin reported during the Great Lakes Expo in Grand Rapids, Michigan, last December on a four-year tart cherry integrated orchard management program funded by the U.S. Environmental Protection Agency’s Risk Avoidance and Mitigation Program (RAMP). RAMP focused on developing strategies for reduced-risk pest and disease management.

The big issue in terms of managing diseases—particularly cherry leaf spot—was a lack of materials, Sundin said. When a traditional fungicide such as Bravo (chlorothalonil) is eliminated from the program, it not only means that less effective alternatives must be used, but it leads to overuse of materials with the highest risk of resistance development.

For example, when the sterol demethylation inhibitor (SI or DMI) fungicides Elite (tebuconazole) and Indar (fenbuconazole) were used season-long in trials at the Northwest Michigan Horticultural Research Station, they provided good control at first, but control diminished over time as resistance built up. In cases of resistance such as these, some strains of the pathogen are still sensitive to the products, some are resistant, and some are highly resistant. When fungicides are applied to those populations, they kill the sensitive strains of the pathogen and leave behind the resistant strains, so that the resistant strains build up until there’s a control failure.

Sundin said that of about a thousand leaf-spot isolates collected from 41 orchards in Michigan, only five were fully sensitive to the SI fungicides. The level of resistance varies depending on the orchard and the fungicides used.

Copper

In the RAMP program, it was important to consider alternative disease controls, he said. Copper is an excellent leaf-spot control because of its persistence on the foliage. If the disease develops late in the season and it’s been a long time since a spray was applied, trees treated with copper have the best disease control because the product is so persistent, he said. However, copper can cause phytotoxicity. If it is used in hot, dry –conditions, leaves can turn yellow and drop. Milder symptoms include bronzing of the undersides of leaves.

Copper has generally been used at 2 pounds per acre, but experiments show that 1.2 pounds of metallic copper per acre can provide the same level of control, even with high disease pressure. Researchers also tested 0.8- and 0.4-pound rates, but found them too low. The new reduced-risk strobilurin fungicides Gem (trifloxystrobin) and Pristine (pyraclostrobin/boscalid) are highly effective leaf-spot materials but are also at high risk of resistance development. Sundin said there have been many reports of disease resistance to strobilurins, and a recent research paper from California reports resistance of alternaria to Pristine.

"We’re very hesitant to rely on these fungicides because of the risk of resistance," he said, adding that—unlike with other fungicides—with these products resistance is all or nothing. "Once we get resistance there, the organism is essentially immune to the fungicide. That’s going to have a major effect on disease control."

Dodine is a highly effective fungicide, which should be used with captan as a hedge against resistance. It’s a tool that may not have been used much lately, but could be used as an alternative chemistry, he said.

"We still need some of those materials that are not reduced risk at this point in order to prevent the onset of resistance. We’re hoping either that we can get new materials, or, if not, we have alternatives that can be used to protect the materials we have."

Resistant cultivars

MSU cherry breeder Dr. Amy Iezzoni is working to develop leaf-spot-resistant cherry cultivars, using two sources of resistant genes.

"It’s very promising work for us down the road," Sundin said.

Iezzoni has a large number of seedlings obtained from the crosses she has made, and several of the most promising ones are planted in replicated field trials at the Northwest Michigan Horticultural Research Station, he said.

"Our hope is that we get some of these down the road, and we can incorporate them into our cherries that are grown commercially."