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Fungal pathogen affecting apple trees found in Minnesota for first time

Japanese apple rust requires both junipers and apples or crabapples to complete its life cycle, with spores carried by wind or rain to infect other plants.

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Japanese apple rust, a disease affecting apple and crabapple trees, as well as junipers, left this spot on the leaves of a tree. The finger-like projections on the back of the leaf produce fungal spores. Minnesota agriculture officials reported this month that it's been detected in the state for the first time.
Courtesy / Minnesota Department of Agriculture via MPR News
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ST. PAUL -- A fungal pathogen that affects apple and crabapple trees, as well as junipers, has been found in Minnesota for the first time.

The state Department of Agriculture reported Japanese apple rust was found during inspections of orchards and nurseries in Scott, Rice and Dakota counties. Surveys are being conducted to see how far the disease has spread.

The fungal pathogen can cause defoliation of trees. Minnesota Department of Agriculture plant pathologist Michelle Grabowski said affected trees may struggle to produce fruit.

“What we don't know right now is how susceptible Minnesota's apple varieties are to the Japanese apple rust,” she said. “We have been seeing leaf spots on the trees (but) we have not been seeing defoliation yet this year. But again, it's brand new to us so we want to watch it for a while.”

The department says it'll work with apple growers and the nursery and landscape industries to learn more about the disease. They are “cautiously optimistic” there will be no significant impact to apple production.

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Japanese apple rust requires both junipers and apples or crabapples to complete its life cycle, with spores carried by wind or rain to infect other plants.

“We do know when these infected trees drop their leaves this fall that the fungus will be broken down along with those leaves,” Grabowski said. “And so it won't survive on the apple leaves; it actually will only survive on the junipers from one season to the next, is what we've seen in other areas.”

The apple rust causes red, orange or yellow spots on apple or crabapple leaves, as well as finger-like projections on the bottom of the leaves that produce spores.

Infected junipers — which haven't yet been observed in Minnesota — “have small woody galls that produce a bright orange gelatinous fungal mass during spring rains,” the department reported. Officials say a juniper species native to Minnesota — eastern red cedar — is not susceptible to Japanese apple rust. But some other species used in landscaping in the state can be infected.

While it's new to Minnesota, the rust has been found on trees and shrubs in the northeastern U.S. for a decade or more. It was confirmed in Wisconsin in 2021.

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