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New apple tree fungus discovered

ST. PAUL — Japanese apple rust, a disease affecting apples, crabapples and junipers, has been confirmed in Minnesota for the first time.

The disease is caused by the fungal plant pathogen gymnosporangium yamadae. MDA staff recently discovered the fungus through surveys and routine inspections at several orchards and nurseries in Dakota, Rice and Scott counties.

Survey efforts will continue to help officials better understand the distribution of Japanese apple rust in the state.

Although new to Minnesota, this rust has been in the northeastern U.S. for 10 years or more. It was found in Wisconsin in 2021. Gymnosporangium yamadae is native to parts of Asia where it is a significant pest of apples and can result in defoliation of susceptible cultivars.

“Discovering a new plant pathogen, like Japanese apple rust, is always a concern for our gardeners and farmers in Minnesota,” said Mark Abrahamson, Director of MDA’s Plant Protection Division. “However, as we’ve observed on the East Coast where Japanese apple rust has been prevalent for over a decade, there have been no significant impacts to apple production. We are cautiously optimistic that the same will be true in Minnesota.”

Japanese apple rust requires two different host plants to complete its life cycle, juniper and apple or crabapple. The MDA has observed leaf spots on many different varieties of both crabapple and apple in Minnesota.

The leaf spots may be deep red, orange, or yellow with a red ring. Finger-like projections, which are fungal spore producing structures, can be seen on the lower leaf surface just below the spots. The spores produced in leaf spots on apple and crabapple trees do not reinfect apple trees. Rather, these spores are carried by wind and infect junipers.

Japanese apple rust has not yet been observed on junipers in Minnesota. Infected junipers have small woody galls that produce a bright orange gelatinous fungal mass during spring rains. Spores produced in juniper galls are carried by wind and rain to infect the leaves of apples and crabapples.

Minnesota’s native juniper, the eastern red cedar (juniperus virginiana), is not susceptible to Japanese apple rust. Several junipers commonly used in landscapes in Minnesota are hosts, including Chinese juniper (Juniperus chinensis), Savin juniper (Juniperus sabina), Sargent juniper (J. chinensis L. var. sargentii), flaky juniper (J. squamata), and garden juniper (J. procumbens).

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