Article written byTiffany Enzenbacher, Researcher, enzen010@umn.edu; Dr. Matt Clark, Associate Professor; and Dr. Brandon Miller, Assistant Professor
August is the time to scout apple and crabapple trees for red star rust. Red star rust is caused by the fungal pathogen Gymnosporangium yamadae.
Red star rust was recently detected in Minnesota in 2022 and has since been reported in 11 counties surrounding the Twin Cities metro, including Anoka, Carver, Dakota, Goodhue, Hennepin, Le Sueur, McCleod, Ramsey, Rice, Scott, and Washington Counties.
This disease is unique in that it affects two unrelated types of plants – apple (Malus spp.) and juniper (Juniperus spp.). Both plants are needed for completion of the red star rust life cycle.
Apple and crabapple
Red star rust causes large spots or blotches on the upper leaf surface that are readily observed in mid to late summer. Brightly colored spots are typically irregular in shape with red borders. Bristly fungal projections develop on the underside of leaves in late summer. These structures contain fungal spores. Spores are blown in the wind to junipers.
The impact on apple and crabapple trees includes early defoliation, reduced plant vigor, a decrease in yield, and infections on fruit. Trees in the landscape, nursery, and orchard are susceptible to red star rust.
Images: Crabapple leaves with red star rust from left to right: Top: ‘Cashmere,’ ‘Dolgo,’ Firebird®; Bottom: Marilee®, ‘Prairifire,’ and ‘Purple Prince.’ The color of red star rust spots differs by cultivar. Cultivars with white flowers often have yellow spots. Cultivars with dark pink or red flowers usually have red spots.
Images: A red star rust spot on a Zestar!® ‘Minnewashta’ apple leaf in late summer (left). Long, tapered, fungal projections on the underside of the leaf (middle). Fungal projections on a ‘Flame’ crabapple leaf (right). Spores inside these projections are blown in the wind to junipers.
Juniper
Red star rust causes small (0.4 inches), woody galls on stems. A gall is a swelling or abnormal growth of tissue. Rain and warm spring temperatures signal orange, gelatinous projections to emerge from galls. These projections contain fungal spores, which are blown in the wind to apple leaves, continuing the red star rust life cycle.
Juniper hosts include:
Chinese juniper (Juniperus chinensis)
flaky juniper (J. squamata)
garden juniper (J. procumbens)
Sargent’s juniper (J. chinensis var. sargentii)
Red star rust causes yellow (chlorotic) needles, galls, and swelling of tissue on twigs and leaf bases, but there is no significant damage to juniper. Junipers in the landscape and nursery are susceptible to red star rust.
Images: Red star rust gelatinous spore projections, which emerge from small juniper galls in spring following a rain event. Note the broad, flat shape of the projections.
Management
Red star rust on landscape trees may be managed with cultural control methods. Do not plant juniper hosts within several hundred yards of apples. Keep apple trees properly pruned to promote air circulation within the canopy. Redirect sprinklers away from apple and crabapple leaves.
Small amounts of galls can be pruned out of infected junipers and treated as yard waste.
Preventative fungicide applications may be necessary in nursery and orchard situations.
Fungicides labeled for cedar apple rust may also treat red star rust. Red star rust is similar to our native cedar apple rust, which is caused by G. juniperi-virginianae.
Sterol inhibitor fungicides (S.I.’s FRAC code 3) and mancozeb are effective for rust management.
If you suspect red star rust, email MDA’s Report a Pest (reportapest@state.mn.us) with a location and photos.
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