Most Un-Wanted: Cedar–Apple Rust
August 8th, 2010
Wanted For: defoliating apple leaves & for just being gross!!
Description: bright yellow-orange-red spots on apple leaves and orange galls with ‘horns’
Hangout: juniper (eastern red cedar) & apple trees including crabapples
Kryptonite: Lime –Sulfur Spray, Home Orchard Spray
The fungus that causes this disease moves back-and-forth between Eastern red cedars (actually junipers) and both fruiting & ornamental apple trees. In order to complete its life cycle this fungus must spend part of its life on both. Therefore, it is possible to eliminate the disease by eliminating the cedars within the given area of the apple tree. However, the spores can be wind borne up to 2-3 miles, so eradication of the disease is often impossible or impractical. Nonetheless, if cedars are not too numerous on your property, the removal of junipers around the immediate apple tree can certainly reduce the spores reaching the apple foliage & fruit.
The spores drift through the air mostly at night and early morning. If it lands on the apple’s leaves & fruit and remains wet for 4-6 hours, the spores infect the leaves of the apple tree. After several weeks, yellow spots develop on the infected apple leaves and fruit. By mid-summer the spots appear slightly raised and light-colored & are visible on the underside of the leaf. Spores are released and the wind carries them to nearby junipers. Infected juniper needles and twigs show no symptoms of infection initially. Inspect them closely the next spring for small, green swellings (immature galls). The galls reach full size by the end of the next growing season but do not mature and develop the orange gelatinous tendrils until the following spring (about 20 months after infection.) This bizarre-looking structure is actually the fruiting stage of the fungus. The “horns” release the spores that infect the apple trees.
Many sulfur fungicides are effective against rust. Begin spraying in spring and repeat throughout the summer as needed. Spraying the juniper is not recommended because cedar-apple rust rarely causes significant damage to this host. Prune dormant galls from juniper during the fall, winter and early spring before the orange tendrils begin to erupt from galls.
planting for the future.
-By Maggie Oldfield Thayer Nursery
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