Cedar-apple rust is a disease caused by fungi of the genus Gymnosporangium. It affects the following common orchard and ornamental trees, as well as the invasive eastern red cedar:
- Crab apple.
Due to its complicated life cycle, this disease is dependent on both deciduous and coniferous trees to survive:
- Galls on junipers and eastern red cedars release spores in the spring in response to warm rains.
- Spores travel by wind, sometimes as far as two miles, to infect deciduous host trees.
- Spores are released from leaves of deciduous trees in the summer (typically June and July in Kansas).
- Spores travel by wind to reinfect junipers and cedars.
On deciduous trees:
- Reduced tree vigor.
- Round spots on leaves, with the top side yellow and the bottom brown.
- Cup-shaped structures on leaves.
- Yellowed leaves.
- Leaf drop.
- Reduced fruit production.
- Brown spots on fruit.
- Cracked or misshapen fruit.
- Premature tree death.
On junipers and cedar:
- Brown round galls hanging from branches.
- Orange gelatinous “horns” on galls.
Treatment consists of spraying fruit trees with fungicides designed to control cedar-apple rust. Organic producers can use copper or sulfur sprays. Garlic sprays also show promise.
Make it a habit to check your ornamental cedars and junipers from time to time and pick off all the galls.
To prevent the further spread of the disease, rake up all fallen leaves and needles and destroy them.
Ideally, susceptible fruit trees should not be kept on the same property as cedars and junipers. For ornamental evergreens, use species that do not get cedar-apple rust if you intend to grow apples. Destroy all eastern red cedars within half a mile of the orchard if possible. A two-mile radius is even better. If this is not possible, select apple and crab apple varieties that are resistant to the disease.
Another important aspect of preventing cedar-apple rust is maintaining general plant vigor. Be sure that each tree receives enough water and nutrition.