Question: What disease affected my apple trees last year and what do I do about it?
Diagnosing plant diseases can be challenging, but in this case, there are distinctive clues to help you find the answer! If cedar-apple rust is the culprit, you can see the evidence on red cedar trees even in the winter time. And, early spring will show evidence on your apple trees.
If the leaves on your apple tree are exhibiting yellowish to orange spots or you find black spots on the back of the leaves, your tree is likely infected with cedar-apple rust. Cedar-apple rust is caused by a fungus that spends parts of its life cycle on two hosts: apple trees and red cedar trees. The fungus, Gymnosporangium juniperi-virginianae, can infect the leaves and fruit of most apple cultivars in the eastern US, with the exception of the nearly immune ‘Delicious’ apple tree.
Cedar-apple rust appears on apple fruit first as bright orange, slightly raised lesions (tissue damage), but may become brown and cracked as the fruit grows. Usually some of the orange color remains at harvest. Damage to the fruit is less common than on the leaves. Stem infection causes a slight swelling of the stem and may result in fruit drop.
On the red cedar tree, cedar-apple rust produces brown, globular galls (swellings on exterior of plant) ranging in size from 1/4 inch to nearly 2 inches in diameter. These galls are dimpled like a golf ball in the dormant season but produce gelatinous, orange spore horns (horn-like protrusions) during spring rains. A video produced by Virginia Cooperative Extension shows how to recognize cedar-apple rust on your cedars. Check out the video here.
The fungus overwinters on galls on the cedar tree. Rainy weather in the spring keeps galls wet, promoting growth of the spores (the reproductive bodies of the fungus), which are carried to the apple tree, infecting leaves and fruit during extended wet periods. Lesions appear 10 to 14 days after infection. In late summer, spores produced on the lower surface of infected apple leaves re-infect foliage of nearby cedar trees. These infections develop into galls, which produce spores in the spring following the next full growing season, well over a year after the initial infection. A cedar-apple rust gall produces spores for only one season. All the lesions seen on the apple tree result from spores produced on the cedar, with no secondary infection of the apple tree.
To control the disease on your apple trees, consult the 2021 Pest Management Guide from the Virginia Cooperative Extension for recommendations on a fungicide that targets cedar-apple rust. Protectant sprays are required periodically from the pink stage of bud development (when the buds swell and turn pink) through early summer to protect the emerging leaves and developing fruit. Removing cedars located within a two-mile radius of the orchard interrupts the life cycle of the fungus and makes control with fungicides easier. Removing all cedars within four to five miles of the orchard provides complete control. By the way, control of cedar apple rust made its way into American jurisprudence through an early 20th century U.S. Supreme court case, Miller v. Schoene. A discussion of the case can be read here.
References:
“Cedar Apple Rust,” Adapted by Alan R. Biggs, West Virginia University, Aug 22 2019.
“Cedar Apple Rust,” Sara Villani, NC State Extension, Apple Pathology Factsheets, June 6 2018.
“Pest Management Guide: Home Grounds & Animals,” Virginia Cooperative Extension, 2021.
