Cedar-apple rust is the most common and economically important rust disease of apple in Kentucky. Symptoms of this disease are beginning to appear across the state. The pathogen overwinters as galls on cedar and juniper. Removal of these pathogen sources on cedar can later reduce disease incidence on apple. Once apple trees become infected limited management options are available.
Cedar-Apple Rust Facts
- Leaf symptoms begin as small, pale yellow spots on upper leaf surfaces (Figure 1). Spots enlarge to become bright yellow-orange, often with reddish borders (Figure 2). In late spring or early summer, small orange-yellow tubular fruiting bodies project from lower sides of leaf spots (Figure 3). Severely infected leaves may drop prematurely. Infected fruit exhibit similar symptoms with spots appearing near the blossom end; however, spots on fruit are much larger than those on leaves. Fruit may also become deformed and drop prematurely.
- Cedar-apple rust in known to infect apple, crabapple, hawthorn, mountain ash, pear, and serviceberry.
- The pathogen requires more than one host to complete its life cycle. Cedar and juniper serve as alternate hosts. In mid-late summer, spores from apple spots are blown to cedar and juniper. Infection results in the formation of galls (swellings), which expand over a period of 2 years before producing spores that infect apples.
- Primary infection of apples begins in early spring during leaf expansion and pink bud stage. Infections can continue until 30 days after bloom.
- Damp conditions with temperatures between 50-60°F favors disease development.
- Caused by the fungus Gymnosporangium juniper-virginianae
- Select apple and juniper cultivars that are resistant or immune to cedar-apple rust.
- Maintain plant health with proper nutrition and irrigation practices.
- Destroy nearby unmanaged, abandoned, or wild hosts.
- Prune and destroy galls found on juniper and cedar.
- Fungicides may be used preventatively beginning at pink-bud stage through 30 days post- Homeowners may use fungicides that contain myclobutanil or mancozeb. Always follow label directions when utilizing fungicides.
- Apple Rust Diseases (PPFS-FR-T-05)
- Backyard Apple Disease & Pest Management Using Cultural Practices (with Low Spray, No Spray & Organic Options) (PPFS-FR-T-21)
- Commercial Midwest Fruit Pest Management Guide (ID-232)
- Disease and Insect Control Programs for Homegrown Fruit in Kentucky including Organic Alternatives (ID-21)
- Fruit, Orchard, and Vineyard Sanitation (PPFS-GEN-05)
- Simplified Backyard Apple Spray Guides (PPFS-FR-T-18)
By Kim Leonberger, Extension Associate and Nicole Ward Gauthier, Extension Specialist