Cedar Apple Rust (CAR) is a common fungal disease caused by the pathogen Gymnosporangium juniperi-virginianae. The fungus requires two distinct host plants to survive: an evergreen from the Juniperus genus, such as the Eastern Red Cedar, and a deciduous member of the rose family, typically apple or crabapple trees. The disease manifests as visually striking, bright orange growths on the cedar host and vibrant yellow-to-red spots on the apple host, a sight that often prompts concern among homeowners.
Direct Assessment of Human and Pet Health Risks
The primary concern for Cedar Apple Rust is exclusively horticultural, as the fungus poses no known health threat to humans or common household pets. The fungus is an obligate plant pathogen, meaning it only survives by feeding on living plant tissue and cannot infect or thrive in mammalian hosts. This biological specialization ensures the fungus is non-toxic and non-infectious to people. Contact with the galls or the fungal spores is not known to cause skin irritation or allergic contact dermatitis in the general population.
While the spores are released into the air and may be inhaled, they are generally considered non-allergenic, unlike common molds that trigger respiratory issues. However, individuals with severe pre-existing sensitivities to airborne fungal spores may experience mild, temporary allergic reactions similar to hay fever. For household pets like dogs and cats, the fungus itself is not toxic. Ingestion of a large quantity of any fungal mass, including the gelatinous cedar galls, carries a small risk. This risk is limited to mild gastrointestinal upset, such as vomiting or diarrhea, rather than any systemic poisoning.
Identifying Cedar Apple Rust and Understanding Its Life Cycle
The appearance of Cedar Apple Rust is dramatically different depending on which of its two required host plants it is currently inhabiting. On the juniper or cedar host, the fungus overwinters inside hard, woody, spherical galls often referred to as “cedar apples.” These galls can range from half an inch to two inches in diameter and appear as lumpy, brown growths attached to the twigs.
During warm, wet conditions in the spring, the mature galls absorb moisture and extrude spectacular, bright orange, gelatinous horns called telial horns. These spongy structures, containing teliospores, can swell to several times the size of the original gall. The telial horns release basidiospores that are carried by the wind to infect the susceptible apple or crabapple foliage.
Once the basidiospores land on a young apple leaf, symptoms appear within one to three weeks. The initial sign is the development of small, greenish-yellow spots on the upper surface of the leaves, which rapidly enlarge and turn a vibrant orange-yellow color. These spots often develop concentric red bands, making them easy to identify. Later in the summer, tiny black dots (spermogonia) appear, followed by hair-like projections called aecia on the underside of the leaf. These aecia release spores that travel back to infect the cedar host, completing the two-year cycle.
Protecting Plants: Management and Prevention Strategies
Since Cedar Apple Rust poses no danger to health, management efforts focus entirely on protecting susceptible apple and crabapple trees from damage and defoliation. The most effective long-term strategy is to select and plant resistant varieties of both apple and juniper, which eliminates the disease cycle entirely.
Cultural controls involve disrupting the life cycle by physically removing the fungal mass from the landscape. Homeowners can prune out the woody galls from juniper trees during the late winter or very early spring before the orange gelatinous horns emerge and release their spores. Similarly, removing and properly disposing of infected apple leaves as soon as the yellow-orange spots appear can reduce the amount of spores that travel back to the juniper host later in the season.
To create an environment less favorable for the fungus, homeowners should focus on improving air circulation around susceptible trees. Pruning dense branches to open the tree canopy helps foliage dry more quickly after rain or morning dew, which is a key factor in preventing spore germination and infection. Avoiding overhead watering and ensuring proper tree spacing are also practical steps to limit the moist conditions the fungus requires.
Chemical management requires precise timing, as fungicides are preventative and must be applied to the apple trees to protect newly emerging foliage from the airborne spores. Application should begin during the apple tree’s “tight cluster” bud stage and continue at regular intervals, often every seven to fourteen days, until two weeks after the apple petals fall. Fungicides containing active ingredients such as myclobutanil are commonly recommended to protect the leaves and developing fruit during this susceptible period.