Cedar Apple Rust (CAR) is a fungal disease affecting many ornamental and fruit trees. Caused by the fungus Gymnosporangium juniperi-virginianae, its striking symptoms often raise concerns among homeowners and gardeners about the long-term health and survival of infected trees. This article clarifies the nature of this unique fungus and addresses the severity of its impact, specifically whether Cedar Apple Rust leads to tree death.
The Dual-Host Nature of Cedar Apple Rust
Cedar Apple Rust (CAR) is classified as a heteroecious rust, meaning the fungus requires two completely different host plants to complete its two-year life cycle. The first host is typically a juniper, such as the Eastern red cedar (Juniperus virginiana), where the fungus overwinters. The second host is a plant in the Rosaceae family, primarily apple or crabapple trees (Malus species).
The fungus spends 18 to 20 months on the juniper host, forming distinct growths called galls. In early spring, these galls swell and produce gelatinous, orange, horn-like structures that release basidiospores. These spores are carried by the wind to infect the newly emerging leaves and fruit of susceptible apple and crabapple trees. The cycle must alternate between the two distinct hosts; the fungus cannot spread directly between two trees of the same species.
Assessing the Threat: Does CAR Cause Tree Death?
For most mature trees, the rust fungus rarely kills established trees directly. The primary damage inflicted by Cedar Apple Rust is defoliation on the apple host and cosmetic galls on the juniper host. Junipers, including Eastern red cedar, are highly tolerant of the disease, and the galls do not cause significant harm to the evergreen’s health.
The apple or crabapple host suffers more noticeable consequences, particularly premature leaf drop. Repeated, severe defoliation over consecutive seasons reduces the tree’s ability to photosynthesize and store energy. A chronic infection weakens the tree, making it susceptible to secondary pests, diseases, or environmental stressors like drought, which can lead to decline or death. This indirect process is primarily a concern for young or already stressed trees, not healthy, established specimens.
Recognizing the Symptoms on Apple and Juniper Hosts
Identifying the specific symptoms of Cedar Apple Rust is the first step in managing the disease, and the signs differ significantly between the two host types. On apple and crabapple leaves, the first visual evidence appears shortly after bloom as bright yellow-green spots on the upper leaf surface. These spots enlarge, developing concentric rings of color, eventually turning bright orange or red with a distinct red border.
Later in the summer, small, raised, black dots form in the center of the spots on the upper leaf surface. Tiny, hair-like tubes develop on the underside of the leaf, releasing a second type of spore that travels back to infect the juniper host. Infected fruit may also exhibit yellow-orange lesions, causing the fruit to become deformed or drop prematurely.
The symptoms on the juniper host are most dramatic during wet spring weather. The fungus forms reddish-brown, golf-ball-sized galls on the twigs over an 18-month period. When the galls mature, they swell and erupt with striking, orange, gelatinous horns. These horns are the spore-producing structures; after releasing their spores, they dry out, collapse, and fall off the branch.
Managing and Preventing Future Infections
Effective management of Cedar Apple Rust relies on preventative cultural practices and timing. Homeowners can disrupt the fungal cycle by physically removing the sources of spores. This involves pruning the galls from juniper trees during late winter or early spring before the orange gelatinous horns emerge.
Raking and destroying infected apple leaves in the fall is a helpful cultural control method, as it reduces the fungal material available to infect junipers later. For chemical control, preventative fungicide applications containing ingredients like Myclobutanil are effective. They must be applied to the apple or crabapple tree in early spring when the buds are first emerging, coating the susceptible new growth and preventing wind-blown spores from germinating.
A long-term strategy involves planting resistant varieties of both apple and juniper species, which are genetically less susceptible to the fungus. Because spores can travel long distances, simply removing a nearby host may not be fully effective. Separating susceptible apple trees from juniper hosts by several hundred yards can significantly reduce the severity of the infection pressure.