Cedar Apple Rust
Cedar apple rust affects crabapples and junipers, as they are alternating hosts.
What Is It?
Cedar apple rust is the result of the plant pathogen Gymnosporangium Juniperi-virginianae. The cedar apple rust pathogen can be highly disfiguring, causing discoloration and growths to occur.
As the name implies, cedar apple rust appears in apples, ornamental crabapples (both in the Malus genus), as well as the Eastern Red Cedar (Juniperus Virginiana). Closely related diseases (cedar hawthorn rust) have been known to appear in hawthorn trees.
How To Identify:
Apples & Crabapples: Infection appears on the leaves, fruit, and young twigs. Small, yellow spots will appear on the upper surface of infected areas. These spots later turn orange, red, or black. Late in the summer, horn-like structures develop on the undersides of the leaves.
Junipers: Infection first appears by producing several reddish-brown galls that are less than an inch in diameter. At maturity, these galls will start developing multiple small horns in a gelatinous mass. These horns will elongate and look like bright orange tendrils.
As described above, this disease alternates between two different hosts. Spores that mature on crabapples affect junipers, while spores that mature on junipers affect crabapples. Cedar apple rust over-winters on junipers in the form of stem galls. In mid-spring, these galls develop multiple orange horns – usually after a warm rain – and from these horns the fungal spores are released. The spores are carried through the air. Once the spores attach to their new host, yellow spots will begin to appear in late spring. At maturity, the infections on the upper sides of the leaves will develop tube-like structures on the underside of the leaves. From these tube-like structures, spores will be released and infect a new juniper host. Once attached to the new host, they will form stem galls and the cycle begins again.
If Left Untreated:
If the disease cycle is not interrupted, cedar apple rust can greatly affect the appearance of the crabapple trees in the landscape. It can also cause small amounts of stem die-back on junipers.
We can control cedar apple rust by applying both a contact and a systemic fungicide spray. To break this disease cycle, we prefer to spray just prior to when the cycle affects crabapples. The timing is very similar to the way we control apple scab, which also affects crabapples. With this treatment method, we can effectively control multiple diseases during our visit.