Those of you with crabapples may have noticed bright yellow spots like those pictured above appearing on leaves. Director of Urban Forestry Marc Welch thinks it’s the beginning stage of cedar apple rust, a common occurrence on crabapples and mostly a cosmetic issue, not a threat to tree survival. He recommends keeping up with weekly watering, and raking and disposing of leaves when they fall, to limit spread of the fungus.
Cedar apple rust is an interesting fungal disease because it requires two hosts to complete its two-year life cycle. Spores produced on cedar trees infect apples and crabapples, and vice versa. You can read more about it on the Missouri Botanical Garden and Cornell University websites, among others.
We’ve seen this before on our Newton Tree Conservancy crabapples. This photo from 2011 shows a later stage, the horn-like structure that grow on leaves.