What is responsible for the yellow spots on my crabapple leaves?

FAQ
Question: 

What is responsible for the yellow spots on my crabapple leaves?

Answer: 

Cedar-apple rust is likely responsible for the yellow spots on the crabapple leaves.  Cedar-apple rust is a fungal disease.  The fungus  requires both a crabapple or apple and a cedar (juniper) to complete its life cycle. Crabapple and apple cultivars differ in their susceptibility to cedar-apple rust.  Some cultivars are susceptible, others are resistant.  

On cedar, the fungus produces reddish brown galls that are up to golf-ball size on young twigs. During wet spring weather these galls swell and begin to push out bright orange gelatinous tubular structures.  Wind carries fungal spores from these gelatinous structures to susceptible crabapple or apple cultivars.  On crabapples and apples, cedar-apple rust produces yellow spots on the foliage.  Cedar-apple rust on crabapples and apples is most severe when there are frequent rains in spring.

Fortunately, cedar-apple rust does not cause serious harm to crabapples.  No control measures are necessary.