Prominent orange spots that have appeared on the leaves of some of your crab apple and apple trees may look interesting, even decorative, but they're no good for the plant.
They're symptoms of cedar-apple rust disease. And if you think rust is decorative, enjoy it now because things are going to get ugly.
The fungus culprit is growing, and leaves are going to get dead spots as well as ugly blobs on their undersides at each point of infection. Furthermore, those dead spots can't use sunlight to make food, and infected leaves tend to fall before their time. The result is a weakened tree or, in an extreme case, a dead tree.
Don't bother reaching for a pesticide, because it won't do any good now.
The 'cedar' in the name cedar-apple rust indicates that this disease needs two host plants - a susceptible cedar and an apple - to survive. To complete its life cycle, the fungus has to hopscotch back and forth between the two plants.
Cedar trees sent out the spores that infected crab apples and apples during a brief period this spring. Back then you may have noticed some brown, marble- to golf ball-sized swellings on cedar branches. With warm, wet weather these swellings grew gelatinous orange tentacles that shot out spores in search of apple leaves.
Toward summer's end, crab apple and apple trees return the 'favor' and, from more sedate-looking swellings on the undersides of their leaves, send a different kind of spore back to infect cedars.
'Chop down cedar trees!' might be the battle cry of cedar-apple rust fighters. But such carnage would have little effect because spores can waft for miles from cedars in search of apples.
A more effective offensive might be to create an environment within your crab apple or apple tree that is inhospitable to cedar-apple rust. Give trees a sunny site and prune to expose branches to drying breezes.
Sprays will work against cedar-apple rust, but only certain pesticides and then only if applied at the correct rate and time. Crab apples and apples pick up infections only from the time fruit buds just begin to show until a couple of weeks after bloom.
The best way to deal with cedar-apple rust is to avoid it altogether by planting a rust-resistant variety of crab apple or apple.
Rust-resistant crab apples include Adams, Golden Gem, Beverly, Centennial, Snowdrift and Profusion.
Rust-resistant apples include Liberty, William's Pride, Nova Easy Grow and Macfree.
Bonus point: Cedar-apple rust is not the only serious disease threatening crab apples and apples, and the above varieties are resistant to the other major diseases, too.