Question
I have an ash tree that is about 7 years old.  I recently noticed several leaves with a splotch of bright orange dust on them.  What could this be?

Answer
Sounds like Ash Leaf Rust. The "dust" will be short lived, al the tree will recover.
SYMPTOMS AND DISEASE CYCLE:
Ash leaf rust is one of the most spectacular rust diseases of trees in the Northeast. The fungus, Puccinia sparganoides, is a heteroecious rust that requires two different kinds of host plants to complete its life cycle. These are several species of ash (Fraxinus spp.) and several cord or marsh grasses (Spartina spp.). This disease appears to run in cycles of approximately 5-7 years.

Ash leaf rust is most severe along the New England coast where the fungus reproduces and overwinters on grasses in salt marshes. In spring, clouds of rust spores move inland from the marshes. Severe rust often develops on ash after periods of foggy weather with onshore airflow. In epidemic years, ash leaf rust has been noticed as far as 48 km inland. Infection of ash occurs on leaves, petioles, and green twigs during May and June. Leaves are conspicuously distorted and diseased tissues swell, often causing sharp bends in the petioles and elliptical, wart-like galls on green twigs. Spots on leaves may enlarge to several millimeters in diameter. Heavily infected trees can look scorched as petiole infections lead to withering and browning of leaves in early summer. Defoliation can result when infection is severe. Spores of this rust fungus appear as bright orange or yellow, powdery masses in minute, cup-like structures. These spores are incapable of reinfecting ash trees, but reinfect the marsh or cord grass hosts in July and August. The fungus multiplies and builds up on these grass hosts in late summer, often reaching epidemic levels since rust infections on these hosts are scarcely noticed. The rust fungus then overwinters in the marshes on these grasses. In spring, the cycle begins again as spores produced on the grass hosts move inland to infect the ash hosts.

CONTROL:
Ash leaf rust is rarely destructive enough to warrant special control measures. Defoliation may be heavy in some years, but it usually occurs in early summer and is thought to do no significant damage to otherwise healthy trees. However, successive years of severe rust infections can weaken trees, make them susceptible to winter damage, and cause dieback. Infected trees should be fertilized and watered during periods of drought to promote vigor and growth.