Pathogen(s) causing disease:

 Puccinia recondita f. sp. tritici

Symptoms:

Leaf rust causes very small (about 1/32 inch long by 1/64 inch wide), orange pustules that erupt through the leaf surface. In some cases, pustules are surrounded by a narrow yellow or white halo. The pustules contain masses of powdery orange spores of the rust fungus. Spores may spill out of pustules and form a grainy orange dust on the leaf surface around the pustule. As leaves age, pustules begin to produce dark black spores instead of orange spores. These black pustules look like tar spots and are most easily seen on the lower leaf surface and leaf sheaths. Leaf rust pustules occur randomly across the leaf; this distinguishes leaf rust from stripe rust, which has narrow yellow stripes of pustules. Leaf rust typically occurs uniformly across a field.

Conditions:

 Infection favored by moisture and cool temperatures (59-72 F).

Management:

Resistant varieties, seed treatment fungicides, and plant early-maturing cultivars. Controlling volunteer wheat in the summer reduces leaf rust inoculum, but it does not guarantee freedom from rust blowing in from distant sources. Delayed planting reduces fall rust infections, but delayed maturity in the spring may lead to higher final rust levels.

Sources:

Kansas State University
Purdue University