Does the level of Corn Disease justify spraying?

The answer here is no, unless you are working with a susceptible hybrid or variety and the disease is present. The most damaging diseases for Ohio would be Gray Leaf Spot and Northern Corn Leaf Blight. This year does seem to be providing the proper environment for rust and we are seeing high levels reported on popcorn and sweet corn which have low resistance to rust. Most all of the field corn have a good rust resistance built in. Information on corn rust can be found in the factsheet AC 0031-01 Common Corn Rust found below. All other scouting results have shown other common diseases to be at very low levels.

So why spray corn with fungicides? Bob Nielson, Purdue University Corn Agronomist has put together a compilation of some articles on this topic from around the country in an article entitled "Corn Fungicide Hoopla 2008". Dr Nielson provides the definition of hoopla and then links to several articles addressing the third party suggestions and evaluations of the practice.
If you want to get right to the yield data the best summary of midwestern data is assembled by the University of Illinois Making Profitable Fungicide Applications in Corn.

Common Corn Rust

AC-0031-01

Common corn rust rarely causes significant yield losses in Ohio field (dent) corn. However, occasionally field corn has been damaged when environmental conditions favor the development and spread of the disease. Sweet corn is generally more susceptible than field corn. Common rust can usually be found in corn fields sometime during the growing season, but generally does not appear before tasseling. In those years with exceptionally cool summers, and especially on late-planted fields or sweet corn, yield losses may be expected when the leaves above the ears become severely diseased.

Symptoms

Common rust of corn can be easily recognized by the development of dark, reddish-brown pustules scattered over both the upper and lower surface of the corn leaves. Pustules appear oval to elongate in shape, are generally small, less than 1/4 inch long, and are surrounded by the leaf epidermal layer, which appears as a whitish covering. These pustules may appear on any above ground portion of the plant, but are most abundant on the leaves.

Causal Fungus

Puccinia sorghi is the fungus causing common rust in corn. The reddish-brown color of the pustule is actually the coloration of the repeating spore or uredospores. These spores are produced throughout the summer and infect new leaf tissue and are responsible for the spread of the disease. As the corn plant matures the pustules turn a brownish-black color due to the development of the darker pigmented spores or teliospores. In tropical regions teliospores infect the rust's alternate host, wood sorrel (Oxalis species). In temperate areas, including the U.S. cornbelt, the fungus does not infect wood sorrel and the teliospores have no real function.

Disease Cycle

The rust fungus overwinters as uredospores on corn or as other spore forms on wood sorrel in subtropical and tropical regions. Spores capable of infecting corn are blown northward during the growing season and become established on the corn crop. Rust development and spread is favored by prolonged cool temperatures ranging from 60° to 74°F and high relative humidity. When these protracted cool, humid conditions exist, susceptible corn hybrids and sweet corn varieties can become seriously affected.

Control

Although rust is frequently seen on corn in Ohio, very rarely has there been need for control on field corn. In field corn, highly resistant hybrids are available and most hybrids possess some degree of resistance. Pop and sweet corn can be quite susceptible. In seasons where considerable rust is present on the lower leaves prior to silking and the weather forecast is for unseasonably cool, wet weather, chemical control may be necessary. Fungicides are available for rust control. Consult label recommendations for rates and application timing. Early application is necessary for effective disease control.