Symptoms of the first wave of corn leaf disease are subsiding in most of the state, but symptoms of mid-season diseases will appear soon. The wet spring resulted in very prevalent anthracnose and holcus leaf spot. Recent drier weather should slow down the development of these two diseases.
Early symptoms of gray leaf spot.
Holcus leaf spot is caused by a bacterium, Pseudomonas syringae. The bacterium overwinters in crop residue and splashes onto leaves, where it infects the plant by entering leaf stomata. Symptoms are round spots, initially water-soaked, that turn yellow and then a light tan. This disease is often mistaken for contact herbicide injury because of the appearance of spots. It usually appears on lower leaves following wet weather. This disease also is prevalent in Illinois and Indiana this year.
Anthracnose, caused by the fungus Colletotrichum graminicola, has become a familiar disease in Iowa and is usually the first leaf disease to appear. Lower leaves develop oval spots one-eighth to one-half-inch long that start out yellow but turn brown and have a darker brown or purplish border. Sometimes a yellowed zone can be seen outside the dark border. Spots often appear along the leaf margin and can grow together, causing the entire leaf to wither. The fungus produces its spores on these spots, and the spots also contain tiny black spines that can be seen with a hand lens.
Both diseases probably will not cause economic damage, especially with drier weather. Holcus leaf spot rarely persists through mid-season and no control measures are warranted. Anthracnose leaf blight can cause yield loss if it persists and moves up the plant, which seems unlikely at this point.
Other mid-season leaf diseases also will be appearing soon, or have already been observed. Symptoms of common rust and eyespot are being seen already. Symptoms of gray leaf spot usually appear around the middle of July, usually as yellow flecks that turn into small, angular, tan spots. The long, rectangular shape does not develop until later. If these diseases are appearing on susceptible seed corn inbreds, it might be beneficial to control them with a fungicide (For more information, see an article on page 115 in last year's ICM newsletter).
Generally, there is no immediate treatment recommended for these diseases in field corn. Management includes use of partially resistant hybrids, crop rotation, and tillage. However, it may be profitable in some hybrid fields to control gray leaf spot with a fungicide. Tilt is the only product specifically labeled for gray leaf spot. At today's corn prices, a 5- to 6-bushel yield increase would pay for a single application.
It is feasible to obtain a yield response in this range or greater when treating gray leaf spot with a fungicide. In field trials last year, we had yield increases of 0 to 27 bushels/acre (depending on location) with a single application at tasseling. The trick is predicting which fields will have enough disease to benefit from a Tilt application. We do not have adequate data to accurately predict this, but reduced-tillage, corn-on-corn fields planted with a susceptible hybrid in southeast Iowa would be the most likely fields to show an economic benefit (if symptoms are present at tasseling). A single application at tasseling is probably most cost-efficient. Tilt cannot be applied after 50 percent of the plants have silked.
This article originally appeared on pages 124-126 of the IC-476(18) -- July 15, 1996 issue.