Source: Iowa State University
There have been several recent reports of leaf spots showing up on corn in addition to the eyespot and Goss's wilt reported earlier this growing season in Iowa. Some of these are expected — Northern corn leaf blight and gray leaf spot, but some such as southern rust are a little less common, and others are just weird.
Identifying characteristics of gray leaf spot are rectangular lesions that start on the bottom leaves of the plant. Gray leaf spot can severely impact yield. Susceptibility varies among hybrids. Infection is favored by warm, muggy conditions.
Large, cigar-shaped lesions are characteristic of northern corn leaf blight. Usually this disease also starts on the lower leaves of the plant, but in 2009, disease started high in the plant canopy.
Southern rust was reported by Pioneer Hi-Bred in southeast Iowa last week. Southern rust can develop rapidly, so it is always a good idea to keep a watchful eye on this disease.
Other spots seen in corn include pinhole punctures caused by the adult corn blotch leafminer (CBL). These flies deposit eggs in the leaves and the larvae create transparent "window-pane" strips that can be confused with disease lesions.