An article on the Wallaces Farmer website today stated some farmers and crop scouts are reporting signs of potassium deficiency on plants in cornfields. They also reported some fields showing symptoms of anthracnose leaf blight.
Brian Lang, Iowa State University Extension field agronomist provided the following information.
As corn begins to move to its rapid growth phase (beyond V6 stage), you may see potassium, or K, deficiency 
symptoms show up on corn in localized areas of the field (see right hand image). Areas showing potassium deficiency are usually associated with two situations:
1) Soil K levels are low and initial seedling roots and/or
2) The mesocotyl had premature rotting and contributed little to the rooting area for absorption and movement of K to the topgrowth.
As the permanent root system advances in development, the symptoms of K deficiency will often subside, but not always. The following link provides a series of photos that walk you through a description of anthracnose and K deficiency symptoms as well as corresponding rooting problems found in some fields with these issues.
Anthracnose leaf blight is a disease of corn that’s fairly normal
Most cornfields will exhibit some scattered anthracnose lesions on lower leaves (see image below). Corn routinely grows through this stage, however, some fields may exhibit necrosis of entire leaf margins on the lower two to three leaves. One way to distinguish this symptom from K deficiency or leaf burn injury from 
postemergence herbicides is to look for the acervuli on the lesions. These are small black spiny structures that are visible with a hand lens on the dark-bordered lesions on the corn leaves.
If the field becomes a significant problem, the following management options are considered for the field’s immediate future:
- Crop rotation to a nonhost crop like soybeans, alfalfa, etc.
- Select a more resistant corn hybrid next year.
- Utilize tillage to bury crop residue which can harbor the fungus over winter.
Click here to read the full article.
Photos courtesy of Iowa State University