The field is a complex environment with many factors that can interact to influence the growth of a corn plant. These factors can be in the form of insects, diseases, nematodes, and weeds (biotic); or, they can be factors such as weather, nutrients, or chemicals (abiotic). Under optimum conditions, production of field corn can exceed over 300 bushels of corn per acre with little or no stress; under totally unfavorable conditions, every corn plant can die.
This guide was prepared to help identify problems so the proper corrective measures can be taken to minimize or prevent yield losses. For any corrective action to be successful, early detection is essential.
The Florida Extension Plant Diagnostic Clinics can help determine disease and insect diagnostic problems that cannot be easily identified. The clinic works primarily through UF/IFAS Extension offices in Florida. You may contact your local UF/IFAS Extension office with your unidentified disease or pest problem.
There are also websites where pictures can be found, such as https://fyi.uwex.edu/fieldcroppathology/files/2015/06/Corn-Diagnostics-Quick-Guide.pdf; http://extension.uga.edu/publications/detail.html?number=B1221; https://www.aganytime.com/dekalb/tools/Documents/CornDiagnosticGuide.pdf. Some problems (pest or nutrient) have similar symptoms and, therefore, observation alone as described in this document is not a definitive diagnosis, but serves as an initial guide.
Growth Stage Definitions
Before Emergence (Seed planted)—Germination of seed may require from 4 to 30 days depending upon soil temperature.
Two Leaf Stage (Two leaves fully open, collar visible)—Leaves have emerged, but the growing point is still below the soil surface.
Early Whorl (4–6 leaves fully emerged)—Plants are in the 5 leaf stage and larger.
Mid Whorl (8–10 leaves fully emerged)—This is a period of rapid leaf formation and the beginning of rapid stalk elongation. The tassel and ear shoots are developing.
Late Whorl (12–16 leaves fully emerged)—Leaf enlargement is complete, and brace roots are developing. The potential number of kernels on the ear is determined by this time. The stalk is rapidly growing, and the tassel is almost full size.
Tasseling (Tassel emerging)—Final stalk elongation occurs during this stage.
Early Silk (Silks emerging, pollen shedding)—The tassel is fully emerged, and stalk development is complete. The ear shank and husks are growing rapidly. Ovules are enlarging, and the silk from each ovule is near the tip of the ear and emerging. The number of ovules that will be fertilized and develop into kernels is being determined at this stage.
Blister Stage (Brown silk, cob full size, watery kernels)—The cob, husk, and shank are fully developed. Starch has begun to accumulate and the kernels are rapidly increasing in size. The plants continue to absorb soil nitrogen and phosphorus, but much of these nutrients are being supplied from other plant parts. High water uptake period for grain fill.
Soft Dough (Kernels milky with some starch)—Starch is accumulating and embryo formation in each new kernel is underway.
Hard Dough (Few kernels with dents)—Embryo growth is rapid, and kernels are denting.
Physiological Maturity (Black layer formed, grain mature and drying)—Dry matter accumulation has ceased, and moisture loss begins. The husks and some leaves are usually no longer green. Most kernels are dented, and "black layer" formation is complete. Moisture is between 30%–35%.
Diagnosis of General Plant Problems by Growth Phase.
Herbicide injury guide.
Guide to nutrient deficiency symptoms of corn.