Farmers, consultants, and university extension agronomists will all agree that problems with crop growth of some sort will develop in most fields this year. These problems will most likely be caused by too much or too little fertilizer, too much or too little water, temperatures too high or low, crust over the planted row, insects, diseases, herbicide drift or carry over, and combinations of these.
To enhance plant health, growth, and subsequent yield, the cause(s) of these problems should be diagnosed quickly so action can be taken to reduce the problem from becoming worse this year or prevent it from developing next year. Diagnosis of crop problems can sometimes be easy, but it is more often difficult. Dr. Laura Sweets, Dr. Wayne Bailey, Dr. Kevin Bradley, and I developed University of Missouri Extension Guide G4050 that contains information to help individuals more quickly diagnose the causes(s) of field crop problems, and it is online at http://extension.missouri.edu/explore/agguides/crops/g04050.htm. This following is a very brief summary of the material in this guide.
First, determine the variety and the age of the plant. An investigator should identify the plant variety because some are more resistant or susceptible to certain diseases, insects, and herbicides, and this information may be very useful when diagnosing the cause of the problem.
Second, identify all the symptoms affecting the leaves, stems, roots and fruit. An investigator should observe all parts of abnormal plants when troubleshooting a field crop problem including the leaves, stems, fruit and roots as well as the tissue inside roots and stems. Frequently, the point of injury to the plant is not where the symptoms appear. For example, leaves on one or several branches may be discolored and withered because of a canker on a lower branch or a borer in the stem. Nutritional deficiencies and injuries from herbicides may damage both roots and leaves. Examine individual plants in detail and determine the location of symptoms on the plant. Are symptoms on old or young leaves, upper or lower stems, or perhaps on one side of the plant? Look for insects and insect feeding damage. Cut stems to check for discoloration inside the stem and for insect feeding. Hold leaves up to the light to check for mosaic, other viral symptoms, or the presence of webbing and mites. Investigators should look for leaf abnormalities in color, size, shape and texture. Also, carefully dig up roots and examine them. Check for galls, rot, abnormal root color and feeder root condition, and assess root growth. While probing the soil, check for soil compaction, soil structure, texture and organic matter, and the presence and depth of hardpans. Also take note on fertilizer placement and the depth of planting and other recently completed cultural practices.
Third, estimate the percentage of plants damaged in the affected part of the field. Were all plants in an area or only 10 percent affected? Symptoms of injury due to insects and disease may appear on every plant in an area, but this is unusual. Symptoms of injury due to herbicides, improper placement of fertilizer, and lightning will usually appear on every plant in an area.
Fourth, determine the distribution or pattern of the problem in the field. Look at the entire field to determine where the problem appears. Determine the distribution of the problem in the field as it relates to field characteristics such as areas with light soil, and drainage patterns. Is the problem only in wet areas? Take notice of whether the problem is associated with certain rows or areas of lower or higher elevation.
Fifth, evaluate whether the crop and weeds in the field share similar symptoms. Examine the weeds in the area where the crop is injured and in nearby fence rows. Symptoms caused by nutritional disorders are usually not plant specific. For example, most plants growing in low-pH soils, including crops as well as weeds, will be stunted. However, diseases are usually plant specific, and weeds in the area are normally not affected by the same diseases that can attack corn or soybean.
Sixth, determine the history of the problem. Ask when the problem was first noticed, and whether crop problems were observed in the same area during previous growing seasons.
The answers to these questions may provide clues that could be useful in diagnosing the causes of field crop problems.
REVISED: October 11, 2011