P&PDL Picture of the Week for
October 17, 2005

The wonderful world of crop forensics

Glenn Nice, Weed Diagnostician, Department of Botany and Plant Pathology, Purdue University and
Bill Johnson, Associate Professor of Weed Science, Department of Botany and Plant Pathology, Purdue University

Yield is the driving force for hybrid and cultivar performance and selection. This is the behind-the-scene thought when one considers using a particular cultivar or hybrid. An example of this is the selection of a disease resistant cultivar. A resistant cultivar is selected with the idea that yield will be optimized if that specific disease is expected to be an issue. So, it is no surprise that when something goes wrong with yield at the time of harvest, the questions start flying.

Causes of yield loss at harvest can be a tricky thing to pin down in most situations. Many things can result in crop performance issues and yield loss. Any stress that corn or soybeans may experience over the growing season could potentially result in reduced yield. These stresses can be environmental, induced by pesticide chemical injury, disease, insects, nematodes, or competition with weeds. In most cases it is often combinations of these things that result in stress to row crops. We are dealing with living organisms that respond in many ways to many variables in a growing season.

Unfortunately yield problems that result from herbicide injury can be difficult to identify at harvest. In addition herbicide injury symptom diagnosis is particularly difficult at harvest. The world of crop forensics leaves little bit to be desired. In many cases even high priced analytical procedures are not effective. By harvest time many of the herbicides are no longer detectable in tissues. Very few symptoms may be present at the time of harvest. Comparisons of yield with neighbors or even between two fields can be a precarious situation. All variables, soil type, timing, hybrid/cultivar, seed lot, etc. would have to be held constant. Only the herbicide treatment could be different to identify it as the problem. In a field research setting, researchers go through a lot of effort trying to hold variables the same between two plots just so that they can say, “this herbicide performed different than that one.” Try as we might, variability can still creep into a study.

Diagnosing herbicide injury is predominantly done by observing foliar symptoms such as chlorosis, speckling, and other visual observations. Some symptoms seen at harvest, like those seen from growth regulator injury (fused brace roots, lodging, and callus growth) or ALS injury (bottle shaped ears) are accompanied with foliar symptoms at the time or shortly after application. In some cases the injury or yield loss seen at harvest can also be caused by other factors or stresses as well as possible herbicide injury making causes inconclusive.

The time to diagnose herbicide injury is during the growing season when visual symptoms are present and tissue samples can still be analyzed. Documenting symptoms and collecting images of unusual growth or appearance of plants will help reach a conclusion. Keeping accurate records is always essential in identifying problems that may have occurred during the growing season.

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Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service