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.