A wide range of things have occurred the past couple weeks in Iowa cornfields. Fields in areas of west-central Iowa got hit by wind, tornados, hail and flooding on Labor Day weekend. Several other areas of the state also have had wind damage, heavy rain and flooding. "That's frustrating after you've had a good-looking crop all the way through August, and then the crops get decimated late in the season," notes Clarke McGrath, an Iowa State University Extension field agronomist. He writes a column each month in Wallaces Farmer magazine. In the September issue, he covers cornstalk rot.
A dead ear leaf is symptomatic of Diplodia ear rot.
"For farmers lucky enough to have dodged the latest round of storms, it may not take much wind to knock down a lot of corn the rest of this fall," says McGrath. "I'm seeing a lot of stalk rot beginning to hit and we aren't even to black layer stage of corn growth in many fields yet. We've seen significant foliar disease present on the leaves in the mid-to-upper canopy this year, caused by several pathogens. Those include northern corn leaf blight, gray leaf spot, common rust, physoderma, eyespot and even some anthracnose top dieback and late arriving Goss's wilt. When we see significant leaf disease pressure in the upper canopy of the corn plant, the risk of stalk rot increases."
Conditions are right in many fields for stalk rot to show up
Stress from a wide range of factors that have occurred this growing season can further increase the risk of stalk rot and help it set in earlier, and have more impact on yield, drydown and standability, says McGrath. Factors causing stress on corn plants include nutrient deficiencies, soil compaction, excess soil moisture, hail damage, root damage, or not enough sunshine.
This is Diplodia ear rot. Note the dense white mold growing from the base of the ear. The tiny black specs, called pycnidia, may also be present on the husks.
Now is time to scout cornfields for stalk rot and decide which fields are priorities for harvest. Iowa State University Extension plant pathologist Alison Robertson recommends the following procedure to assess your fields prior to harvest.
If you are scouting for stalk rot, look for lower stalk discoloration, and check stalk firmness by pinching the lower internodes. Simply pinch the stalk between your thumb and fingers. Healthy stalks are firm and won't compress easily; if a node can be "squished" or if it otherwise feels soft, that means stalk rot has set in and risk of lodging goes up. Instead of this "pinch test," some agronomists and farmers prefer using the "push" test, but either way works fine.
Check at least 100 plants per field; 20 plants in five spots. Better yet, try to test each of your hybrids, with special attention given to any that have low stalk rot and/or standability scores. Try to sample the different "management areas" you have in a field, assess them separately: various tillage systems, crop rotations, drainage issues and fertility histories. Prioritize scouting towards fields that showed stress first, especially if they've had significant foliar disease this summer. If around 10% or greater of the stalks have issues, you need to do your best to get those hybrids harvested first to reduce the risk of significant lodging.
Figure 2: Tiny black specks (known as pycnidia) embedded in the rind are a diagnostic symptom for Diplodia stalk rot.
Future management steps to control stalk rot
Do you need to identify the particular pathogen? For example, is it anthracnose stalk rot, diplodia stalk rot, gibberella stalk rot, pythium stalk rot or maybe a bacterial stalk rot? "I used to say I didn't care what the pathogen was," notes McGrath. "I felt that once we got to the point we were seeing stalk rot issues in a cornfield, usually multiple pathogens were present and deciding which one was the initial culprit was difficult to do. However, in talking with seed companies and ISU plant pathology experts recently, I've had a change of heart."
Hybrid disease resistance ratings have gotten much more accurate in the last few years and disease resistant genetics are improving at an increasingly rapid pace, he notes. Taking a little more time to identify what type of stalk rot is predominant in the field this year can help you with hybrid selection in subsequent years.
Most stalk rot diseases are caused by fungal organisms, and behave by primarily infecting injured, stressed or maturing plants, adds McGrath. Typically, multiple stalk rot organisms are present at initial infections, and then a particular disease is favored by a certain set of environmental conditions. In other words, you may see several different kinds of stalk rot if it shows up, but most likely one will dominate. Take note of it and use that information for hybrid selection the next time the field will be planted to corn.
What can you do to reduce incidence of stalk rot in future years? Management to reduce stalk rots for future years is pretty consistent among pathogens. With a few of them you can reduce the disease pressure by "burying" crop residue, but that can create other issues if soil structure and health is compromised. "In some regions and fields in Iowa, using tillage is a good option but I rarely recommend doing that in most of Iowa with our slopes and erosion control challenges," says McGrath. He suggests taking the following steps to manage stalk rot in future fields:
* Plant corn hybrids with the genetics that have good stalk strength ratings.
* Identify the primary pathogens and select genetics with good scores against these pathogens when you plant the field back to corn.
* Also, look for hybrids with resistance to the more common leaf diseases; consider applying foliar fungicides if necessary.
* Rotating crops will help a lot. If you must plant corn on corn, talk to your seed dealer about rotating genetic packages.
* Sample your soil and have it tested to ensure soil fertility is optimum.
* If your field tends to be wet, consider drainage management.
* Fine-tune your management to optimize soil structure and planting depth; be sure that corn rootworms are controlled.