Iowa farmers are moving into "harvest mode" as silage chopping started last week, seed corn harvest has begun and grain harvest of corn and soybeans will start soon. The 2015 crop year has had its share of highs and lows. A cold, wet spring set us up for season long disease challenges, and the weather in June, July and August was prime for diseases in much of the state.
"I've been cutting a fair number of corn plants open the past two weeks," says Clarke McGrath, and Iowa State University Extension field agronomist. "While most of the corn plants are holding up well, I am seeing more pith deterioration in the stalks than I'd like to see this time of year."
Be careful in choosing which fields to let stand longer
Given that most of the corn in Iowa got planted later than we typically plant, and we are a little behind on heat units… this year's corn crop likely will need to stand a little longer than we'd like this fall in order to reach maturity, and it may not dry down as well as we'd like. "We will want to really watch this stalk situation as the corn matures and we are tempted to leave it out there to field dry", says McGrath. "Some fields and areas may not have the stalk integrity that we need to safely field dry the corn."
Thus, cornstalk rots are likely to be an issue this growing season. "We have seen significant blighting of the leaves in the mid-to-upper canopy from several pathogens, notes McGrath. Those include northern corn leaf blight, gray leaf spot, common rust, physoderma, eyespot and anthracnose top dieback. "When we see significant leaf disease pressure in the upper canopy, the risk of stalk rot occurring increases."
When plants are stressed, stalk rot can set in earlier
Stalk rot is present in almost all Iowa cornfields to some degree each year, as the pathogens that move in are common and opportunistic, attacking corn as it starts to mature. Fortunately for farmers, in many years stalk rot diseases set in relatively late and don't have much impact on yield, drydown and ease of harvest. However, when plants are stressed from a wide range of factors (nutrient deficiencies, soil compaction, shortage of soil moisture or excess of soil moisture, hail damage, root damage, not enough sunshine, excess heat, etc.) stalk rot can set in earlier and have more impact on yield, drydown and standability.
Most of the more severe stalk rot infections result in substantial deterioration of the inner "pith" tissue inside the cornstalk, leaving plants much more prone to falling over in storms or even moderate winds, says McGrath. The abundance of recent rains will only tend to make the stalk rot pathogens worse.
What kind of stalk rot?
Here are a few things to think about regarding stalk rot, when you are scouting fields and prioritizing them for harvest.
* 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 that I didn't care what the pathogen was," says 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 tough to do. However, in talking with seed companies and ISU plant pathology experts recently, I've had a change of heart."
The disease resistance ratings for corn hybrids have gotten much more accurate in the last few years and disease resistant genetics are improving at an increasingly rapid pace. Taking a little more time to identify what stalk rot is predominant in the field this year can help 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, he explains. 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.
Management considerations to control stalk rot
To control stalk rot in the future, McGrath offers these steps to take, some of them depending more on your particular field and conditions.
* 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 rootworm and stalk pests such as corn borers are controlled.
An Iowa State University Extension publication, "Corn Stalk Rot in Iowa", IPM 50, which describes the symptoms and management of stalk rot in more detail, is available at store.extension.iastate.edu.
Decide which fields to harvest first
Now is time to scout 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.