A favorable corn growing season has transitioned into a challenging ripening time in many parts of Pennsylvania and surrounding states.
Rain and persistent humidity over the past month have created a great environment for pathogens, even in those fields that have received fungicide treatments.
With grain harvest right around the corner, now is the time to scout fields and find out if your corn has any surprises waiting for you.
Crown and stalk rots
The first signs that your corn may be struggling is lodging and premature drying from the top down in scattered areas of a field.
Lodging will become most apparent after storms with high winds, but early plant death may be more obvious earlier. These are both indicators of potential crown and stalk rots.
Paul Esker, field crop pathologist with Penn State, says he’s been getting lots of questions from growers about stalk rots this season and has also found evidence of gibberella ear rot in numerous fields and on the Penn State research plots.
Notable crown and stalk rots include:
Anthracnose stalk rot. Symptoms often first appear around physiological maturity, although infection can also occur during vegetative growth stages. One symptom is disintegration of the pith tissue, giving the appearance that the stalk is shredded. Another is the distinctive blackening of the stalk rind. Finally, a top dieback can also occur, whereby the flag leaf may be yellow, purple or dead, which then affects the tassel.
Charcoal rot. Typically, charcoal rot is diagnosed by noting that the pith and stalk rind tissue have a silvery gray appearance because of the development of black microsclerotia, which leads to the pith tissue being disintegrated. It is the appearance of these microsclerotia that distinguish charcoal rot from other corn stalk rots.
Diplodia stalk rot. Diagnosis is based on the identification of small, black pycnidia located on the lower stalk rind. These pycnidia are the size of a pinhead and are embedded in the rind tissue, which means that they are easily scraped away. This can be used as a method to distinguish this stalk rot from other corn stalk rots, although proper identification may require a laboratory analysis. When conditions are very wet, you might see a white mold on the stalk with internal tissue being discolored and shredded.
Fusarium stalk rot. This will cause the internal pith tissue to shred and may also cause a discoloration that, as the pith tissue continues to rot, becomes pinkish or salmon colored. In the field, the plants may suddenly die before maturity, whereby leaves are wilted and have a dull green or grayish color, along with a stalk that is straw colored. Lack of pycnidia is one way to differentiate fusarium stalk rot from diplodia stalk rot.
Gibberella crown rot and stalk rot. Favored by plant stress with warm and wet conditions shortly after silking, gibberella stalk rot can be identified by the presence of perithecia, which are small, round, black fungal structures found on the internodes and nodes. Very important is that these fruiting structures can be easily scraped away from the stalk using a fingernail. Internally, it is very common to see rotted pith tissue that has a light-to-dark pink color.
Some hybrids with purported resistance to stalk rots may still suffer from a low incidence of disease in a high-pressure year. This resistance is more likely to be overcome in areas of a field that are prone to flooding or drought.
When incidence seems randomly dispersed across a field, however, this may be due to genetic differences between the primary hybrid and the conventional hybrids included in “refuge-in-a-bag” type seed packages.
The presence of stalk rots is a good reason to move a field to the top of your harvest priority list to avoid further lodging and a difficult harvest for you and your combine.
Kernel sprouting and ear rots
Once you start examining some ears, you may find multiple issues stemming from the weather conditions.
Check for kernel sprouting. Then, look for fungal growth on the ear that could be indicative of one or several of these problematic corn ear rots:
Gibberella ear rot. Caused by Fusarium graminearum and favored by cool and humid weather conditions, symptoms include a reddish discoloration that begins at the tip of the ear where a red-to-bright-pink mold develops toward the base of the ear. It may contaminate corn with the mycotoxins deoxynivalenol (DON or vomitoxin) and zearalenone that can cause feed refusal and reproductive problems in farm animals, respectively.
There are many online resources with useful tools for identification for gibberella and sampling corn for DON.
Trichoderma ear rot. Caused by Trichoderma viride and favored by excessive rain, symptoms include a dark green mold growth that covers most of the ear (on and between the kernels) and can be observed on the husks. No mycotoxin issues have been associated with trichoderma ear rot.
Fusarium ear rot. Signs include a brown discoloration or a white mold on scattered kernels. Affected kernels may also show a starburst pattern — white streaks radiating from the point where the silk was attached. Fusarium ear rot may lead to contamination with mycotoxins called fumonisins, which are highly toxic to horses and swine.
Penicillium ear rot. Signs include a green-blue powdery mold between kernels near the tip of the ear. Different Penicillium species affecting corn can produce mycotoxins. However, these are considered to be a greater problem in silage.
Scout before harvest, and pay close attention to areas in your field that have been exposed to the most severe weather conditions. If the risk of mycotoxins is high, contact your crop insurance agent and take a proper sample for mycotoxin testing.
If your corn tests positive for mycotoxins that does not mean you have to dispose of it right away. Depending on the levels of mycotoxins, there are maximum limits and recommendations that can guide you on what to do with the contaminated grain.
There are also recommendations for next growing season that can help you reduce the risk of mycotoxins that include hybrid selection, insect protection and residue management.
A field with significant levels of mycotoxins means you will have to make some critical harvest and storage decisions depending upon where that grain may be accepted and what it can be used for.
This new leaf disease for the region has been found in several counties in south-central Pennsylvania. Be on the lookout as you scout your fields.
At this late stage, it is not a threat to most of the current crop, but the pathogen will likely survive in crop residue to cause problems next season. It is worth detecting so you can prepare to fight it in the future.
Researchers at Purdue Extension believe the fungus that causes tar spot overwinters in infected corn debris on the soil surface. As a result, growers are often told to rotate away from corn the following year to allow the residue to decompose, or to incorporate tillage to bury the inoculum.
The Crop Protection Network’s Corn Disease Working Group has developed ratings for how well fungicides control major corn diseases in the U.S., including tar spot.