Various stalk rot diseases have been confirmed in samples from corn fields across Nebraska. Stalk rot diseases and the pathogens that cause them are common in corn. They can weaken corn stalks leading to stalk lodging and harvest difficulties for producers. These diseases also can impact yield if corn plants are killed prematurely.
It’s important to be aware of their incidence in fields to better understand the risk of harvest difficulties if plants lodge. In addition, knowing which diseases are present in individual fields can help producers anticipate, manage and prevent them in subsequent growing years.
Unfortunately, the symptoms that stalk rotting pathogens cause can be misleading and make it difficult to diagnose. For example, some of the earliest symptoms of stalk rot disease may be the discoloration of leaves and rapid plant wilting and/or premature death.
In recent weeks, several samples of dead leaves have been submitted to the UNL Plant and Pest Diagnostic Clinic. Microscopic observation and testing of these samples has failed to identify any pathogens or provide a diagnosis we can be confident of, leading us to suspect stalk rot diseases.
Crown and Stalk Rot Sampling Tips
As you continue to scout fields late in the season, it is important to collect the right kind of sample to ensure proper diagnosis. Over the course of several days, the leaves can become necrotic with symptoms resembling sunscald (Figure 1).
However, splitting the stalk and crown can reveal the real cause of leaf dieback (Figures 2 and 3). Certain stalk rots, such as Anthracnose, are managed through resistant varieties. Others, like Fusarium spp., are controlled through cultural practices that lessen plant stress – balanced fertility, proper planting populations, and insect control.
Even though the noticeable symptoms appear on the leaves, the pathogen is not present in the leaves. Additionally, it may be easier to cut the plant a few inches above ground level and leave the crown and roots, but the main difference between crown and stalk rots is where the pathogen is infected.
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If the crown and roots are left and the disease has not progressed up the stalk, it is difficult to get an accurate diagnosis. Submitting entire plants is the best way to get an accurate diagnosis. Proper management starts with a proper diagnosis.
Please do not send dead plants. It is difficult to ID the exact cause of death on a plant once other, opportunistic pathogens and microorganisms have had a chance to infect the weakened plant. Ideal samples are ones that are just starting to show symptoms. Keeping soil on the root ball is helpful in keeping the plant alive a little longer as we work through our diagnosis.
Please enclose the root ball in a separate plastic bag to prevent soil from spreading to the rest of the plant. In order to fit the entire plant into a bag or shipping container it is fine to cut, or bend them to fit into a trash bag or large zip lock bag. More information regarding sample submission can be found here.
As always, please fill out the sample ID form with as much information as possible.
Numerous stalk rot diseases occur in Nebraska and other states and your sample analysis results can identify one or more common pathogens in samples submitted to the UNL Plant & Pest Diagnostic Clinic.
Risk Factors for Stalk Rot
Crop stress during the growing season contributes to the development of some stalk rot diseases. Pay special attention to fields that have one or more of the following risk factors for stalk rot diseases and lodging:
- Higher yielding hybrids
- Lost leaf area (due to leaf diseases, hail, etc.)
- Excessive rainfall/ponding
- Stalk wounding, usually by hail or insect feeding
- High planting populations
- Thin stalks
Evaluating Stalk Strength Prior to Harvest
Walking through a field, randomly select a minimum of 100 plants representing a large portion of the field. To test for stalk rot you can PUSH the plant tops away from you approximately 30 degrees from vertical. If plants don’t snap back to vertical, the stalk may have been compromised by stalk rot disease. An alternative method is to PINCH the internodes of the lower stalk.
If the stalks crush easily by hand, their integrity is reduced by stalk rot and they are prone to lodging. If more than 10% of plants exhibit stalk rot symptoms, harvesting that field first should be a priority over others at less risk in order to reduce the chance of plant lodging prior to harvest.
Several fungi are common in our production fields and can opportunistically cause stalk rot diseases in stressed plants. Some of the most common stalk rot diseases this year are listed below:
- Charcoal rot is one of the few diseases that is more common during drought conditions, and so, is more likely to affect corn in non-irrigated fields or pivot corners. The disease is characterized by the presence of many minute black round structures inside the stalk that can give it a gray to black appearance (Figure 3a). In addition, the fungus that causes charcoal rot, Macrophomina phaseolina, has a wide host range and can cause the same disease in several crops, including corn, soybean, sorghum, and alfalfa.
- Fusarium stalk rot is especially common during damp conditions, but may occur anywhere. One of the most common symptoms is the development of crown rot inside the lower stalk, below the soil line, near where roots attach to the stalk (Figure 4). The pathogen, Fusarium verticillioides, can sometimes be visible as white fungal growth on the outside of stalk nodes. Eventually, the disease may cause a pink or salmon discoloration to the inside of stalks.
- Anthracnose stalk rot can also cause a leaf disease and is a common cause of top rots in corn. In more advanced stages the disease can cause the development of black lesions visible on the outside of the stalk (Figure 5) and is caused by the fungus Colletotrichum graminicola.
Managing for Stalk Rots in Corn
There is nothing to be done at this point in the season to stop stalk rots as affected stalks will continue to degrade over time, further weakening them. Growers can minimize losses by identifying which fields have the worst stalk rot diseases and adjust the harvest order of those fields.
Consider harvesting or chopping fields heavily impacted by stalk rots first or earlier than others to minimize losses that can occur after lodging. Some seed companies provide ratings for their hybrids’ reactions for a few stalk rot diseases that may be helpful in selecting hybrids for fields with chronic stalk rot disease.