Oct. 30, 2012

The historic drought of 2012 is having lasting impressions and effects on farmers and their corn crop. The drought and resulting crop stress has contributed to development of several diseases, including stalk rots.

Producers should consider checking for stalk rot disease and weakened stalks in any corn still standing. Last week’s sustained high winds led to lodging in corn (Figure 1) where stalks have become weakened by stalk rot diseases and other problems.

Affected plants often have stalks that are hollow and easily crushed by hand or bent using the “push or pinch” test. Stalk rots can occur at any point in the stalk from the crown at/below the soil line all the way to the tassel. Rotting that occurs at an upper node and kills only the upper plant parts is referred to as “top rot” and does not necessarily cause lodging of the whole plant. However, degradation of the stalk below the ear can lead to plant lodging and losses during harvest.

Scouting for Stalk Rot Diseases — Push or Pinch Tests

While walking through a field, randomly select a minimum of 100 plants representing a large portion of the field. To test for stalk rot you may choose to push the plant tops away from you approximately 30° from vertical. If plants fail to snap back to vertical, then the stalk has been compromised by stalk rot.

An alternative method is to use the pinch test to evaluate plants for stalk rots. Pinch or squeeze the plants at one of the lowest internodes above the brace roots. If the stalks crush easily by hand, then their integrity is reduced by stalk rot and they are prone to lodging. If more than 10% of plants exhibit stalk rot symptoms, harvesting of that field should be a priority over others at less risk in order to reduce the chance of plant lodging and the potential for yield loss.

Figures 3a and 3b. Fusarium stalk rot can be evident on the outside of some stalks as white fungal growth at the nodes of affected plants (Figure 3a). Inside, rotted stalks may eventually turn pink to salmon in color (Figure 3b).

Several fungi that are common in our production fields can cause stalk rot diseases. 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 non-irrigated crops. 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 (hence the name). 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 soybean, sorghum, and alfalfa (Figure 2).
   

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• Fusarium stalk rot is especially common during damp conditions, but may occur anywhere, including in irrigated fields this year. The pathogen, Fusarium verticillioides, can sometimes be visible as white fungal growth on the outside of stalks at the nodes (Figure 3a). Eventually, the disease may cause discoloration of the inside of stalks to pink or salmon (Figure 3b).
   
• Anthracnose stalk rot also can 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 4) and is caused by the fungus Colletotrichum graminicola.

Management

At this time in the season there are no management options for stalk rot. Stalks will continue to degrade and weaken over time. But, you can scout fields and identify those most affected by stalk rot diseases. Consider harvesting fields that are heavily impacted by stalk rots first to minimize losses after lodging.

Resources

For more information see these UNL Extension publications:

• Stalk Rot Diseases of Corn
• Corn Disease Profiles II: Stalk Rot Diseases

If you are in doubt about the identity of a disease or cause of a plant problem, you may submit a sample to the UNL Plant and Pest Diagnostic Clinic (P&PDC) for diagnosis. For more information about plant diseases or for submission instructions and forms for the P&PDC, visit UNL's Plant Disease Central website.

Tamra Jackson-Ziems
Extension Plant Pathologist