Some of the 2016 Nebraska corn crop experienced prolonged stress and sometimes wounding earlier this season. While most of the crop looks good, we are beginning to see early evidence of problems developing.
In particular, stalk and crown rot diseases, including Anthracnose top dieback symptoms, are beginning to develop. Producers should watch for early symptoms and consider monitoring high risk corn fields for stalk rot diseases as harvest approaches.
Crop stress during the growing season has and will likely contribute to the development of some stalk rot diseases. These can lead to weakened stalks, already evident in some corn, can be vulnerable to lodging under high wind conditions. Pay special attention to fields that have one or more of these risk factors for stalk rot diseases and lodging:
- Higher yielding hybrids
- Nutrient deficiencies, especially N loss
- Lost leaf area (due to leaf diseases, hail, etc.)
- Excessive rainfall/ponding anytime during season
- Stalk wounding, usually by hail or insects
- High planting populations
- Thin stalks
Scouting for Stalk Rot Diseases
The first indication of a problem could be the early, and sometimes rapid, discoloration of the corn plant turning from green to brown or gray. Individual plants or patches of several plants may be affected. 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.
To scout for stalk rots, walk through a field, randomly selecting a minimum of 100 plants representing a large portion of the field. To test for stalk rot push the plant tops away from you approximately 30° from vertical. If plants don’t snap back to vertical, 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 plant at one of the lowest internodes above the brace roots. If the stalk crushes easily by hand, that’s a sign its integrity has been reduced by stalk rot and it’s prone to lodging. If more than 10% of plants exhibit stalk rot symptoms, harvesting 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.
Several fungal and bacterial pathogens can cause stalk rot diseases. The more common ones are summarized below.
- Anthracnose stalk rot is caused by the fungus Colletotrichum graminicola, which can also cause a leaf disease and is a common cause of top rot or dieback disease of corn. In more advanced stages the disease can cause the development of black lesions on the outside of the stalk. This disease causes the sudden browning and death of the upper plant parts, called top dieback, and has been confirmed in a number of Nebraska corn fields this year.
- Charcoal rot is one of the few diseases 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 (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 corn, soybean, sorghum, alfalfa, and others.
- 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. Eventually, the disease may cause discoloration of the inside of stalks to pink or salmon.
- Diplodia stalk rot can cause either/both an ear and stalk rot. The fungus causing Diplodia stalk rot reproduces with microscopic spores inside minute raised black structures (pycnidia) that can give the stalk a rough/sandpaper-like feeling (Figure 3).
- Physoderma brown spot most commonly causes a leaf disease, but under some conditions can cause weakening of stalk tissues. The pathogen causing Physoderma needs free moisture and is more common this year following the frequent rainfall events that occurred earlier this year. Although not technically a stalk-rotting disease, infections concentrated at one or more nodes can weaken the outer rind leading to stalk breakage.
There is nothing to be done at this point in the season to stop stalk rot diseases; affected plants will continue to degrade over time further weakening them. But, you can work to minimize your losses by identifying which fields have the worst stalk rot diseases and harvest or chop heavily impacted fields first to minimize losses from lodging. When making hybrid selections for later years, consider choosing hybrids with better resistance to one or more stalk rot diseases to help reduce disease pressure.
If you are in doubt about the identity of a disease or cause of another plant problem, you can submit a sample to the UNL Plant and Pest Diagnostic Clinic (P&PDC) for diagnosis. For more information about these and other plant diseases or for submission forms and submission instructions for the clinic, visit the Clinic website in CropWatch.