As corn fields start to reach maturity across the state of Iowa, we need to start thinking about stalk and ear rots, particularly in terms of scheduling harvest and grain storage. Read on for more details on stalk rots and ear rots and how to scout for them.
Stalk rots
The wet conditions earlier this growing season have many folks concerned about crown and stalk rots. Stalk rot is more common when stressful conditions, like severe foliar disease or cool and cloudy weather, have limited photosynthesis and the corn plant has cannibalized the sugars in the stalk to fill grain. This causes the corn plant to be more susceptible to stalk rot pathogens since these pathogens are opportunistic.
Premature death of corn plants is often an indication of crown or stalk rot. Remember, like most diseases, symptomatic plants will occur singularly or in small groups and disease severity will range from minimal to severe.
Scouting: It is recommended to scout fields around physiological maturity (black layer) for stalk rot issues using the pinch or push test method described below. With either of these methods, assess approximately 100 plants randomly throughout the field.
- Pinch test: Pinch the lower part of the stalk. If it collapses, it fails the pinch test.
- Push test: Push the corn stalk forward about 8 inches (30-degree angle) and release. If the plant does not spring back to its upright position when released, it fails the push test.
If greater than 10 percent of the plants fail the pinch or push test, it may be prudent to schedule the field for an early harvest.
Questions and observations from the field:
- I have had several questions about whole fields dying from the top down due to anthracnose top dieback, which may indicate anthracnose stalk rot. However, it is extremely rare for a disease to occur across an entire field. A situation like this is more likely to be a characteristic of the hybrid maturing. Here’s a link to an earlier article discussing anthracnose top dieback.
- This year Physoderma has been very widespread. Physoderma brown spot affects corn leaves, but on some hybrids, Physoderma can cause node rot that leads to lodging. Node rot is easy to recognize. It usually occurs at node 7, although other nodes may be affected. When gently pushed, the plants will snap and break cleanly at the node which will be rotted (Figure 1). The pathogen can survive for several years in the soil. Frequent, heavy precipitation from V3 through V8 favors infection. Hybrids appear to vary in their susceptibility to node rot and Physoderma brown spot.
Figure 1. Physoderma node rot of corn
Ear rots
Ear rots affect grain quality. Some ear rots produce mycotoxins, so it is important to get the grain out of the field, and dried and cooled as soon as possible to prevent growth of the mold and production of mycotoxins. Consequently, fields should be scouted at maturity to assess the prevalence and type of ear rots.
Scouting: Scouting for ear rots can easily be done in conjunction with scouting for stalk rot issues. At several random locations in the field, peel back the husks and assess for ear molds. If greater than 10 percent of the ears are moldy, the field should be scheduled for an early harvest.
Questions and observations from the field:
- I have seen, read, and heard reports of a lot of insect damage to ears this growing season. Ear rots are often associated with insect injury.
- Ear rots are often identified based on their color. This season ear rots are pink, white, green, black and blue-green. The Crop Protection Network has a great publication describing ear rots and their identification based on color and where they occur on the ear.
- Fusarium ear rot: white to pink mold scattered around the ear, is common. (This one is of the most concern since is produces a mycotoxin called fumonisin.
- Diplodia ear rot: dense white mold that usually starts at the base of the ear
- Trichoderma ear rot: green mold in between the kernels
- Penicillium ear rot: green-blue powdery mold in between the kernels near the tip of the ear. Affected grain should be tested for mycotoxins.
- Cladosporium ear rot: dark green or black powdery mold between the kernels and black streaks on kernels
Acknowledgements: Thanks to Meaghan Anderson and Rebecca Vittetoe for helpful suggestions and edits.
Alison Robertson Professor of Plant Pathology and Microbiology
Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...