As the 2021 growing season comes to a close, what should you be thinking about in terms of corn disease and how that might impact your harvest plans?
Crown and stalk rots
Have you noticed scattered, individual plants in a field turning brown, appearing to have reached physiological maturity? Chances are these “ghost plants” have crown and/or stalk rot. Using a knife to split the plants from the ear down to the crown will provide a diagnosis. Plants with crown rot will, as the name implies, have a discolored, rooted crown (Figure 1). Plants with stalk rot will have shredded, and/or discolored pith in the internodes, and discolored nodes (Figure 2). Pink to red discoloration may indicate Gibberella or Fusarium species as the cause. Dark discoloration could be anthracnose, and peppering (tiny black spherical sclerotia scattered throughout the pith), charcoal rot. While not much can be done to manage the disease now, taking good field notes is important to plan for subsequent years.
Crown and stalk rots can compromise standability as infected plants are more prone to lodging. Two simple tests for lodging potential are the ‘push’ and ‘pinch’ tests that you can read more about here. Fields in which more than 10 percent of the plants have crown or stalk rot should be scheduled for an early harvest.
Figure 1. Characteristic symptoms of crown rot (left); healthy plant (right).
As a side note, I have also started to notice anthracnose top dieback: random, scattered plants that are dying from the top. Plants with top dieback usually develop stalk rot. Thus, if you notice fields with top dieback symptoms, it’s a good idea to scout these for stalk rot too.
Ear rots
Although I have not noticed any ear rots in my trials or heard reports of ear rots, I suspect I will start to soon. I have noticed a lot of insect damage to kernels, which can lead to ear rots. Additionally, while the precipitation events that have occurred across most of the state through August have provided much needed moisture for the crop, they also provide a more favorable environment for fungal pathogens that cause various diseases of corn, including ear rot. It is important to scout your fields for ear rot because some fungi that cause ear rots produce mycotoxins. Similar to crown and stalk rot, if more than 10 percent of the plants have ear rot, schedule an early harvest, and get the grain dried and cooled as quickly as possible to prevent further fungal growth and mycotoxin production. Always store grain harvested from a field with ear rot in a separate bin from grain from healthy fields to prevent more grain from being contaminated.
The Crop Protection Network has an excellent publication on identifying ear rots.
Update on tar spot
I continue to hear reports of tar spot in Iowa. For the most part, the disease remains at pretty low levels, but there have been some fields along the eastern border of Iowa where the disease has become a concern. A fungicide application at around silking appears to have done a good job at reducing disease in some fields. Plants with high levels of tar spot die prematurely and lodging can be an issue. While it’s too late to do anything for tar spot this year, now is a good time to get out and make note if you see hybrid differences, which can help you plan for next year and also help you prioritize fields for harvest.
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...