Issue No. 22, Article 4/September 4, 2009
Fungal Ear and Stalk Rots
Given the recent weather, it is essential to be wary about Diplodia and Fusarium ear and stalk rots. We've had a lot of moisture, and unfortunately some of our ear and stalk rot fungi find that situation invigorating. Although the season should be winding down, late planting has led to later-maturing fields again this year. This has many producers worried about ear and stalk rots and even seed germinating on the cob. Diplodia ear rot and Fusarium ear rot are our most typical ear rots in a nondrought year. Interestingly, both Diplodia and Fusarium also can cause stalk rots.
Fusarium ear rot, normally the most common ear rot found in Illinois and the Midwest, is characterized by pinkish to salmon-colored fungal tissue growing on the kernels. The fungus can be scattered on the cob or, often, seen toward the middle of the ear. The color is distinctive. Fusarium ear rot produces mycotoxins called fumonisins. Fusarium is favored by hot and dry conditions at pollination and high humidity. So while it is a disease endemic to our area, weather conditions were not exceptionally conducive to its development this year.
Diplodia ear rot, on the other hand, has had an optimal season for development. Diplodia ear rot is first noticeable in the field by a bleached appearance of the husk. When you peel back the husk, you see a white, fluffy fungus. The good news is that the Diplodia fungus will not produce toxins in the grain; the bad news is that kernels will be very lightweight, shriveled, and of very poor quality. Diplodia is starting to be observed and will likely be our most common ear rot this season.
What is the right moisture at which to harvest corn that has had fungal ear rot problems? The answer really depends on several important issues. First, what ear rot do you have in the field? Second, what weather is expected? Ear rot fungi will continue to develop in the field or in storage at moisture above 18%. If dry weather is expected, you can try to save some drying costs and leave the grain to dry a bit longer in the field. If you have moderate infection, though, and wet weather is expected, harvesting and drying to at least 18% is probably your best option.
Do you really have to dry to 18% moisture? Well, that depends on what you are planning to do with the grain. If you are planning on long-term storage, you actually should get the moisture down below 15% to 16%. Diplodia is not your biggest worry for storage, but the many species of another of our ear rots, Aspergillus, produce very serious grain toxins, including aflatoxin, and are a concern as they like to grow from about 14% to 18% moisture.
Diplodia, Fusarium, and many other fungi also cause stalk rots in our area. They produce symptoms such as white, black, and pink stalk discoloration, but their main impact is decreased standability of the stalk.
Scouting for stalk rots is a fairly easy endeavor. Evaluate 20 plants at each of five locations in a field. Use the common zigzag scouting pattern to accurately evaluate stalk rot incidence. Begin scouting when kernels are at 30% to 40% moisture. You can use either of two methods to evaluate stalk integrity. The first is to lightly grasp the stalk at waist level and push it about 15 degrees from the vertical. A second method is to pinch the base of the stalk below the first node. Stalks that lodge or collapse when pinched should be marked positive for stalk rot. Fields can endure stalk rot incidence up to 10%. However, incidence above 10% to 15% calls for an early harvest to prevent further damage and lodging. You can investigate ears for ear rot just by peeling back the husk at the same time you are scouting for stalk rots.--Suzanne Bissonnette