Issue No. 23, Article 5/October 3, 2008
Mold in Your Ears? Diplodia and Fusarium Ear Rots
We've had a lot of moisture lately. Although the season should be winding down, late planting has led to later maturing fields, which 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 normal year. Both Diplodia and Fusarium also can cause a stalk rot.
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.
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 and shriveled and of very poor quality. Diplodia has been rampant across the middle part of Illinois this late season.
White fungal growth of Diplodia on corn ears (photo courtesy of the department of Crop Sciences).
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 above 18% moisture. 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 expect to do with the grain. If you are planning on long-term storage, you actually should get the moisture down below 15% or 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, such as aflatoxin, and are a concern as they like to grow from about 14% to 18% moisture. Diplodia, Fusarium, and many other fungi cause stalk rots in our area. They produce symptoms, such as white, black, or pink stalk discoloration, but their main impact is decreased standability of the stalk.
Scouting for stalk rots is fairly easy. Evaluate 20 plants at each of 5 locations in a field. Use the common zigzag scouting pattern to accurately evaluate stalk rot incidence. Begin scouting when the 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 the ears for ear rot just by peeling back the husk at the same time you are scouting for stalk rots.
Aspergillus--a special ear rot case. We have not had reports of Aspergillus this season. In areas that have been droughty, Aspergillus infection should always be a concern. Worries center around the aflatoxin that can be produced by Aspergillus spp. infection. Aflatoxin is just one member of the larger group of chemicals called mycotoxins, which are toxins produced by fungi. Aflatoxins are some of the most potent carcinogens on earth, which certainly gets everyone's attention. The allowable amount of this toxin in grain for food or feed is highly regulated by the FDA. Testing is aimed at detection of 20 ppb (parts per billion) of the toxin in grain intended for human consumption and immature animals.
If you think your corn may have been at risk, how can you minimize the chances of fungi and toxin development, both now and in storage? First, harvest drought-stressed and insect-infested grain at early maturity as soon as the moisture content allows minimum grain damage: for shelled corn, 23% to 25% moisture; for ear corn, 25% to 30%. Set the blower on the combine high. Adjust the combine header speed to minimize cracking and reduce the content of trash, fines, and small broken or mold-infected kernels, especially those near the tips, where mold infestation is most likely to be present. Upward of 50% reduction in existing aflatoxin levels (or below 20 ppb) can be achieved in some fields by careful monitoring of combine cylinder, screen, and air-flow levels.
Drying is then essential. You need to prevent further fungal growth. Dry all grain to at least 13% to 14% moisture as rapidly as possible, not to exceed 24 to 48 hours after harvest. Safe long-term storage can be accomplished at a uniform moisture level of 13%. Moisture may be 14% if grain is to be moved or sold in a shorter period. For slow drying, the grain should contain no more than 18% to 20% moisture in full-bin drying; this may not be the best method when toxins are a concern. Another possibility is high-temperature drying until the grain reaches 18% to 20% moisture, followed by low-heat drying to 13% moisture. Avoid air-drying mold-damaged corn without heat. Research has shown aflatoxin increases of 100 to more than 2,000 ppb in three days when recently harvested field corn was stored at high moisture levels.
Delays in transit to the storage bin or buying point need to be minimal. Aflatoxins have been shown to increase in truckloads of contaminated corn by as much as 6% per hour of delay. Cool the grain after drying, and maintain dry storage conditions. When possible, continue cooling until the grain temperature reaches 35° to 40°F. Thoroughly screen and clean the grain and all bins before storage to remove dirt, dust, other foreign matter, crop debris, chaff, and cracked or broken seeds and kernels. Most of the contaminated corn is in the small and broken kernels, which will drop through a screen. Store the grain in water-, insect-, and rodent-tight structures. Continue periodic aeration and probing for hot spots at intervals of 1 to 4 weeks throughout the storage period.
Finally, you know that if you have crop insurance the crop is insured while it's in the field, but what about after? Contact your insurer before harvest to learn what coverage you have if aflatoxin is detected in your grain.--Suzanne Bissonnette