DON Threatens Wheat

Spores Overwinter In Corn Refuse, Attack Next Wheat Crop

MidAmerica Farmer Grower

   Two sub-topics of a larger issue, Fusarium head blight, were examined in detail recently by Dr. Don Hershman, outgoing extension plant pathologist at the University of Kentucky. He discussed Deoxynivalenol or DON and wheat head scab relative to different production scenarios. 
   “Last year in Kentucky, and I believe parts of southeast Missouri as well, there wasn’t a great deal of head scab symptom expression in fields but apparently there were enough late infections to result in a situation where some of the grain was surprisingly contaminated with a mycotoxin called Deoxynivalenol or DON,” he said. “DON is a serious issue to grain producers and the industry because it results in discounts at the elevator and can negatively affect end use of contaminated grain; a significant amount of the value of the crop can be lowered at the grain elevator because of this DON.”
   Many producers have heard about DON but probably know very little about it. Hershman explained what DON is, how it relates to head scab; he also described some of the nuances of how DON could result in what looks like a relatively good grain in the field with very little Fusarium head blight or head scab.
   The second part of his message was to explain how corn fits into the wheat head scab picture. In fact, the same fungus that causes head scab also causes an ear rot and a stalk rot of corn; also the  fungus that causes wheat head scab overwinters in the residue of corn. Most plant pathologists feel that the increase in the number of acres of corn production in this country and also the expansion of traditionally non corn areas, has really been the reason that there has been more head scab through the years.

 Dr. Don Hershman, outgoing extension plant pathologist at the University of Kentucky, discussed Deoxynivalenol or DON and wheat head scab relative to different production scenarios.
 Photo by John LaRose, Jr.

   “Nationally and even regionally the amount of corn has played a big difference in the fact that we’re seeing more head scab,” Hershman said. “Planting wheat behind corn in a no-till system, as many Kentucky producers do, appears to be the worst case scenario. We do a really good job of producing our soft red winter wheat crop in Kentucky. The question is how that could happen if planting wheat no till behind corn is the worse case scenario; as it turns out we did some research on this, particularly through a survey in the late 1990s. Then in the last five years a group of scientists from across the country has started to look at this issue. Many people are realizing that you can do a really good job of growing wheat behind corn and they want to understand how that can happen.”
   There are two scenarios. One is where you live in a state where corn is concentrated in a particular part of the state but not in another where they grow wheat, and in those situations planting wheat behind soybeans is the preferred plan. The corn residue does appear to have a very significant  impact on whether this head scab and then DON also develop.
   But, with the second scenario, for example, in Kentucky and many other states where there’s widely scattered corn acres, no matter where you plant your wheat crop there’s going to be an old corn crop a stone’s throw away. In that situation the amount of the spores of the head scab fungus that blow into a field tends to be more important than the amount of fungus that overwinters in a particular field.
   “The point is that regardless of crop rotation, what crop you’re planting wheat behind in places where there’s a lot of corn produced, whether you’re planting wheat behind soybeans or behind corn, whether it’s conventionally tilled or no-tilled really has a minor impact on the development of head scab. You’re going to have spores blowing into those wheat fields no matter what you do,” he said.
   “However, in places like North Dakota where the wheat would tend to be concentrated and separated from the corn acres, then it would make a big difference. What you do in a particular field might have more importance then. So there are two scenarios and in places like Kentucky, Missouri, Illinois, Indiana and surrounding states, there’s just a lot of corn, statewide, and this is where the head scab fungus survives; and when the conditions are right, when you have the proper conditions for sporulation of the fungus in the residue, you’re going to have a lot of spores produced no matter what you do,” Hershman explained.
   “In this situation, there’s no reason for a farmer to get too concerned about planting wheat behind corn or behind soybeans or anything else, because no matter what there will be a lot of spores flying around when the conditions are favorable, and that really is my take home message.”
   Another issue is how DON could be at a significant level and little or no head scab is apparent. Hershman explained that just by looking at the situation one can usually analyze how it happens.
   ”You can actually do a really good job raising wheat as long as you do other things that are required to manage head scab,” he said. “These efforts include growing varieties that have some resistance, using fungicides when necessary, and then also planting different varieties that might flower at different times which would allow for some escape of individual fields.”
   DON is called a trichothecene toxin and there are actually different toxins in the same category. However, the main fungus that causes Fusarium head blight or head scab in the United States is Fusarium graminearum and this DON is the primary mycotoxin that’s produced by that particular fungus.
   In other countries, there are different fungi and different toxins but DON is the one prevalent here. There are a couple of ways to measure DON, which is in the grain and is a product of the fungus during the infection and the disease development process. So every time you see head scab in a field there is going to be some element of DON in the harvested grain and it can be measured.
   “Many times it’s measured using various kinds of kits that are available at the grain elevator,” he explained. “That’s sort of a down and dirty way to know more or less what sort of concentrations of the mycotoxins there are, typically talking of parts per million. Anything below two parts per million in grain is acceptable and would not be docked and anything above two to four parts per million will result in a dockage. Some years when the concentration is above four and certainly at five, six or 10 parts per million, if there’s other wheat that is available in an area, some elevators will reject that wheat, so it can be a real significant problem. There really is nothing else you can do with that once it’s rejected because it wouldn’t be acceptable for feed and certainly wouldn’t be useful in other ways.”
   There are some marketing strategies but it’s a problem no matter what. Chemists in the lab have a very accurate way to measure DON, but it’s very expensive and takes time to complete the test, so most producers and grain elevators just use the kit format. Kits are good, just not as accurate as the gas chromatography method used in scientific laboratories.
   “DON is an issue, its not actually a poison, but it does cause feed refusal in animals; it can cause some physiological changes that can result in death of swine and other animals if exceeded. From a human perspective, probably the most significant symptom it produces is reduced hunger in children which might result in malnutrition and all the negative consequences that come from that. DON also causes beer to gush, so when you open a bottle of beer it just gushes out uncontrollably and it’s not something that you would desire. Then just from a safety standpoint, most of the industry is very keen on not having any DON in their products, because wheat is one of those products that is often consumed in raw material. In whole grain wheat, that’s obviously not a very good thing. The industry is very, very tuned into DON, probably more than the producers, but ultimately that’s the reason there’s a dockage associated with excessive DON in grain.”
   There is no way to get rid of DON, the only way is to manage Fusarium head blight; when you manage the disease, that’s going to result in less DON in most cases. The best efforts are to use resistant varieties, fungicides, multiple varieties, do the best job you can to result in less head scab.
   “Regardless, there are situations where you do the best you can possibly do and you still end up with DON and there’s really no way to get rid of it,” Hershman said. “Sometimes you can blend grain that doesn’t have DON with grain that has some DON and the average situation then brings that DON down to acceptable levels.
   “Most growers do not segregate their grain for different logistical reasons so that’s really difficult unless you are segregating your loads of grain. It’s a complex thing. The ideal situation is to be sure you don’t let DON develop in the first place. The ultimate determiner really is weather. If you have a lot of moisture during the flowering period in the spring, the risk of head scab and DON skyrockets, and that’s when it becomes difficult to manage. In four out of five years, head scab and DON is not a significant problem because the weather simply isn’t favorable for spore production or infection; hopefully, next year will not be a Fusarium head blight year and we’ll be good to go,” Hershman summed. ∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower

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