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8/27/2012 - 9/2/2012

Black Mold on Corn

By Alison Robertson, Department of Plant Pathology

There have been several reports of a black mold covering the leaves and stalks of corn plants across the state. Walking in these fields turn a white shirt black quickly. Not surprisingly, combining this blackened corn is also very dirty. 

The black mold is saprophytic fungi – microorganisms that feed on dead plant material. The wet weather over the weekend followed by warm, humid weather and morning dews have favored growth of these organisms. They are not known to produce toxins, and the harvested grain should look relatively clean.

Individuals with allergies or respiratory problems are encouraged to wear dust masks to reduce breathing in masses of spores. These saprophytic fungi are a big contributor to the mold portion of the pollen and mold counts. It is also important to keep combine engines and can filters clean.


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. She can be reached at 515-294-6708 or e-mail

Aflatoxin Detected in Fields in Central and Southern Iowa

By Alison Robertson, Department of Plant Pathology, and Charles Hurburgh, Department of Ag and Biosystems Engineering

Within the past two weeks, there have been several reports of aflatoxin detected in southern Iowa and also a few reports from central Iowa.  Levels of aflatoxin have ranged from 8 ppb to almost 200 ppb. The FDA action level for aflatoxin in grain is 20 ppb.

Thus far, the problem does not appear widespread; however, fields across the state are at risk for aflatoxin considering the hot, dry conditions we have had during pollination and are having now as much of the crop reaches black layer (see Aspergillus ear rot and aflatoxin production). 

To determine if a field is at risk for aflatoxin, scout for aspergillus ear rot at black layer. Downed corn and more stressed areas of the field are a good place to start scouting. This ear rot is easily identified as an olive green powdery mold that usually occurs at the ear tips (Figure 1).

Figure1.  Typical signs of Aspergillus ear rot from a field in southeast Iowa.


If aspergillus ear rot is detected, call your insurance adjuster immediately. Corn will only be adjusted in the field. Once the grain is in the bin, it is no longer covered (see Crop quality issues from the drought of 2012).

Harvest the corn as soon as possible. The goal is to cool (below 50F) and dry (<15 percent moisture) the grain as quickly as possible to prevent the fungus from growing and producing aflatoxin (see Aspergillus ear rot and aflatoxin production). Some companies are offering discounts on drying this growing season.

Elevators will use up to three methods to check for aflatoxin. The black light method is used to detect glowing particles in the grain, which indicate a potential for aflatoxin. A specific fluorescence denotes the presence of kojic acid and, therefore, actively growing Aspergillus flavus, the fungus that produces aflatoxin. Other tests kits may be used to qualitatively (yes or no) or quantitatively (ppb) detect aflatoxin. These kits require a 5lb sample of grain to be collected and ground and then a subsample of ground grain is tested. Sampling error for aflatoxin is known to be large. A list of GIPSA approved test kits may be found at


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. She can be reached at 515-294-6708 or e-mail Charles Hurburgh is a professor in the Department of Ag and Biosystems Engineering. He can be reached at 515-294-8629 or e-mail

Identifying the Cause of Yellow Patches in Soybeans

By Daren Mueller, Department of Plant Pathology

Over the past week, we have observed yellow patches developing in soybean fields across Iowa (Figure 1). There are several different causes, including spider mites and soybean cyst nematode. However, we will look at three additional problems that are causing these patches: sudden death syndrome, charcoal rot and top dieback.

Figure 1 – Yellow patch in a soybean field.


For all three diseases, the symptoms can be very similar, but there are a few clues to distinguish them from each other. All three can have yellowing leaves in the upper canopy. For top dieback (Figure 2), the yellowing occurs on the outer margins of the leaves in the top of the canopy. There was a very thorough article written about top dieback a few years ago. It has been associated with potassium deficiency, but there is no clear-cut situation that precludes disease development.

Figure 2 – Top dieback in soybean.


Charcoal rot has been reported in the past, but this season it is showing up earlier than usual. The pathogen causing charcoal rot can survive many years in the soil. Conditions experienced in 2012 – hot and dry – are conducive for development of this disease. Interveinal yellowing of the leaves, an early symptom, can look a bit like sudden death syndrome (see Figure 3).

To distinguish charcoal rot from sudden death syndrome, there are a couple of things that can be done. First, you can look at the lower stem for microsclerotia (Figure 4). These can be found in the outer or inner stem tissue. A second way to tell the diseases apart is to wait until imminent death occurs. Plants with sudden death syndrome drop leaves but the petioles remain attached to the stem. Charcoal rot-infected plants that die may have leaves attached to the plants.

Figure 3 – Early symptoms of charcoal rot.


Figure 4. Microsclerotia in the soybean stem.


Figure 5 – Advanced symptoms of charcoal rot (photo courtesy of Carl Bradley).

There are no management strategies for this season for all three of these diseases. But knowing what is causing these spots may affect management in future years. For example, if you have top dieback, you may consider checking soil for potassium levels or soybean cyst nematode. Management of SCN or applications of potassium may alleviate this problem. Selecting resistant cultivars will help with sudden death syndrome. Finding ways to alleviate stress to future soybean crops can reduce charcoal rot.


Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at (515) 460-8000 or by email at

Crop Quality Issues from the Drought of 2012

By Charles Hurburgh, Department of Ag and Biosystems Engineering, Alison Robertson, Department of Plant Pathology, and Connie Hardy, Value Added Agriculture

As we approach harvest, the impact of the drought on grain quality is becoming clearer. Corn in many areas to the west and east of Iowa reached maturity earlier. Most of Iowa's corn will be past black layer by Labor Day weekend. Soybeans have stopped and started with late rains, but expect pod count and seed size to be pretty well set by Labor Day as well. Regardless of crop and quality issue, please talk to your crop insurance company before harvest. Some quality issues are covered; others are not. Quality issues are resolved in crop insurance by deducting an additional percentage of actual production before calculating the settlement.



The primary corn quality issues are low test weight/small kernels, significant mold pressure of all kinds in the many acres of downed corn and a general potential for aflatoxin at some level. Stalk strength is poor so expect more downed corn if we get wind or storms in September.


Test weight-kernel size

The drought- retarded grain fill makes kernels less dense and, therefore, lower in test weight. Low test weight from drought is not the same as low test weight from frost or wet weather (remember 2009) in that protein and oil levels will likely be average or even above. This is good news for feeding. Historically, test weights down to 45 lbs/bu have not had lower energy per unit of weight. The small kernels may partially offset the lighter density of each kernel because more small seeds fit in the test weight volume cup than larger seeds.

As always, low test weight grain will break to a greater degree in handling and will have shorter storage life at a given moisture. Table 1 shows the typical storage time by moisture and temperature. If test weights are 52 lb/bu and lower, cut these numbers in half as in 2009.

Table 1.


Do not hold this year’s crop at moistures above 17 percent. Even if it is necessary or advantageous to harvest early, dry immediately. The 2012 corn should not be put on top of or blended with older corn if you expect to store the corn.


Downed corn

Storms in August put corn on the ground in several areas of Iowa. The downed stalks will be in humid, high mold situations. Studies from the 2009 crop showed that, aside from toxin risks, mold damage can reduce feed energy values by 5 percent or more. This year the toxin risks are also high. This corn should be taken out of the field and dried as soon as possible.

Corn laying on the ground can have more toxin risks than just the aflatoxin, for which there is general potential. This corn should be tested for the series of mycotoxins of most concern – aflatoxin, vomitoxin, fumonisin and zearalenone. Testing laboratories are listed on the Iowa Grain Quality Initiative website; the Iowa State University Veterinary Diagnostic Lab will also test samples for mycotoxins.



Aflatoxin, a toxic secondary metabolite of Aspergillus flavus, is possible this year because of the hot, dry weather persisting from pollination through grain fill. The Aug. 2 issue of the ICM Newsletter explained the biology and conditions required for aflatoxin. Now is the time to be scouting fields for the presence of the fungus. Schedule affected fields for first harvest.

Aflatoxin will be adjusted in the field, not after the crop has gone into the bin. It is important to harvest and sample the insurance check strips as close to field harvest as possible. With the forecasts for more hot weather (above 90F) this week, the toxin levels in standing strips can increase. Aflatoxin testing has a 25-50 percent sampling error; only a USDA approved third-party lab can determine aflatoxin in crop insurance samples.

If you receive a crop insurance settlement for aflatoxin, the corn must be directed, with documented proof, to an approved feed use. Corn settled for aflatoxin should not be offered back to the general market without notice; the new food safety legislation creates significant liability if a downstream issue can be traced back to a production source.

Table 2.


The general tolerance for aflatoxin in interstate commerce is 20 parts per billion. Aflatoxin is classified as an adulterant in U.S. Food and Drug regulations.

The most sensitive industries for aflatoxin are dairy (because of pass through to milk), pet food (pets are very sensitive to aflatoxin) and ethanol/processing plants (the toxin concentrates by a factor of three in the feed co-products).

Expect grain buyers, both elevators and processors, to be monitoring for aflatoxin. If the overall level of aflatoxin in a region does not approach the 20ppb limit, then the buyer may elect to test composite samples of all loads in a period (half-day, day, etc) to verify that, on average, the grain is acceptable. If the composites show a regional or local issue, then individual load testing may be needed. This is time consuming and costly. A future issue of the ICM Newsletter will discuss testing options. If you feed your own corn, a test would be a worthwhile investment.

Aflatoxin is not removed by drying or freezing, but does not usually increase in storage. A flavus is not a strong storage mold; it is quickly crowded out by others. Storage at 18 percent moisture and above, with temperatures above 70F,  could cause an increase in aflatoxin, but normal grain cooling and drying practices will be effective in controlling further production. Natural air, stirred and other bin drying methods will work if the wet grain is not held warm awaiting drying. Evaporative cooling (check the dewpoint temperatures) normally keeps air drying systems cool enough. Do not fill heated air bin systems full so that wet grain is “held” at higher moistures and temperatures. Below 70F and below 18 percent moisture are the targets.



Soybeans are not susceptible to field-induced mold toxins. The primary soybean quality impact of the drought will be small and, perhaps, flat and shriveled soybeans. In the drought of 1988, shriveled and wrinkled (shrinkled) soybeans occurred. A definition was created by USDA-GIPSA, as shown below.

Image 1.

These soybeans do not fall through the small foreign material screen and are not considered splits. Price discounting will be at the discretion of the buyer, if at all. Shrinkled soybeans have a reasonably good protein and oil profile, but do not crack into pieces as required for efficient oil production at solvent extraction soybean plants. Soybean protein content overall may be low because dry weather does not favor nitrogen fixation.

Future issues will describe testing procedures for aflatoxin and will provide updates on grain management conditions as they develop.


Charles Hurburgh is a professor in the Department of Ag and Biosystems Engineering. He can be reached at 515-294-8629 or e-mail Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. She can be reached at 515-294-6708 or e-mail  Connie Hardy is an extension specialist in the Value Added Agriculture Program.  She specializes in food science aspects of business development, and can be reached at 515-294-8519 or

This article was published originally on 9/3/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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