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8/22/2011 - 8/28/2011

Crop Minute - Aphids in Corn

Erin Hodgson, ISU Extension entomologist, is monitoring the rapid aphid infestation in Iowa corn fields. During the weekly crop minute, she tells where the aphids have been found and what justifies treatment at this point in the growing season.

Reduce Risk of Mycotoxin Contamination by Scouting Fields for Ear Rot

Alison Robertson, Department of Plant Pathology and Microbiology; and Charles Hurburgh, Iowa Grain Quality Initiative

Hail storms damaged several corn and soybean fields in parts of Iowa last week. In some areas, the corn and beans are completely lodged as a result of the storm. In other areas, leaves are significantly stripped, but the grain seems relatively undamaged. 

During the 2009 growing season, approximately one million acres of crops from Sac to Grundy Counties were damaged by a single hail storm. Most of the corn crop was at growth stage R2.  We conducted a survey to assess the impact of hail damage on grain quality (Robertson et al., 2010). We found that hail damage to kernels increased the risk of ear rot and mycotoxin contamination.

Scout for ear rot

The corn that was damaged in the hail storms last week was further along in development (growth stage R5) than the grain damaged in 2009, but it still may be at risk for ear rots and associated mycotoxin contamination. Fields that were damaged need to be scouted in the next 10 to 14 days for ear rot. If more than 10 percent of the ears in a field are moldy, the field should be scheduled for an early harvest. Check with your insurance company regarding their requirements for claims. Most companies will want to assess the field before it is harvested.

Fields that were not damaged by hail should also be scouted for ear rot, since the hot, dry weather with occasional rain that has occurred recently is favorable for Aspergillus and Fusarium ear rot development. Symptoms of Aspergillus ear rot are a powdery olive-green mold that develops on damaged kernels (Figure 1). High temperatures (80 to 100 F) and high relative humidity (85 percent) favor the growth of Aspergillus in the field. Note that the presence of Aspergillus ear rot does not necessarily indicate aflatoxin contamination. Aflatoxins are produced under certain conditions, and are most often a problem when night temperatures remain above 70 F. The U.S. Food and Drug Administration regulates aflatoxin levels in food and livestock feed. An "action level" of 20 parts per billion (ppb) for aflatoxin in corn has been established for interstate commerce.

Fusarium ear rot symptoms are characterized by white to light pink mold that usually occurs on damaged kernels (Figure 2).  High temperatures (above 77 F), drought stress before or after silking and mechanical damage favor infection and the development of Fusarium ear rot. Mycotoxins associated with this ear rot are fumonisins, and the optimum temperature for fumonisin production is 75 F (which is cooler than that for aflatoxin). Bush et al (2003) found fumonisin concentrations increased from physiological maturity, thus early harvest may help reduce the level of contamination. The U.S. Food and Drug Administration (FDA) has guidelines for safe levels of fumonisins in corn used for foods and animal feeds. Fumonisins are acutely toxic to animals (especially pigs and horses), and have been linked to increased cancer rates and other human health problems.

aspergillus ear rot
Figure 1. Symptoms of Aspergillus ear rot are a powdery olive-green mold that develops on damaged kernels.


Fusarium ear rot
Figure 2. White to light pink mold is a characteristic of Fusarium ear rot.


Bush et al.  2004.  Phytopathology 94:88-93
Robertson et al. 2010.  Agronomy Journal 103: 193-199. 


Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact her at or phone 515-294-6708. Charles Hurburgh is a professor of Agricultural and Biosystems Engineering and professor in charge of the Iowa Grain Quality Initiative; contact him at or 515-294-8629.

Survival of the Goss’s Wilt Bacterium and Management Implications

By Alison Robertson and Gwyn Beattie, Department of Plant Pathology and Microbiology

Goss’s wilt is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn). 

The survival of Cmn in soil and crop residues was examined by Schuster (1975). Pure cultures of the bacterium in soil did not survive for long (less than two weeks), however the bacterium was able to survive for up to 10 months in infested surface crop residue. When the crop residue (leaves, stalks, cobs and ears) was buried at 4 inches or 8 inches, the bacterium was only detected in stalks residue after 10 months. Thus, conservation tillage practices that partially bury infested crop residue should reduce survival of the Goss’s wilt bacterium. Any tillage done must take into account soil conservation. Rotating to a non-host crop, such as soybean, will allow time for infested residues to breakdown and inoculum levels to decrease. 


What effect does ensiling have on the survival of Cmn?

No research has been done on the effect of ensiling on the survival of Cmn. During silage production, Cmn would be exposed to high temperatures, other microorganisms and low pH.

Although the effect of heat on the survival of Cmn has not been studied, it has been examined in closely related bacteria. Turner et al. (1983) concluded that survival of C.m. subsp. michiganensis (Cmm) was effectively reduced during anaerobic digestion at 95 F. Similarly Kaemmerer (2009) found C.m. subsp. sepedonicus was sensitive to heat during anaerobic digestion in biogas producers. Heat treatment at 127 F is used to control the sugar cane pathogen Clavibacter xyli subsp. xyli. Many bacterial plant pathogens are eradicated by a constant temperature of 140 F for one hour, in plant material (Noble et al., 2009). Thus, heat generated during silage production may negatively impact Cmn survivability.

The population of C.m. subsp. sepedonicus was negatively impacted by competition from other microbes in cattle manure slurry (Roozen and Vanvuurde 1991). Similarly, composts have been shown to reduce the survival of Cmm presumably due to competition although heat could also be involved (Yogev et al. 2009). Thus, competition from other microbes during silage production may reduce the survival of Cmn.

Low pH reduced the survival of Cmm (Ozdemir 2009). Thus, the low pH associated with silage may reduce survival of Cmn.


Survival of Cmn in bedding straw

There are no reports on the survivability of Cmn in bedding straw. If bedding straw is very dry, Cmn growth is likely to be limited, as it is for most non-sporeforming bacteria. If the bedding straw is moist, survival may be better, although growth is likely to be low due to Cmn’s highly specific nutrient requirements, including the need for multiple vitamins for growth (Vidaver 1982), which may explain the general view that host plants are virtually the only habitat for this organism. Its survival on straw is also likely to be limited due to competition from other microbes associated with manure.


Kaemmerer, D.   2009. Journal of Plant Diseases and Protection 116: 10-16.
Noble et al.  2009 Bioresource Technology 100: 3431–3446
Ozdemir, Z.   2009. Journal of Plant Pathology 91: 221-224.
Roozen et al.   1991.  Netherlands Journal of Plant Pathology 97: 321-334.
Turner et al.  1983.  Agric. Wastes 6: 1–11.
Vidaver. 1982. Annu. Rev. Microbiol. 36:495-517.
Yogev et al.  2009.  Crop Protection 28: 97-103.


Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact her at or phone 515-294-6708. Gwyn Beattie is a professor of bacteriology for research and nomenclature in the Department of Plant Pathology and Microbiology. She can be reached at 515-294-5571 or

New Stink Bug Identification Card Available

By Laura Jess, Plant and Insect Diagnostic Clinic and Erin Hodgson, Department of Entomology

A new, pocket-sized identification card for stink bugs in the Midwest will make identification of these insects much handier. Iowa State University Extension has just released the new identification card from authors Laura Jesse, Erin Hodgson, Donald Lewis, Matt O’Neal and Adam Sisson. The card was made possible with funding from a 2011 North Central IPM Center mini-grant.

In 2011, the brown marmorated stink bug (BMSB) was confirmed in Iowa. These were dead specimens; no live BMSB have been confirmed this year. ISU entomologists are actively looking for this pest throughout the state because this invasive species has the potential to damage corn, soybean and many other plants. In addition, BMSB is a home invader similar to the multicolored Asian lady beetle.

BMSB nymphs and adults have unique body characters (Fig. 1), but are easily confused with other stink bugs and incidental insects. This new card will help you distinguish some of the most common look-alikes in Iowa. In addition, a previous ICM News article gives longer descriptions for stink bugs.

Brown marmorated stink bug nymph (left, photo UGA1460050 by David R. Lance, USDA APHIS PPQ) and adult (right, photo David Shetlar, the Ohio State University).

To learn more about BMSB in Iowa, visit our Stink bug Web page. If you think you see BMSB, please send a high-quality photo by e-mail to or mail the specimen to the ISU Plant and Insect Diagnostic Clinic at I 327 Bessey Hall, Ames, IA  50011.

Printed copies of the Stink bugs of the Midwest can be ordered from the ISU Extension Online Store at or by calling 515-294-5247.


Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at or by phone 515-294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847.

This article was published originally on 8/29/2011 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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