CROP INSIGHTS Gibberella Ear Rot of Corn by Gary Munkvold and Steve Butzen

fungus. This article will describe Gibberella ear rot Summary development, disease symptoms, associated mycotoxins, and management practices to help control the disease in the field  Gibberella ear rot, a pink-colored mold that usually begins and grain bin. at the tip of the ear, can occur throughout North America, but especially thrives in northern corn growing areas where summer conditions are relatively cool and wet. Disease Development Like most ear rots, Gibberella zeae spores are produced on  Mycotoxin contamination of grain may or may not accom- crop residue and spread to corn ears by wind and splashing pany Gibberella ear rot symptoms. The primary myco- rain. The same fungus causes head scab of wheat, so wheat toxin is deoxynivalenol (DON), also called vomitoxin. residue can contribute to the severity of the corn disease and vice versa. Infection occurs through the young silks and is favored by relatively cool, wet weather during pollination  Selecting more tolerant hybrids can help reduce Gibberella and afterwards. ear rot and associated mycotoxins. Rotation and residue management may also provide some benefit. Insects typically play only a minor role in the disease cycle of Gibberella ear rot, although increased infection may  Scouting fields prior to harvest is essential to identify sometimes result from ECB activity. severe disease outbreaks. Affected fields should be harvested as soon as grain moisture allows. Disease Symptoms  Harvest and grain handling practices can minimize kernel damage, reducing further Gibberella infection and Gibberella ear rot can be most readily identified by the red or improving storability of grain. Infected grain should be pink color of the mold. Early infected ears may rot dried to 15% moisture or less. completely, with the husks adhering tightly to the ear and the mold growing between the husks and ear. In some cases, the  Suspect grain should be tested for DON, zearalenone and mold color is very pale and appears to be white, causing it to other trichothecene mycotoxins – this is the only way to be confused with Fusarium ear rot. Gibberella almost always truly assess the risk to animal health. begins at the ear tip, and progresses from there, while Fusar-  For on-farm use, blending with clean grain can be an ium is usually more generally scattered throughout the ear or effective means of grain utilization. Feeding to more tol- localized on injured kernels. erant animals such as ruminants and poultry is suggested.  When blending, knowing initial DON levels can help pre- vent contamination of even more grain. Feeding mold- affected grain as soon as possible is usually less risky than storing it.

Gibberella ear rot is typically a problem in parts of the northern and eastern Corn Belt (both U.S. and Canada) where humidity is high, moisture is plentiful, and temperatures are moderate. Mycotoxin contamination of grain may or may not accompany Gibberella mold symptoms. Occurrence and impact of associated mycotoxins is not completely understood, but is thought to be highly dependent on weather. This makes outbreaks of high mycotoxin levels sporadic and unpredictable. To guard against possible mycotoxin contamination, growers should strive to limit infection of corn ears by the Gibberella

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Gibberella ear rot almost always begins in the tip of the ear. Fusarium graminearum, but the symptoms it causes are distinct from Fusarium ear rot.) DON is part of a family of Gibberella ear rot can sometimes be confused with Diplodia mycotoxins called trichothecenes, which includes the T-2 ear rot. Both can rot the whole ear and mummify it with toxin. DON causes feed refusal and poor weight gain in white mold. But Diplodia usually starts at the base of the ear, livestock, especially swine. In high enough doses it can tends to be gray rather than pink, and produces black result in vomiting and hemorrhage (thus its common name, pycnidia between the moldy kernels or on the husk. vomitoxin), but in actuality, this rarely occurs. In addition to DON, G. zeae can produce other mycotoxins such as zearalenone, but they occur much less frequently than DON.

Management of Gibberella Ear Rot Hybrid Selection Plant breeders have made steady progress in developing hybrids with lower occurrence of Gibberella ear rot. Breeders have selected for physical traits that limit disease development, as well as genetic tolerance. Physical traits: Early studies demonstrated correlations between Gibberella ear rot occurrence and physical traits, such as husk tightness. Tight husks can hinder the rate of grain drying, maintaining higher moisture contents favorable to Gibberella ear rot. In addition, hybrids with ears that do not remain upright after maturity experience less ear rot.

Because higher moisture favors Gibberella development, growers should avoid late-maturing hybrids that retain ear moisture longer in the fall. Selecting adapted hybrids with good ear drydown characteristics is an important management practice to help reduce the level of Gibberella ear rot. Genetic tolerance to Gibberella ear rot has also been identified, and incorporated into currently used hybrids. By selecting for desirable physical traits and tolerance, corn breeders have eliminated very susceptible genotypes. Most widely grown hybrids are not excessively susceptible, but unacceptable mycotoxin levels still occur when weather conditions favor severe outbreaks of the disease. Most commercial hybrids score between 4 and 6 for Gibberella tolerance, on a 1 to 9 scale. For this disease, it has been difficult for researchers to identify highly resistant hybrids that meet acceptable standards for yield and other traits. The sporadic nature of disease outbreaks has

complicated the process of screening hybrids and limited Top – Left: Corn husks drying prematurely from Gibberella resistance development efforts. infection. Right: Severe Gibberella ear rot on corn ear. Bt Hybrids: A few research studies in Canada and Europe Bottom: Fusarium (left) and Diplodia (right) ear rot on corn. have reported on comparisons between Bt and non-Bt hybrids for DON and other trichothecenes, as well as zearalenone. Results have shown that there are usually only Mycotoxins Produced by Gibberella minor reductions in some of these toxins with Bt hybrids, because Gibberella zeae does not rely on insect injuries as a Gibberella zeae , the fungal pathogen that causes Gibberella pathway to kernel infection. ear rot, can produce the mycotoxin deoxynivalenol (DON), also called vomitoxin. (This fungus is also known as

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Crop Residue Management physiologically active. Grain with significant ear rot symptoms from the field should be dried at high temperature Crop residues are clearly the most important source of as quickly as possible to 15% or less to minimize the risk of inoculum for Gibberella ear rot, and managing crop residues mycotoxin development. The lower the moisture content in through rotation or tillage is often suggested as a control storage, the lower the risk of mycotoxin development. measure. However, some research studies have shown that tillage practices did not affect the incidence of this disease. Corn planted following corn or wheat is at a higher risk for Storage Gibberella ear rot. However, even if inoculum sources Gibberella zeae rarely develops in storage if proper practices within the field are minimized through tillage or rotation, are used (low grain moisture, aeration, proper storage airborne spores from distant fields are often sufficient to temperature and insect control). If these factors are not cause economic disease levels. controlled, Gibberella and other molds can still develop in Scouting and Harvest Timing storage because of moisture variability within the grain mass or moisture migration that results from rapid grain cooling Management of Gibberella and its mycotoxins requires late- along the bin walls. season scouting in order to make informed decisions about harvest timing, postharvest grain handling, storage and Observe Grain Frequently utilization. Timing of harvest can have major consequences Because of the unpredictability associated with stored grain, for the ultimate level of mycotoxin accumulation. Earlier no matter how carefully it is dried and aerated, frequent harvest results in lower concentrations of DON. While grain observations are necessary to head off developing problems dries slowly in the field, moisture content remains high with molds and mycotoxins. Weekly observation is enough to allow continued development and toxin recommended during warm months, and every two weeks production by Gibberella fungi that infect kernels preharvest. during the winter. Observations should include inspection Gibberella zeae can continue to grow and produce for overall temperature, crusts or mold on the grain, moisture mycotoxins while the grain is above 20% moisture content. in the bin, moldy odor, and warm spots. If any problems are detected, steps should be taken immediately to reduce the Growers should schedule fields for harvest based not only on temperature, aerate the bin, break up hot spots, or remove grain moisture, but also on crop condition, including severity spoiled grain. of Gibberella ear rot. If extensive mold development is Testing for Mycotoxins observed (for example, 10% of ears with more than 20% mold) harvest as soon as moisture content allows. Drying Extension Plant Pathologists from five universities have costs will probably be less expensive than loss of crop jointly issued the following procedures for testing for quality and resulting feeding problems. Make decisions on mycotoxins in harvested grain (DeWolf et al. 2003): handling moldy grain before it is harvested. Once harvested, the grain condition can worsen quickly if the moisture  The presence of a fungus known to produce toxins is not content is not low enough. proof that the grain contains injurious levels of toxin. It may be a good investment to collect a representative Combining and Handling sample and send it to a laboratory for chemical analysis. Physical damage to grain during harvest and handling allows  The first step in mycotoxin determination is sampling of ready invasion and colonization by Gibberella zeae, and the grain. Particular attention should be given to the increases the potential for mycotoxin development.. sampling procedure because sampling error will be the Potential damage can be reduced by adjusting the combine’s greatest source of variation in the analytical procedure. cylinder speed and clearance. Furthermore, harvested grain This variation is primarily due to the uneven distribution quality can be improved by increasing the combine fan speed of the mycotoxin contaminated kernels within a lot of so that the low-density, moldy kernels are discarded through grain or feed. The ideal sampling procedure should assure the back of the combine. This strategy has obvious the highest probability of detecting mycotoxins even when limitations -- discarded grain is an economic loss, so it is contamination is low. undesirable to discard very much grain.  One method of sampling grain is to use a probe sampler. Grain Drying Since mold growth usually occurs in spots in the grain lot, best sampling is done on recently blended lots of grain. After harvest, reducing grain moisture by artificial drying is a valuable tool for arresting Gibberella development and mycotoxin production. The objective of grain drying is to  Another method is to collect small samples from the reduce moisture content to the extent that molds, both moving stream of grain as it is moved in or out of the bins. toxigenic and nontoxigenic, are not able to grow or remain With both sampling methods, the collected grain is pooled

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pooled into a large aggregate sample that represents the 1Research suggests that grain at or below these levels is safe for lot. feeding, but see section on “Advisory Levels” above.  For shelled corn, it is recommended that the aggregate Future Genetic Resistance? sample be about 10 pounds. The aggregate sample should be coarsely ground. Most analytical procedures use only Scientists are focusing considerable resources on finding a about 25 grams (0.9 ounces) of ground corn, so it is genetic solution to Gibberella ear rot. In one study, important that the aggregate sample be thoroughly mixed researchers determined that resistance to infection through after grinding. A one or two pound sub-sample is then the silks and resistance to spread of the fungus among the taken and more finely ground. From this sub-sample a kernels are under separate genetic control. In addition, at final sample is taken for analysis. least one major gene for resistance has been identified within the corn genome. Corn parent lines and hybrids expressing  A number of commercial, university and government this gene have low severity of Gibberella ear rot and their laboratories perform mycotoxin analyses for a fee. Contact grain also has reduced levels of DON compared to the lab to determine the proper way to obtain and ship the susceptible genotypes. sample. For general information see: (http://www.oardc.ohio- state.edu/ohiofieldcropdisease/wheat/mycotoxin%20text2.htm) Whether or not this particular gene proves to be successful, scientists are continuing to screen for resistance within the corn genome. Pioneer is evaluating numerous potential  Blending is not an approved practice by the FDA for sources of resistance both independently and through col- interstate commerce. Blending is a practice intended to laboration with public research institutions. Contemporary reduce toxins to acceptable levels in small lots only for on- approaches to molecular genetics are assisting researchers to farm use. utilize native resistance, but major-gene resistance is not yet  If the mycotoxin in the contaminated feed is known, it available in commercial hybrids. Transgenic resistance may be a good idea to channel the feed to animals that are strategies are also being pursued intensively, but the fruits of more tolerant. this research are at least several years away.

Advisory Levels: The U.S. Food and Drug Administration Conclusions has determined advisory levels for deoxynivalenol in grain and feed for different livestock species (Table 1). Grain Outbreaks of Gibberella ear rot with potentially harmful testing at or below the advisory level is believed to be safe levels of mycotoxins occur sporadically in corn, and are based on research results. However, occasional field difficult to predict. This makes management a challenge. observations suggest that detrimental effects may be seen at Cultural practices used in the field have limited capabilities, lower levels. Some of these observations may reflect the but can possibly be manipulated to reduce the risk of grain presence of other mycotoxins in combination with low DON contamination by DON. Appropriate practices in harvesting, levels. If grain tests near the advisory level for DON, or if handling and storing grain are also critical tools in managing untested grain is suspected of possible mycotoxins, consider mycotoxin levels. Pioneer continues to work toward feeding the grain in small batches only, while carefully developing improved hybrid tolerance to the disease. Direct monitoring animal health. genetic resistance is the best long-term strategy for dealing with Gibberella ear rot and its mycotoxins. Silage: DON can be present in silage coming from the field, but the fungus cannot develop further or produce mycotoxins in an anaerobic environment. Although References problems associated with feeding silage are often attributed Del Ponte, E. M., D.A. Shah, and G.C. Bergstrom. 2003. to DON, this mycotoxin rarely occurs at high enough levels Spatial patterns of Fusarium head blight in New York wheat in silage to cause animal health effects. fields suggest role of airborne inoculum. Online. Plant Health Progress doi:10.1094/PHP-2003-0418-01-RS. Table 1. FDA advisory levels1 for DON in grain and feed. DeWolf, E., G. Kuldau, P. Lipps, G. Munkvold, P. Vincelli, Livestock DON in DON in Grain C. Woloshuk, and D. Mills, 2003. Moldy grains, mycotox- Type Final Diet Portion of Diet ins and feeding problems. OARDC-Ohio-State Website. http://www.oardc.ohio- Beef cattle and 5 ppm 10 ppm state.edu/ohiofieldcropdisease/Mycotoxins/mycopagedefault.htm chickens Munkvold, G.P. 2003. Cultural and genetic approaches to Swine 1 ppm 5 ppm managing mycotoxins in maize. Annu. Rev. Phytopathol. 2003. 41:99–116. All other animals 2 ppm 5 ppm

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