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

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CROP INSIGHTS Gibberella Ear Rot of Corn by Gary Munkvold and Steve Butzen 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 CROP INSIGHTS VOL. 14 NO. 12 PAGE 1 ®Registered trademark of Pioneer Hi-Bred International, Inc. ©2004, PHII 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 manage- ment 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 CROP INSIGHTS VOL. 14 NO. 12 PAGE 2 ®Registered trademark of Pioneer Hi-Bred International, Inc. ©2004, PHII 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
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