Prepared byGary P. MunkvoldIowa State University Dept. of Plant PathologyRichard L. HellmichUSDA-ARS, Corn Insects and Crop Genetics Research Unit, and Iowa State University Dept. of Entomology
Munkvold, G.P. and Hellmich, R.L. 1999. Genetically modified insect resistant corn: Implications for disease management. APSnet Features. Online. doi: 10.1094/APSnetFeature-1999-1199
In the current atmosphere surrounding Bt corn production, the need for investigation into all potential risks and benefits of Bt technology is more critical than ever. Approval by EPA carries with it assurance that these products are safe, but additional data may be needed so consumers can make informed choices and convey their preferences to policymakers. One aspect of risk/benefit analysis is the influence that Bt technology may have on corn diseases and mycotoxin-producing fungi in corn.
European corn borer.Click image for enlargment.
Corn earworm.Click image for enlargment.
Southwestern corn borer.Click image for enlargment.Stalk tunneling.Click image for enlargment.
The associations between these insects and corn diseases result from several types of host-insect-pathogen interactions. One type of interaction is a vector relationship. European corn borer larvae carry spores of Fusarium species from the plant surface to the surfaces of damaged kernels (Sobek and Munkvold, 1999) or to the interior of stalks, where infections are initiated. Viable spores can be found externally, internally, and in the frass of European corn borer larvae. Similar relationships exist between corn earworm or southwestern corn borer and Fusarium or Aspergillus spp. (Dowd, 1998). A second type of interaction is the formation of entry wounds for the fungi when larvae feed on stalks or kernels. Even when the larvae do not directly carry the fungi into the stalks, spores subsequently deposited on the wounded tissue are very likely to infect the plant. Additionally, root and stalk damage by insects causes stress that predisposes the plants to stalk rot development. For these reasons, management of these insects can play a major role in corn root and stalk rot management.
Fusarium ear rot.Click image for enlargment.
Fumonisins are a group of mycotoxins that can be fatal to horses and pigs, and are probable human carcinogens (Munkvold and Desjardins, 1997). The importance of fumonisins in human health is still a subject of debate, but there is evidence that they have some impact on cancer incidence in some parts of the world (Marasas, 1995). Fumonisin concentrations in corn are or will be under regulatory scrutiny in many parts of the world (Miller, 1999). Symptoms of Fusarium ear rot are often highly correlated with ear damage by European corn borer and corn earworm larvae (Table 1) (Christensen and Schneider, 1950; Munkvold et al., 1999; Smeltzer, 1958). Several Fusarium species can infect kernels without causing visible symptoms, but still affect grain quality and produce mycotoxins.
Fusarium ear rot severity (kernels/ear)
Fumonisin B1 (µg/g)
The stalk rot complex comprises the most serious, widespread disease problem in corn. Yield losses occur as a result of premature plant death and lodging. Stalk rot-affected fields usually are damaged by more than one fungal species, but Gibberella stalk rot, caused by Gibberella zeae, Fusarium stalk rot, caused by Fusarium verticillioides (F. moniliforme), F. proliferatum, or F. subglutinans, and anthracnose stalk rot, caused by Colletotrichum graminicola are the most frequently reported (Smith and White, 1988). The development of stalk rot is greatly affected by plant stress (Dodd, 1980) and may or may not be associated with insect damage.
Kernel rot.Click image for enlargment.Gibberella stalk rot.Click image for enlargment.Anthracnose stalk rot.Click image for enlargment.
In these studies, differences among types of Bt genes (or Bt events) have become evident. All Bt events are not alike. Currently available Bt hybrids express either CryIA(b), CryIA(c), or Cry9C, all members of a group of delta-endotoxins originally produced by some strains of the bacterium Bacillus thuringiensis. Table 2 shows some characteristics of currently available Bt events. The expression of Cry proteins in specific corn plant tissues is dependent on the gene promoter used in each transgenic genotype. Proprietary cryIA(b) transformations BT11 and MON810 (YieldGard®) use a CaMV 35S gene promoter that results in season-long expression of CryIA(b) in all plant tissues, whereas cryIA(b) transformation 176 (marketed as Knockout® and NatureGard®) uses a combination of two maize-derived, tissue-specific promoters: a phosphoenolpyruvate carboxylase promoter that results in gene expression only in green plant tissues, and a pollen-specific promoter. Kernel expression of CryIA(b) appears to be an important factor determining the amount of kernel feeding by European corn borer larvae and subsequently the intensity of Fusarium infection.
FIGURE ONE.Click image for enlargment.
Ear samples from a 1997 field trial.Click image for enlargment.
Another limitation of Bt corn hybrids is their spectrum of activity. Currently available events are very effective against European corn borer but not as effective against corn earworm and fall armyworm. In the southern United States, where aflatoxin problems are chronic, these species and southwestern corn borers are the primary lepidopteran pests feeding on corn ears. Damage to ears of Bt hybrids by these insects probably leads to A. flavus and F. verticillioides (F. moniliforme) infection and mycotoxin contamination.
Debate surrounding the use of genetically modified crops should be based on an assessment of all risks and benefits that can be measured, including environmental impacts, livestock impacts, and potential human health threats. Available data show that Bt transformation of corn hybrids enhances the safety of the grain for livestock feed by reducing its vulnerability to mycotoxin-producing fungi. These mycotoxins also are likely to be detrimental to human health, so the lower concentrations of mycotoxins in Bt corn potentially have implications for food safety. Lower mycotoxin concentrations represent a clear benefit to consumers of Bt grain, whether the intended use is for livestock or human foods. Consumers and regulatory agencies should consider these factors in decisions regarding Bt corn use.
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