Gary P. Munkvold

Comparison of Fumonisin Concentrations in Kernels of Transgenic Bt Maize Hybrids and Nontransgenic Hybrids

Maize hybrids genetically engineered with genes from the bacterium Bacillus thuringiensis (Bt maize) express CryIA(b) and other Cry proteins that are toxic to certain insects, particularly the European corn borer (Ostrinia nubilalis). Maize kernel feeding by O. nubilalis often leads to infection by fungi in the genus Fusarium, including the fumonisin-producing species F. verticillioides and F. proliferatum. In field experiments in 1995, 1996, and 1997, transgenic maize hybrids and near-isogenic, nontransgenic hybrids were manually infested with neonatal European corn borer larvae. Manual infestation increased Fusarium ear rot severity and fumonisin concentrations in kernels of nontransgenic hybrids. Transgenic hybrids with kernel expression of CryIA(b) consistently experienced less insect feeding on kernels and less Fusarium ear rot than their nontransgenic counterparts. In manually infested treatments, these hybrids also exhibited lower concentrations of fumonisins in kernels compared with their nontransgenic counterparts. In manually infested treatments in 1995, mean fumonisin B1 concentrations were 8.8 μg/g in the nontransgenic hybrid and 6.7 or 3.0 μg/g in transgenic hybrids. In 1996, mean fumonisin B1 concentrations in manually infested treatments were 4.9 μg/g (range 2.3 to 8.8) for nontransgenic and 1.2 μg/g (range 1.0 to 1.3) for transgenic hybrids with kernel expression. Mean total fumonisin concentrations (fumonisin B1 + B2 + B3) were 7.0 μg/g (range 3.0 to 12.2) for nontransgenic and 1.7 μg/g (range 1.5 to 1.9) for transgenic hybrids with kernel expression. In 1997, mean fumonisin B1 concentrations in manually infested treatments were 11.8 μg/g (range 7.6 to 17.3) for nontransgenic and 1.3 μg/g (range 0.8 to 2.2) for transgenic hybrids with kernel expression of CryIA(b) or Cry9C. Mean total fumonisin concentrations were 16.5 μg/g (range 10.7 to 24.0) for nontransgenic and 2.1 μg/g (range 1.5 to 3.1) for transgenic hybrids with kernel expression. Transgenic hybrids that do not express CryIA(b) or Cry9C in kernels did not consistently have fumonisin concentrations different from the nontransgenic hybrids. Higher fumonisin concentrations in nontransgenic hybrids were associated with high European corn borer populations during the early reproductive stages of the maize plants. These results indicate that under some conditions, genetic engineering of maize for insect resistance may enhance its safety for animal and human consumption.

Epidemiology of Fusarium diseases and their mycotoxins in maize ears

Fusarium species cause two distinct diseases on ears of maize, Fusarium ear rot (or pink ear rot) and Gibberella ear rot (or red ear rot), both of which can result in mycotoxin contamination of maize grain. The primary causal agent for Fusarium ear rot is Fusarium verticillioides, but F. subglutinans and F. proliferatum are also important. Gibberella ear rot is caused primarily by F. graminearum, but F. culmorum can also be important, especially in Europe. Aspects of the epidemiology of both diseases have been studied for decades, but only recently have efforts been made to synthesize this information into comprehensive models of disease development. Much of the work on F. graminearum has focused on Fusarium head blight of small-grain crops, but some of the results obtained are also relevant to maize. The primary mycotoxins produced by these fungi, fumonisins and deoxynivalenol, have differing roles in the disease-cycle, and these roles are not completely understood, especially in the case of fumonisins. Progress is being made toward accurate models for risk assessment of both diseases, but key challenges remain in terms of integrating models of pre- and post-infection events, quantifying the roles of insects in these diseases, and characterizing interactions among competing fungi and the environment.

Reduced Fusarium Ear Rot and Symptomless Infection in Kernels of Maize Genetically Engineered for European Corn Borer Resistance

ABSTRACT Field experiments were conducted in 1994, 1995, and 1996 to evaluate the incidence and severity of Fusarium ear rot and the incidence of symp-tomless Fusarium infection in kernels of maize hybrids genetically engineered with Bacillus thuringiensis genes encoding for the delta-endotoxin CryIA(b). Treatments included manual infestation with European corn borer (ECB) larvae and insecticide applications to limit ECB activity to specific maize growth stages or mimic standard ECB control practices. Fusarium symptoms and infection were affected by the specific cryIA(b) transformation used in each hybrid that determines tissue-specific expression of CryIA(b). In hybrids expressing CryIA(b) in kernels, incidence and severity of Fusarium ear rot and incidence of symptomless kernel infection were reduced compared with near-isogenic hybrids lacking cryIA(b) genes. In plants that were manually infested with ECB, ear rot incidence was reduced by 87, 58, and 68%; severity was reduced by 96, 54, and 64%; and incidence of kernel infection by Fusarium species was reduced by 17, 38, and 38% in 1994, 1995, and 1996, respectively. Results were similar in treatments that were not manually infested, but differences between transgenic and nontransgenic hybrids were smaller. Most kernel infection was due to F. moniliforme, F. proliferatum, and F. subglutinans (section Liseola) collectively, and it was within this group that transgenic hybrids exhibited reduced infection. Expression of CryIA(b) in plant tissues other than kernels did not consistently affect Fusarium symptoms or infection. Disease incidence was positively correlated with ECB damage to kernels. Insecticide applications also reduced Fusarium symptoms and infection when applied to nontransgenic plants.

Effect of Processing on Fumonisin Content of Corn

Fumonisins (FBs) are a family of mycotoxins produced by Fusarium moniliforme and F. proliferatum, predominant corn pathogens, and are found in most corn-containing foods. The FBs are heat stable, resistant to ammoniation, and unlike most mycotoxins, are water-soluble. The levels in corn and corn-containing foods will be presented ranging from < 20 ppb to > 2 ppm. Washing of contaminated FB-corn with water did not reduce the measured FB levels of significantly. The traditional processing step to make tortilla flour, nixtamalization [Ca(OH)2 cooking] to produce masa, reduced FB levels but produced hydrolyzed FB which was almost as toxic as FB. Retorting sweet corn in brine apparently produced hydrolyzed FB. Fermentation of corn to ethanol did not alter FB levels but distillation yielded FB-free ethanol. Attempts to enzymatically modify FB with several enzymes were unsuccessful. Reactions between FB and reducing sugars (glucose or fructose) to produce Schiffs bases yielded products that were not toxic. The effects of these processing treatments must be evaluated both chemically and biologically.

Climate change impacts on mycotoxin risks in US maize

To ensure future food security, it is crucial to understand how potential climate change scenarios will affect agriculture. One key area of interest is how climatic factors, both in the near- and the long-term future, could affect fungal infection of crops and mycotoxin production by these fungi. The objective of this paper is to review the potential impact of climate change on three important mycotoxins that contaminate maize in the United States, and to highlight key research questions and approaches for understanding this impact. Recent climate change analyses that pertain to agriculture and in particular to mycotoxigenic fungi are discussed, with respect to the climatic factors – temperature and relative humidity – at which they thrive and cause severe damage. Additionally, we discuss how climate change will likely alter the life cycles and geographic distribution of insects that are known to facilitate fungal infection of crops.

Probabilities for Profitable Fungicide Use Against Gray Leaf Spot in Hybrid Maize

ABSTRACT Gray leaf spot, caused by the fungus Cercospora zeae-maydis, causes considerable yield losses in hybrid maize grown in the north-central United States and elsewhere. Nonchemical management tactics have not adequately prevented these losses. The probability of profitably using fungicide application as a management tool for gray leaf spot was evaluated in 10 field experiments under conditions of natural inoculum in Iowa. Gray leaf spot severity in untreated control plots ranged from 2.6 to 72.8% for the ear leaf and from 3.0 to 7.7 (1 to 9 scale) for whole-plot ratings. In each experiment, fungicide applications with propiconazole or mancozeb significantly reduced gray leaf spot severity. Fungicide treatment significantly (P </= 0.05) increased yield by as much as 1.65 t/ha with a single propiconazole application. There were significant (P < 0.05) correlations between gray leaf spot severity and yield. We used a Bayesian inference method to calculate for each experiment the probability of achieving a positive net return with one or two propiconazole applications, based on the mean yields and standard deviations for treated and untreated plots, the price of grain, and the costs of the fungicide applications. For one application, the probability ranged from approximately 0.06 to more than 0.99, and exceeded 0.50 in six of nine scenarios (specific experiment/hybrid). The highest probabilities occurred in the 1995 experiments with the most susceptible hybrid. Probabilities were almost always higher for a single application of propiconazole than for two applications. These results indicate that a single application of propiconazole frequently can be profitable for gray leaf spot management in Iowa, but the probability of a profitable application is strongly influenced by hybrid susceptibility. The calculation of probabilities for positive net returns was more informative than mean separation in terms of assessing the economic success of the fungicide applications.

Aggressiveness of Fusarium Species and Impact of Root Infection on Growth and Yield of Soybeans

Fusarium spp. are commonly isolated from soybean roots but the pathogenic activity of most species is poorly documented. Aggressiveness and yield impact of nine species of Fusarium were determined on soybean in greenhouse (50 isolates) and field microplot (19 isolates) experiments. Root rot severity and shoot and root dry weights were compared at growth stages V3 or R1. Root systems were scanned and digital image analysis was conducted; yield was measured in microplots. Disease severity and root morphology impacts varied among and within species. Fusarium graminearum was highly aggressive (root rot severity >90%), followed by F. proliferatum and F. virguliforme. Significant variation in damping-off (20 to 75%) and root rot severity (<20 to >60%) was observed among F. oxysporum isolates. In artificially-infested microplots, root rot severity was low (<25%) and mean yield was not significantly reduced. However, there were significant linear relationships between yield and root symptoms for some isolates. Root morphological characteristics were more consistent indicators of yield loss than root rot severity. This study provides the first characterization of aggressiveness and yield impact of Fusarium root rot species on soybean at different plant stages and introduces root image analysis to assess the impact of root pathogens on soybean.

Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. corn belt

The use of foliar fungicides on field corn has increased greatly over the past 5 years in the United States in an attempt to increase yields, despite limited evidence that use of the fungicides is consistently profitable. To assess the value of using fungicides in grain corn production, random-effects meta-analyses were performed on results from foliar fungicide experiments conducted during 2002 to 2009 in 14 states across the United States to determine the mean yield response to the fungicides azoxystrobin, pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin. For all fungicides, the yield difference between treated and nontreated plots was highly variable among studies. All four fungicides resulted in a significant mean yield increase relative to the nontreated plots (P < 0.05). Mean yield difference was highest for propiconazole + trifloxystrobin (390 kg/ha), followed by propiconazole + azoxystrobin (331 kg/ha) and pyraclostrobin (256 kg/ha), and lowest for azoxystrobin (230 kg/ha). Baseline yield (mean yield in the nontreated plots) had a significant effect on yield for propiconazole + azoxystrobin (P < 0.05), whereas baseline foliar disease severity (mean severity in the nontreated plots) significantly affected the yield response to pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin but not to azoxystrobin. Mean yield difference was generally higher in the lowest yield and higher disease severity categories than in the highest yield and lower disease categories. The probability of failing to recover the fungicide application cost (p(loss)) also was estimated for a range of grain corn prices and application costs. At the 10-year average corn grain price of

Associations of planting date, drought stress, and insects with Fusarium ear rot and fumonisin B1 contamination in California maize.

0.12/kg (

Plant disease incidence: inverse sampling, sequential sampling, and confidence intervals when observed mean incidence is zero

2.97/bushel) and application costs of