Published in Agron. J. 96:565-570 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
PLANT DISEASE
Aflatoxin Accumulation in Maize Hybrids of Different Maturities
F. J. Betrán*,a and
T. Isakeitb
a Maize Breeding and Genet. Progr., Texas A&M Univ., College Station, TX 77845
b Dep. Plant Pathology and Microbiol., Texas A&M Univ., College Station, TX 77843
* Corresponding author (javier-betran{at}tamu.edu).
Received for publication June 16, 2003.
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ABSTRACT
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The incidence and severity of preharvest aflatoxin is greater under drought conditions, which commonly occur late in the growing season of south and central Texas. To determine if early maturation could be used as a means of disease escape, aflatoxin contamination was measured in early-, intermediate-, and full-season commercial hybrids at two Texas locations, Weslaco and College Station. The early and intermediate hybrids chosen are primarily marketed in midwestern USA while the full-season hybrids are primarily marketed in southeastern USA. Hybrids were evaluated by inoculating ears 6 to 10 d after midsilk with Aspergillus flavus NRRL 3357 using the silk channel technique and measuring aflatoxin in harvested grain using the VICAM Aflatest procedure. Across locations, full-season hybrids had lower aflatoxin (mean = 777 ng g–1) levels than intermediate (mean = 1668 ng g–1) and early (mean = 1899 ng g–1) hybrids. There was an inverse correlation between silking date and aflatoxin levels at both locations (r = –0.59, P = 0.01 at College Station and r = –0.58, P = 0.01 at Weslaco). Early and intermediate hybrids had looser husk coverage than full-season hybrids, a characteristic that was positively correlated with aflatoxin levels at both locations. At both locations, grain yield was lower with early and intermediate hybrids than with full-season hybrids. Early maturation in hybrids was insufficient by itself to reduce aflatoxin contamination, but it should be re-evaluated using early maturing hybrids that have good agronomic adaptation to these two Texas growing conditions.
Abbreviations: DRM, days to relative maturity
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INTRODUCTION
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AFLATOXIN, the mycotoxin produced by Aspergillus flavus Link:Fr., causes aflatoxicosis in livestock and is a hepatic carcinogen in humans (Castegnaro and McGregor, 1998). Preharvest aflatoxin contamination of maize (Zea mays L.) is a chronic problem in the southeastern United States, which limits maize marketability and causes economic losses (Widstrom, 1996; Cardwell, 2001). The mean potential value of feed maize lost to aflatoxin is $225 million per year, out of the estimated $932 million lost to this and other mycotoxins on crops in the United States (CAST, 2003).
Aflatoxin contamination of maize has been associated with drought combined with high temperature as well as insect injury (Payne, 1992). Approaches to reduce aflatoxin include cultural practices and crop management, host plant resistance through breeding and/or genetic engineering, and biocontrol (e.g., atoxigenic strains) (Brown et al., 1998; Widstrom, 1987). Lower aflatoxin contamination has been associated with the expression of secondary traits such as husk coverage and tightness, insect resistance, kernel integrity under environmental stress, and drought tolerance (Lillehoj et al., 1975; Odvody et al., 1997). Genetic variation for response to aflatoxin has been found in maize (Widstrom, 1987; Scott and Zummo, 1988; Campbell and White, 1995; Betrán et al., 2002). Sources of resistance include inbred lines Mp420, Mp313E, Mp715, Tex6, and population GT-MAS:gk (Scott and Zummo, 1992; McMillian et al., 1993; Windham and Williams, 2002). However, the majority of these sources of resistance lack agronomic performance, which precludes their direct use in commercial hybrids. Objectives of several research groups are the transfer of resistance from these sources of resistance to elite inbreds and hybrids and the search for resistance in elite commercial material. There are no commercial hybrids resistant to aflatoxin. Additionally, information about the comparative aflatoxin accumulation of commercial hybrids under field conditions in the southeastern USA is limited.
Seasonal, climatic, biotic, and cultural practices are important factors in the development of aflatoxin, and some of these factors can be manipulated to reduce its incidence in a crop. Recommended cultural practices intended to reduce crop susceptibility include selecting a field suitable to maize production, using adapted hybrids of proper maturity group, maintaining a proper fertilization, using optimum planting densities and an early planting date, and irrigating when necessary (Jones, 1987).
Several studies have addressed the question of hybrid maturity with respect to the aflatoxin accumulation (Jones and Duncan, 1981; Jones et al., 1981). Although hybrids of short, intermediate, and late maturities differed in aflatoxin accumulation, the differences seemed to be affected by other factors such as planting date, location, and climatic conditions. In south and central Texas, drought conditions are common late in the season, which limit maize production and increase the incidence of aflatoxin. Short-season maize could escape growth-limiting conditions of a hot, dry summer and the associated aflatoxin in contrast to the full-season maize. Late maize hybrids can have greater exposure to higher temperatures at flowering and postflowering stages, greater A. flavus inoculum, and increased insect activity compared with early hybrids. The objectives of this study are to compare the aflatoxin contamination of early-, intermediate-, and full-season commercial hybrids at two Texas locations and to estimate the relationship of aflatoxin with maturity and husk coverage.
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MATERIALS AND METHODS
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Twenty-five commercial field and food maize hybrids selected by participating seed companies were evaluated at two locations in Texas. The College Station location at the Texas Agricultural Experiment Station in Burleson County has a humid subtropical climate and 99.3 cm of annual precipitation while the Weslaco location has a semiarid subtropical climate and 58.4 cm of annual precipitation (Griffiths and Bryan, 1987). The hybrids spanned a gradient of different maturities: full season [>115 d to relative maturity (DRM)], intermediate (95–115 DRM), and early (DRM < 95). Plots were 6.4 m long and 0.75 m apart, with a plant density of approximately 60000 plants/ha. An 
-lattice design with four replications was used (Patterson and Williams, 1976). No insecticides were applied after planting. The planting dates were typical for those used by commercial growers: 16 Feb. 2000 at Weslaco and 29 Feb. 2000 at College Station. Drought stress was induced in the aflatoxin trials by withdrawing furrow irrigation 3 wk before flowering to experience an estimated 50% yield reduction.
Yield data was obtained for these hybrids by planting a contiguous experiment at both locations. The design was similar to that used for the aflatoxin evaluation, except two, two-row replicates were used, and no drought stress was imposed. Ears were artificially inoculated with a conidial suspension of A. flavus isolate NRRL 3357. The fungus was grown on 370 g of sterile maize kernels in 2.8-L Fernbach flasks. Kernels were prepared for inoculation as follows. The kernels were soaked overnight in flasks with 400 mL of water, excess water was decanted, and the flasks were autoclaved 20 min and stored at ambient room temperature. Two days later, 100 mL of water was added to flasks, followed by autoclaving for 1 h and then 20 min autoclaving the following day. Flasks were inoculated with several 1-cm2 pieces from an agar culture and incubated at 25°C for 10 to 14 d. Spores were washed from kernels with 600 mL of sterile distilled water containing two to three drops of Tween 20/100 mL and filtered through four layers of sterile cheesecloth to prepare a concentrate. The conidial suspension was adjusted to a final concentration of 107/mL just before use.
Each ear was inoculated 6 to 10 d after silking with 3 mL of the conidial suspension using the nonwounding silk channel inoculation technique (Zummo and Scott, 1989), and 20 ears were inoculated per replicate. Inoculated ears were hand-harvested when kernel moisture in all hybrids was below 15%. Ears were husked, rated for visible fungal colonization, dried, shelled, and bulked. Whole kernel samples from each plot were ground using a Romer mill (Romer Labs, Union, MO), and aflatoxin was quantified from 50-g subsamples using the VICAM Aflatest (VICAM, Watertown, MA). Aflatoxin contents in ng g–1 were log-transformed to equalize variance. Antilogarithmic values were used to report the results.
In addition to aflatoxin content, the following secondary traits were measured in the inoculated trials: silking date, as days from planting to date at which 50% of the plants in the plot exhibited emerged silks, and husk coverage, a visual rating from 1 (good) to 5 (poor) (1 = husk leaves extended more than 2.54 cm from the tip of the ear; 2 = husk leaves covering the tip of the ear between 0 and 2.54 cm; 3 = husk leaves of the same length as the ear, and no grain is exposed; 4 = husk leaves are shorter than the ear, and tip kernels are exposed; and 5 = few husk leaves with more than few kernels exposed). In yield trials, combine grain plot weight was converted to Mg ha–1 and standardized to 15.5 g kg–1 moisture content. Individual analyses of variance were conducted for each trial and stress level with the PROC MIXED procedure from SAS (SAS Inst., 1997). Hybrids and environments were considered fixed effects. Replications and incomplete blocks within replications were considered random effects. Combined analysis of variance across locations and regression analysis of means across locations were conducted with PROC GLM and PROC REG (SAS Inst., 1997), respectively.
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RESULTS AND DISCUSSION
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In areas and seasons where late-season drought stress and high temperatures are present, early maturing maize hybrids occasionally have provided grain yield advantage over late-maturing hybrids (Troyer, 1983). Additionally, hybrids can be exposed to an increased risk of aflatoxin contamination when such environmental stresses occur during flowering and grain filling (Payne, 1992). Thus, early hybrids could conceivably escape aflatoxin contamination fostered by these environmental stresses, which occur frequently in the southern USA, particularly in south and central Texas. However, in our study of hybrids with a range of maturities, early hybrids had higher levels of aflatoxin than late-maturing hybrids (Table 1). Significant differences among hybrids were detected for all traits and environments. Interactions between location and both maturity and husk cover were nonsignificant. There was an overall negative correlation of silking date and aflatoxin levels (Fig. 1)
. This negative correlation between silking date and aflatoxin accumulation was significant in both locations (–0.59** at College Station and –0.58** at Weslaco, where ** indicates significance at the 0.01 probability level).
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Table 1. Aflatoxin (AF) rank and concentration, silking date, husk coverage rating, and grain yield for early-, intermediate-, and full-season hybrids at College Station and Weslaco, TX, during 2000.
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Fig. 1. Relationship between aflatoxin (AF) concentration (ng g–1) and maturity of full-, intermediate-, and early-season commercial hybrids inoculated with Aspergillus flavus across Weslaco and College Station, TX. SD, silking date.
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The association between maturity and aflatoxin found in this study was more pronounced than other studies where a consistent relationship between aflatoxin and maturity was not observed (LaPrade and Manwiller, 1977; Lillehoj et al., 1978, 1980; Widstrom et al., 1978). The timing of the stress in these trials could be a determining factor in aflatoxin production. In our trials, environmental stress caused by high temperatures and limited water supply was appreciable 
2 wk after flowering of the early hybrids. This is reflected in the high aflatoxin levels seen in all hybrids at both locations. The average aflatoxin was 926 ng g–1 (range from 176–2033 ng g–1) at College Station and 1742 ng g–1 (range from 282 to 3550 ng g–1) at Weslaco (Table 1). We hypothesized that the postflowering stress we imposed should favor increased aflatoxin production in late hybrids compared with early hybrids, but the opposite occurred.
Average silking date was 82.9 d (range 75.6–89.1 d) at College Station and 66.7 d (range 61.3–72.0 d) at Weslaco. Significant differences were observed among full-season, intermediate, and early hybrids for silking date. On average, full-season hybrids flowered 3.6 and 6.7 d later than intermediate and early hybrids at College Station and 4.3 and 7.0 d later at Weslaco, respectively.
While earliness is not a sufficient characteristic to reduce aflatoxin, the overall environmental adaptation of a hybrid appears to have importance for minimizing aflatoxin. The early hybrids used in this study are commonly planted in the midwestern USA, an environment of presumed adaptation. Based on yield from our experiments, these early hybrids are generally not adapted to either of the two Texas locations. The linear phenotypic correlation between grain yield and maturity was 0.71**. Grain yield average was 3.5 Mg ha–1 (range 0.7–7.2 Mg ha–1) at College Station and 6.7 Mg ha–1 (range 4.4–10 Mg ha–1) at Weslaco. The full-season hybrids, commonly planted in the southern USA, had higher yields than hybrids that matured earlier (Table 1). There was a negative correlation between grain yield and aflatoxin content of hybrids (Fig. 2)
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Fig. 2. Relationship between aflatoxin (AF) concentration (ng g–1) and grain yield (GY, Mg ha–1) of full-, intermediate-, and early-season commercial hybrids inoculated with Aspergillus flavus across Weslaco and College Station, TX.
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Grain yield in maize is especially sensitive to drought during flowering and early postsilking stages (Grant et al., 1989). In our evaluations for grain yield, plots were irrigated as needed to prevent water stress. High temperatures were present in both locations during flowering and grain-filling periods. At College Station, a hailstorm before flowering caused foliar damage, inducing additional stress and reducing grain yield. Under these conditions, full-season hybrids yielded more than intermediate and early hybrids. Early maturing hybrids have shown lower yield potential than late-maturing hybrids under nonstressed conditions because decreased leaf area, associated with a reduced leaf number and/or size, reduces assimilate supply (Hunter, 1980). Longer-season hybrids normally have more leaves than early maturing hybrids.
Husk coverage is a trait that can influence aflatoxin contamination. Our work presents a quantification of this relationship. The average husk coverage rating was 2.4 (range 1–3.8) at College Station and 3 (range 1.3–4.3) at Weslaco. Husk coverage was greater in full-season hybrids (Table 1). The correlation between husk coverage and aflatoxin content was significantly positive at both locations (0.77** at College Station and 0.76** at Weslaco) and across locations (Table 1 and Fig. 3)
. Early hybrids showed loose husk and poor husk coverage while full-season hybrids had greater husk coverage (Fig. 4)
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Fig. 3. Relationship between aflatoxin (AF) concentration (ng g–1) and husk coverage (HC) rating (1 = well-covered ear; 5 = poorly covered ear) of full-, intermediate-, and early-season commercial hybrids inoculated with Aspergillus flavus in College Station and Weslaco, TX.
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Fig. 4. Example of good husk coverage (top ear, rating = 1) and poor husk coverage (lower ear, rating = 4.5) (Courtesy: Dr. Gary Odvody).
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In the midwestern USA, where maize grain moisture contents at harvest generally exceed 20% and additional costs are incurred by postharvest grain drying, loose husks are preferred to allow faster drying in the field before harvest. By contrast, in the southeastern USA, where grain dries in the field to moisture contents generally less than 15% at harvest, tight and long husks are preferred to reduce or prevent insect damage. Husk coverage has been correlated with less aflatoxin in similar studies in Texas with white- and yellow-grained standards and Quality Protein Maize (QPM) (Betrán et al., 2002; Bhatnagar et al., 2003). In south Texas, hybrids with loose husks were more vulnerable to loss of kernel integrity (Odvody et al., 1997), which can predispose kernels to A. flavus infection. Maturity and husk coverage are closely correlated as later hybrids have more plant and husk leaves than early hybrids. Although the effects of these two traits on aflatoxin accumulation in these hybrids is difficult to separate, less aflatoxin in late-maturing hybrids has been observed in other experiments (data not shown).
The contribution of insect injury to aflatoxin was not measured in this study although no significantly injurious insect activity was observed at either location. At Weslaco, the southwestern corn borer (Diatraea grandiosella) and the corn earworm (Helicoverpa zea) are the most common pests, with the fall armyworm (Spodoptera frugiperda) as an occasional pest. At College Station, the southwestern corn borer and the corn earworm are common pests but are not as injurious as they are in Weslaco. In a study of hybrids from crosses of resistant parents, aflatoxin levels were substantially increased in the presence of the southwestern corn borer (Windham et al., 1999).
Based on data from two Texas locations, we conclude that early maturing varieties used in the midwestern USA are unsuitable for use in the southeastern USA as an approach to manage aflatoxin. If early maturation is considered a viable approach to escape aflatoxin contamination, then varieties developed with this trait should be agronomically adapted for southeastern conditions. The development of intermediate or full-season drought-tolerant hybrids adapted to southern growing conditions, with good husk coverage and insect resistance, should result in a combination of reduced aflatoxin contamination and enhanced grain yield potential.
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ACKNOWLEDGMENTS
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We thank Frank Fojt III, Dennis Transue, Sandeep Bhatnagar, Cindy King, Beto Garza, John Drawe, Marvin Miller, Rick Hernandez, Joe Rivera, and Eugene Jimenez for their assistance conducting the field trials, processing the plot samples, conducting insect injury ratings, and analyzing aflatoxin content. We thank the participating seed companies for providing seed of their hybrids. This research was partly funded by an USDA-ARS competitive grant and the Texas Corn Producers Board.
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REFERENCES
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- Betrán, F.J., T. Isakeit, and G. Odvody. 2002. Combining ability for resistance to aflatoxin accumulation in white and yellow maize inbreds. Crop Sci. 42:1894–1901.[Abstract/Free Full Text]
- Bhatnagar, S., F.J. Betrán, D. Transue. 2003. Agronomic performance, aflatoxin accumulation and protein quality of subtropical and tropical QPM hybrids in southern U.S. Maydica 48:113–124.
- Brown, R., D. Bhatnagar, T.E. Cleveland, and J.W. Cary. 1998. Recent advances in preharvest prevention of mycotoxin contamination. p. 351–379. In K.K. Sinha and D. Bhatnagar (ed.) Mycotoxins in agriculture and food safety. Marcel Dekker, New York.
- Campbell, K.W., and D.G. White. 1995. Inheritance of resistance to Aspergillus ear rot and aflatoxin in maize genotypes. Phytopathology 85:886–896.[Web of Science]
- Cardwell, K. 2001. Scientists weigh costs of mycotoxin-contaminated crops. CSANews 46(10):15.
- CAST. 2003. Mycotoxins: Risks in plant, animal and human systems. CAST Rep. 139. CAST, Ames, IA.
- Castegnaro, M., and D. McGregor. 1998. Carcinogenic risk assessment of mycotoxins. Rev. Med. Vet. 149:671–678.
- Grant, R.F., B.S. Jackson, J.R. Kiniry, and G.F. Arkin. 1989. Water deficit timing effects on yield components in maize. Agron. J. 81:61–65.[Abstract/Free Full Text]
- Griffiths, J., and J. Bryan. 1987. The climates of Texas counties. Nat. Fibers Inf. Cent., The Univ. of Texas, Austin.
- Hunter, R.B. 1980. Increased leaf area (source) and yield of maize in short-season areas. Crop Sci. 20:571–574.[Abstract/Free Full Text]
- Jones, R.K. 1987. The influence of cultural practices on minimizing the development of aflatoxin in field maize. In M.S. Zuber, E.B. Lillehoj, and B.L. Renfro (ed.) Aflatoxin in maize. Proc. Workshop, El Batan, Mexico. 7–11 Apr. 1986. CIMMYT, Mexico, D.F.
- Jones, R.K., and H.E. Duncan. 1981. Effect of nitrogen fertilizer, planting date, and harvest date on aflatoxin production in maize inoculated with Aspergillus flavus. Plant Dis. 65:741–744.
- Jones, R.K., H.E. Duncan, and P.B. Hamilton. 1981. Planting date, harvest date, and irrigation effects on infection and aflatoxin production by Aspergillus flavus in field maize. Phytopathology 71:810–816.
- LaPrade, J.C., and A. Manwiller. 1977. Relation of insect damage, vector, and hybrid reaction to aflatoxin B1 recovery from field maize. Phytopathology 67:544–547.
- Lillehoj, E.B., W.F. Kwolek, E.E. Vandergraft, M.S. Zuber, O.H. Calvert, N. Widstrom, M.C. Futrell, and A.J. Bockholt. 1975. Aflatoxin production in Aspergillus flavus inoculated ears of maize grown at diverse locations. Crop Sci. 15:267–270.[Abstract/Free Full Text]
- Lillehoj, E.B., W.F. Kwolek, W.F. Zuber, A.J. Bockholt, O.H. Calvert, W.R. Findley, W.D. Guthrie, E.S. Horner, L.M. Josephson, S. King, A. Manwiller, D.B. Sauer, D.L. Thompson, M. Turner, and N.W. Widstrom. 1980. Aflatoxin in maize before harvest: Interaction of hybrids and locations. Crop Sci. 20:731–734.[Abstract/Free Full Text]
- Lillehoj, E.B., W.F. Kwolek, W.F. Zuber, O.H. Calvert, E.S. Horner, N.W. Widstrom, W.D. Guthrie, G.E. Scott, D.L. Thompson, W.R. Findley, and A.J. Bockholt. 1978. Aflatoxin contamination of field maize: Evaluation of regional test plots for early detection. Cereal Chem. 55:1007–1013.
- McMillian, W.W., N.W. Widstrom, and D.M. Wilson. 1993. Registration of GT-MAS:gk maize germplasm. Crop Sci. 33:882.[Free Full Text]
- Odvody, G.N., N. Spencer, and J. Remmers. 1997. A description of silk cut, a stress-related loss of kernel integrity in preharvest maize. Plant Dis. 81:439–444.
- Patterson, H.D., and E.R. Williams. 1976. A new class of resolvable incomplete block designs. Biometrika 63:83–89.[Abstract/Free Full Text]
- Payne, G.A. 1992. Aflatoxin in maize. Crit. Rev. Plant Sci. 10:423–440.
- SAS Institute. 1997. SAS proprietary software release 6.12. SAS Inst., Cary, NC.
- Scott, G.E., and N. Zummo. 1988. Sources of resistance in maize to kernel infection by Aspergillus flavus in the field. Crop Sci. 28:504–507.[Abstract/Free Full Text]
- Scott, G.E., and N. Zummo. 1992. Registration of Mp420 germplasm line of maize. Crop Sci. 32:1296.[Free Full Text]
- Troyer, F. 1983. Breeding corn for heat and drought tolerance. p. 128–143. In Proc. Annu. Corn and Sorghum Res. Conf., Chicago. December 1983. ASTA, Washington, DC.
- Widstrom, N.W. 1987. Breeding strategies to control aflatoxin contamination of maize through host plant resistance. p. 212–220. In M.S. Zuber, E.B. Lillehoj, and B.L. Renfro (ed.) Aflatoxin in maize. Proc. Workshop, El Batan, Mexico. 7–11 Apr. 1986. CIMMYT, Mexico, D.F.
- Widstrom, N.W. 1996. The aflatoxin problem with maize grain. Adv. Agron. 56:2129–2280.
- Widstrom, N.W., B.R. Wiseman, W.W. McMillian, W.F. Kwolek, E.B. Lillehoj, M.D. Jellum, and J.H. Massey. 1978. Evaluation of commercial and experimental three-way maize hybrids for aflatoxin B1 production potential. Agron. J. 70:986–988.[Abstract/Free Full Text]
- Windham, G.L., and W.P. Williams. 2002. Evaluation of corn inbreds and advanced breeding lines for resistance to aflatoxin contamination in the field. Plant Dis. 86:232–234.
- Windham, G.L., W.P. Williams, and F.M. Davis. 1999. Effects of the southwestern maize borer on Aspergillus flavus kernel infection and aflatoxin accumulation in maize hybrids. Plant Dis. 83:535–540.
- Zummo, N., and G.E. Scott. 1989. Evaluation of field inoculation techniques for screening maize genotypes against kernel infection by Aspergillus flavus in Mississippi. Plant Dis. 73:313–316.
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ABSTRACT
INTRODUCTION
