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reflect those distributed throughout the crop's extended environment. Within these communities not all fungi are equal. Individual strains may vary in crop preference and the ability to ramify through and rot crop tissues (Cotty, 1989; Brown et al., 1992; Shieh et al., 1997). The divergence in aflatoxin-producing ability is striking, as some isolates produce much higher levels of aflatoxins than others. It is not unusual to find crop components that range in aflatoxin content from < 10 ng/g to several million ng/g (Lee et al., 1990; Mellon and Cotty, 2004). This range partially reflects the diversity present amongst the infecting fungi. Thus relatively minor, in number, components of the infecting fungal community may have major roles in determining the ultimate quantity of aflatoxins in the crop.

Work with the S strain of A. flavus illustrates both the potential importance of etiological knowledge and how minor components of infecting fungal communities can be the most important etiological agents of contamination. The S strain is of great importance both in North America and Africa (Cotty, 1989, 1997; Jaime-Garcia and Cotty, 2006a,b; Probst et al., 2007). The apparent importance of the S strain as a causative agent can be erroneously minimized when researchers either overlook S strain isolates or when they preferentially select "typical", or L strain, isolates during primary isolations from crops. S strain isolates can appear either fluffy white, without sclerotia or spores, early in isolation or primarily black from abundant small sclerotia late in isolation. In Arizona, the S strain infects a relatively low proportion of cotton seed and yet causes the vast majority of the aflatoxin contamination (Table 1; Cotty, 1996). Thus, although the L strain of A. flavus is the most common strain infecting the cotton seed, it is not the most important cause of contamination. The S strain also is the most important etiological agent of aflatoxin contamination in South Texas, even though S strain isolates make up only a minor portion of the Aspergillus section Flavi propagules on harvested crops (Jaime-Garcia and Cotty, 2006a). The S strain morphotype also was the primary cause of the aflatoxin contamination events that resulted in hundreds of deaths in Kenya in 2004 (Probst et al., 2007).

Development of cultivars with reduced susceptibility to aflatoxin contamination is a strategy for limiting contamination that is applicable to many crops. However, selection of the fungi used in the resistance screens does not typically include evaluation of the most important causal agents as they may vary by region (Jaime-Garcia and Cotty, 2006b), and it is not uncommon for researchers to request isolates from other regions, crops, and even continents for such screens. Differential virulence to hosts is well characterized within many fungal species (Agrios, 2004), and it is reasonable to question the assumption that resistance to one strain of A. flavus implies resistance to all strains of A. flavus. Characterization of

Figure 1. Proportion of the overall A. flavus community composed of the S and L strains in the air over agricultural fields in the Sonoran Desert. The most competitive strain varies with season. Redrawn from Bock et al. (2004). ♦ - % S strain observed, □ - % L strain observed.

the most important causal agents might result in cultivar screens that increase host resistance levels. S strains of A. flavus are an important cause of contamination in several areas where contamination is a perennial problem (Cotty, 1989, 1997; Jaime-Garcia and Cotty, 2006a,b), yet there are relatively few host resistance screens that incorporate S strain isolates.

The S and L strains of A. flavus are adapted to distinct ecological niches. This adaptation can be seen in the size and production habit of sclerotia, the variation in hydrolase production (Cotty et al., 1990; Mellon and Cotty, 2004), the distribution by soil type (Jaime-Garcia and Cotty, 2006b), and even in seasonality (Fig. 1; Orum et al., 1997; Bock et al., 2004). In agricultural regions of the Sonoran desert conditions favoring the S and L strains differ, with S strains dominating during the warmest periods and L strains dominating during the winter and spring (Fig. 1). Similar responses to the environment may exist in African deserts including regions bordering the Sahara (Cardwell and Cotty, 2002). Thus, control measures optimized for one environment may not work in others, with management strategies utilizing cultural, chemical, and/or biological control methods particularly sensitive to their environments.

Under some conditions crop rotations and crop mixtures can influence the composition of A. flavus communities in soils, the average aflatoxin producing potential of resident fungi, and, the vulnerability of crops to contamination (Jaime-Garcia and Cotty, 2006b). It is not clear how cropping systems influence fungal communities and whether such influences relate to crop infection, crop debris, or other aspects of the cropping process. However, the results do suggest that by identifying the most important etiologic agents and the influences of agronomic practices on the prevalence of these agents, cropping systems can be altered to favor less toxigenic but similarly adapted fungi, and, in so doing, reduce the aflatoxin-producing potential of fungal communities to which susceptible crops are exposed.

Use of atoxigenic strains of A. flavus to limit contamination

Chemical examination of seeds infected by either the S or the L strains indicated that seed infected by the S strain alone had much higher aflatoxin content than seed coinfected by both S and L strain isolates. This phenomenon has been observed repeatedly: co-infecting fungi modulate aflatoxin production by each other (Cotty, 1990; Cotty and Bhatnagar, 1994; Garber and Cotty, 1997). Infection with a highly toxigenic strain of A. flavus alone results in much more toxin than co-infection by both a high toxin producer and a low toxin producer. Sometimes even a relatively low incidence of the low toxin producer can greatly reduce the extent of contamination. This phenomenon was an important factor in determining which Kenyan maize became contaminated with toxic levels of aflatoxins during the 2004 aflatoxicosis epidemic (Probst et al., 2007). Modulation of aflatoxin biosynthesis by coinfecting strains has led to the development of techniques that exploit this phenomenon to reduce aflatoxin contamination. These procedures utilize the application of atoxigenic strains of A. flavus as biological control agents directed at minimizing aflatoxin contamination. Thus, by detailed exploration of the etiology of aflatoxin contamination, some members of the most important species causing contamination were shown not only not to make aflatoxins but to be useful as tools for limiting aflatoxin contamination.

Two observations were central to the initial development of atoxigenic strains as tools for limiting aflatoxin contamination. First, in the 1980s, investigations of the etiology of contamination resulted in the discovery that neither isolate pathogenicity nor the ability of a fungal isolate to ramify through crop tissues were related to aflatoxin-producing ability (Cotty, 1989). Thus, the aflatoxin-producing ability of an A. flavus strain is unrelated to its success during crop colonization, and, in theory, isolates that do not produce aflatoxins might be effective competitors of aflatoxin producers during crop infection. Some atox-igenic strains can competitively exclude aflatoxin producers during the infection of crop tissues (Cotty and Bayman, 1993), and, in so doing, markedly reduce or eliminate aflatoxin production by highly toxigenic strains during co-infection (Cotty and Bhatnagar, 1994; Cotty, 1990; Garber and Cotty, 1997). Increasing the frequency of this natural interference with seed contamination by atoxigenic strains would reduce the extent to which crops become contaminated. Second, the fungal community resident at one location may differ considerably in aflatoxin-producing potential from the fungal community at second location (Joffe, 1969; Schroeder and Boller, 1973; Cotty, 1997). Therefore, communities with lower aflatoxin-producing potentials exist and reductions in aflatoxin-producing potential might be induced by certain agronomic practices. Crops infected by fungal communities with relatively low aflatoxin-producing potential should be less contaminated with aflatoxin than crops infected by communities with high aflatoxin-producing abilities. If the aflatoxin-producing potential of fungal communities resident in an area could be reduced, then the extent to which crops in that area become contaminated also should be reduced. Breeders and other researchers of aflatoxin contamination routinely practice this principle when they spread substrates colonized by highly toxigenic isolates of A. flavus or A. parasiticus throughout test plots to increase the extent and uniformity of crop aflatoxin content (Batson et al., 1997; Holbrook et al, 2000; Betran et al, 2002). Field tests have shown that application of atoxigenic strains in a similar manner could be used to reduce the aflatoxin-producing potential of fungal communities resident on crops and that these applications could have far reaching influences on the fungi resident in the environment (Fig. 2: Antilla and Cotty, 2002; Cotty and Antilla, 2003).

Figure 2. Proportion of the overall A. flavus community composed of the applied atoxigenic strain (AF36) in the soil prior to application and on the harvested crop. Application (9 kg of colonized wheat seed per hectare) was made to a single 16 hectare field. AF36 is native and occurred in soil prior to treatment. Application influences extended to untreated adjacent fields. Data are the result of 564 vegetative compatibility analyses. Redrawn from Cotty and Antilla (2003).

Figure 2. Proportion of the overall A. flavus community composed of the applied atoxigenic strain (AF36) in the soil prior to application and on the harvested crop. Application (9 kg of colonized wheat seed per hectare) was made to a single 16 hectare field. AF36 is native and occurred in soil prior to treatment. Application influences extended to untreated adjacent fields. Data are the result of 564 vegetative compatibility analyses. Redrawn from Cotty and Antilla (2003).

Strategies that use atoxigenic strains of Aspergillus flavus as biological control agents directed at limiting the production of aflatoxins exploit both the ability of certain atoxigenic strains to modulate aflatoxin biosynthesis during crop infection and the ability of atoxigenic strain applications to displace aflatoxin producers throughout the crop environment and in so doing, reduce the frequency and extent of crop infection by aflatoxin producers (Cole and Cotty, 1990; Cotty et al., 1994; Dorner, 2004). These strategies seek to competitively exclude aflatoxin producers from crops and thus reduce both the incidence of aflatoxin producers in the environment and the level of aflatoxin contamination (Cotty and Antilla, 2003).

Aflatoxin prevention technologies based on atoxigenic strains of A. flavus and A. parasiticus are being developed for several crops in diverse agricultural systems (Doster et al., 2002; Cotty and Antilla, 2003; Dorner, 2004). Most strategies apply relatively low amounts of A. flavus to a food source on which the fungus reproduces and from which distribution to secondary food sources and the crop occurs. Besides innate competitive ability, the applied atoxigenic strains usually have advantages provided by management practices that allow improved competition over aflatoxin-producers resident in the field. These management practices include having atoxigenic strains arrive with a food source formulated for utilization only by the applied strain, and applying the strains to the top of the soil, which eliminates the need for atoxigenics to escape the soil matrix to colonize above-ground crops. Applications are timed to ensure that the environment will support growth and reproduction by the atoxigenic strain and that the resident aflatoxin-producing strains have not previously multiplied and colonized the crop to such an extent that applications would be futile. In theory, application of an atoxigenic strain when overall A. flavus levels are low provides preferential exposure to the crop and an advantage in competing for crop resources. A col laboration between the International Institute of Tropical Agriculture, the University of Bonn, and the U.S. Department of Agriculture's Agricultural Research Service (USDA-ARS) is selecting useful atoxigenic strains from crops grown in Africa and developing strategies to apply atoxigenic strain technology in Africa.

Atoxigenic strains are considered biopesticides and, as such, the use of atoxigenic strains must comply with applicable pesticide laws. Two atoxigenic strains currently have pesticide registrations in the United States. Atoxigenic strains have been used most extensively on cotton crops in Arizona. Development of atoxigenic strains in Arizona has been through a collaborative partnership between the Arizona Cotton Research and Protection Council (ACRPC), a farmer run organization, and the USDA-ARS (Cotty and Antilla, 2003, Antilla and Cotty, 2002). This collaboration has resulted in the construction of a facility for the production of an atoxigenic strain product that is run by ACRPC. Commercial cotton in Arizona has been treated since 1996, first under an experimental use registration and, since 2002, under a full section 3 pesticide registration. All material used to treat fields in Arizona, California, and Texas since 1999 has been produced at the ARS-ACRPC facility with over 20,000 acres per year treated since 2002 (Cotty and Antilla, 2003). In the United States, atoxigenic strains are applied mechanically either by airplane or tractor; however, these procedures should be readily adaptable to smaller scale operations. Treatments increase the incidence of the applied atoxigenic strain in A. flavus communities associated with the crop and the soil. This increase reduces both the average aflatoxin-producing potential of the communities and the quantity of aflatoxins in the crop (Cotty and Antilla, 2003). Treatments do not increase the overall quantity of A. flavus on the crop at harvest or after ginning. There is an inverse relationship between the incidence of the applied strain and the concentration of aflatoxin in the crop (Cotty, 1994).

Changes to the average aflatoxin producing potential of A. flavus communities induced by atoxigenic strain applications typically last for several years. Cumulative benefits are expected due to effects on untreated fields adjacent to treated fields (Fig. 2) and to repeated treatments through sequences of crop rotation. Atoxigenic strain applications may provide area-wide benefits. Many farmers hope that by reducing the aflatoxin-producing potential of A. flavus communities throughout an area, the vulnerability of all of the crops grown in the area may be reduced, as it is common to cultivate multiple susceptible crops in the same region, e.g., peanuts and maize. Atoxigenic strain technologies provide the hope of addressing all of these contamination issues with a single technology and in so doing reduce the burden of contamination over entire areas not just one crop within that area.

Atoxigenic strains, like aflatoxin-producing fungi, become associated with crops in the field during crop production. These fungi remain with the harvested crops after harvest and in storage. Since crop contamination with aflatoxins may occur in the field, in storage or anytime until the crop is consumed, if conditions are conducive for fungal growth, e.g., high humidity and high temperature, then crop infection and contamination will continue as well. Like their aflatoxin-producing relatives, atoxigenic strains also move into storage with the crop and provide residual protection in transport, storage, and processing until consumption (Brown et al., 1991; Dorner and Cole, 2002). Crops infected in the field and already contaminated at harvest accumulate less aflatoxin in storage when treated with an atoxigenic strain just prior to entering storage (Brown et al., 1991). However, postharvest applications lack many advantages of field applications when the atoxigenic strains multiply in competition with the aflatoxin producers and become associated with the crop before extensive infection by aflatoxin producers.

Atoxigenic strain technology provides an opportunity to reduce the overall risk of contamination during all phases of aflatoxin contamination including in the field during crop development, in storage or at any other time after harvest until the mature crop is eventually utilized. Atoxigenic strains are but one example of how improved knowledge of both the contamination process and the etiologic agents can result in improved methods for limiting human exposure to aflatoxins.

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