Atoxigenic Strains Research Articles

Biological control strategies of mycotoxigenic fungi and associated mycotoxins in Mediterranean basin crops

Fungi that belong to the genera Aspergillus , Fusarium , and Penicillium pose serious phytopathological and mycotoxicological risks at pre-harvest and post-harvest stages, as well as in processed food products because they can produce several mycotoxins. Mycotoxins pose a serious problem for animal and human health and have a significant economic impact worldwide. The Mediterranean basin is a large geographical region with a temperate climate supporting the cultivation of a wealth of field and greenhouse crops with a high risk of mycotoxin contamination. The most important mycotoxins that occur in the Mediterranean basin are aflatoxins (B1, B2, G1 and G2) in dried fruits and nuts, ochratoxin A in grapes and raisins as well as trichothecenes and fumonisins in cereals. A variety of chemical, biological and physical strategies have been developed to control the mycotoxigenic pathogens; to minimize mycotoxin production at pre- or post-harvest level; to contribute to decontamination and/or detoxification of mycotoxins from contaminated foods and feeds; or to inhibit mycotoxin absorption in the gastrointestinal tract. Biological control using microbial antagonists either alone or as part of an integrated control strategy to reduce pesticide inputs, has emerged as a promising approach for control of mycotoxins in crops, both pre- and post-harvest. Several organisms including atoxigenic Aspergilli, yeasts, bacteria and fungi have been tested for their ability to reduce both fungal infection and mycotoxin contamination. For instance, atoxigenic fungal strains are being used widely to prevent pre-harvest aflatoxin contamination of crops such as peanuts, pistachios, maize, and cottonseed in several parts of the world including the Mediterranean area. Recent advancements in the use of biocontrol strategies have led to registration of commercial products with increased practical applications for the benefit of growers in several countries.

Identification of genetic defects in the atoxigenic biocontrol strain Aspergillus flavus K49 reveals the presence of a competitive recombinant group in field populations

Contamination of corn, cotton, peanuts and tree nuts by aflatoxins is a severe economic burden for growers. A current biocontrol strategy is to use non-aflatoxigenic Aspergillus flavus strains to competitively exclude field toxigenic Aspergillus species. A. flavus K49 does not produce aflatoxins and cyclopiazonic acid (CPA) and is currently being tested in corn-growing fields in Mississippi. We found that its lack of production of aflatoxins and CPA resulted from single nucleotide mutations in the polyketide synthase gene and hybrid polyketide-nonribosomal peptide synthase gene, respectively. Furthermore, based on single nucleotide polymorphisms of the aflatoxin biosynthesis omtA gene and the CPA biosynthesis dmaT gene, we conclude that K49, AF36 and previously characterized TX9-8 form a biocontrol group. These isolates appear to be derived from recombinants of typical large and small sclerotial morphotype strains. This finding provides an easy way to select future biocontrol strains from the reservoir of non-aflatoxigenic populations in agricultural fields.

Laboratory tests for assessing efficacy of atoxigenic Aspergillus flavus strains as biocontrol agents

Biocontrol by competitive inhibition using atoxigenic Aspergillus flavus strains has been shown to be an effective method for controlling aflatoxin production in peanuts, maize and cottonseed. Selecting biocontrol strains is not straightforward, as it is difficult to assess fitness for the task without expensive field trials. Reconstruction experiments have been generally performed under laboratory conditions to investigate the biological mechanisms underlying the efficacy of atoxigenic strains in preventing aflatoxin production and/or to give a preliminary indication of strain performance when released in the field. The study here described was conducted in order to evaluate the potential of the different atoxigenic A. flavus strains, colonizing the corn fields of the Po Valley, in reducing aflatoxin accumulation when grown in mixed cultures together with atoxigenic strains; additionally, we developed a simple and inexpensive procedure that may be used to scale-up the screening process and to increase knowledge on the mechanisms interfering with mycotoxin production during co-infection.

Influence of the host contact sequence on the outcome of competition among aspergillus flavus isolates during host tissue invasion.

Biological control of aflatoxin contamination by Aspergillus flavus is achieved through competitive exclusion of aflatoxin producers by atoxigenic strains. Factors dictating the extent to which competitive displacement occurs during host infection are unknown. The role of initial host contact in competition between pairs of A. flavus isolates coinfecting maize kernels was examined. Isolate success during tissue invasion and reproduction was assessed by quantification of isolate-specific single nucleotide polymorphisms using pyrosequencing. Isolates were inoculated either simultaneously or 1 h apart. Increased success during competition was conferred to the first isolate to contact the host independent of that isolate's innate competitive ability. The first-isolate advantage decreased with the conidial concentration, suggesting capture of limited resources on kernel surfaces contributes to competitive exclusion. Attempts to modify access to putative attachment sites by either coating kernels with dead conidia or washing kernels with solvents did not influence the success of the first isolate, suggesting competition for limited attachment sites on kernel surfaces does not mediate first-isolate advantage. The current study is the first to demonstrate an immediate competitive advantage conferred to A. flavus isolates upon host contact and prior to either germ tube emergence or host colonization. This suggests the timing of host contact is as important to competition during disease cycles as innate competitive ability. Early dispersal to susceptible crop components may allow maintenance within A. flavus populations of genetic types with low competitive ability during host tissue invasion.

Aspergillus Colonization and Aflatoxin Contamination of Maize and Sesame Kernels in Two Agro-ecological Zones in Senegal

Aflatoxin contamination of major food crops is a serious problem in Senegal. Maize and sesame samples were collected during a survey in five districts located in two agro-ecological zones in Senegal to determine levels of aflatoxin contamination and the distribution and toxigenicity potential of members of Aspergillus section Flavi. Maize samples from the Guinea Savannah zone (SG) exhibited lower aflatoxin content and colony-forming units (cfu) than those collected from the Sudan Savannah (SS) zone. In maize, aflatoxin concentration and cfu of A.flavus varied with cultivars, shelling practices and storage methods. The maize variety 'Jaune de Bambey' had high aflatoxin levels in both agro-ecological zones. Aflatoxin content in machine-shelled maize (120 ng/g) was more than 10-fold higher than that in manually shelled (8 ng/g) or unshelled maize. Aflatoxin content (between 0.1 and 1.2 ng/g) and cfu values (between 13 and 42 000 cfu/g) of sesame were low, suggesting a low susceptibility to A. flavus. In both agro-ecological zones, and in all storage systems, aflatoxin contamination was lower in sesame than in maize. In this study, only three species of Aspergillus section Flavi (A. flavus, A. tamarii and the unnamed taxon S BG ) were observed with the frequency of toxigenic strains remaining below 50% in maize from the SG zone compared with 51% of isolates from samples collected in Sedhiou district in SS zone. The proportion of toxigenic strains isolated from sesame was variable. For both crops, L-strains were the most prevalent in the two agro-ecological zones. Some of the atoxigenic strains collected could be valuable microbial resources for the biological control of aflatoxin in Senegal.

Molecular characterization of atoxigenic strains for biological control of aflatoxins in Nigeria

Aflatoxins are highly toxic carcinogens produced by several species in Aspergillus section Flavi. Strains of A. flavus that do not produce aflatoxins, called atoxigenic strains, have been used commercially in North America as tools for limiting aflatoxin contamination. A similar aflatoxin management strategy is being pursued in Nigeria. In the current study, loci across the 68 kb aflatoxin biosynthesis gene cluster were compared among 18 atoxigenic and two aflatoxin-producing vegetative compatibility groups (VCGs) from Nigeria and an atoxigenic VCG used commercially in North America. Five of the atoxigenic VCGs had large deletions (37–65 kb) extending from the teleomeric side of the aflatoxin biosynthesis cluster. In one VCG (AV0222) the deletion extended through the cluster to the adjacent sugar cluster. The remaining twelve atoxigenic VCGs, including the VCG used for aflatoxin management in North America, contained all the aflatoxin pathway genes, but with defects. Two observations support the long-term persistence of atoxigenicity within A. flavus: first, a comparison of pathway genes revealed more changes in atoxigenic than in aflatoxin-producing isolates relative to the aflatoxin-producing strain NRRL 3357; and second, several non-synonymous changes are unique to atoxigenics. Atoxigenic VCG diversity was assessed with phylogenetic analyses. Although some atoxigenics share relatively recent ancestry, several are more closely related to aflatoxin producers than to other atoxigenics. The current study demonstrates VCGs of A. flavus in West Africa with diverse mechanisms of atoxigenicity and potential value in aflatoxin management programmes.

Health economic impacts and cost-effectiveness of aflatoxin-reduction strategies in Africa: case studies in biocontrol and post-harvest interventions

Advances in health economics have proven useful in evaluating the cost-effectiveness of interventions, where the benefit usually takes the form of improved health outcomes rather than market outcomes. The paper performs health-based cost-effectiveness analyses of two potential aflatoxin control strategies in Africa: (1) pre-harvest biocontrol, using atoxigenic strains of Aspergillus flavus competitively to exclude toxigenic strains from colonizing maize in Nigeria; and (2) post-harvest interventions in a package to reduce aflatoxin accumulation in groundnuts in Guinea. It is described how health benefits gained from each intervention, in terms of fewer aflatoxin-induced hepatocellular carcinoma cases, can be compared with the costs of implementing the interventions. It is found that both interventions would be extremely cost-effective if applied widely in African agriculture. That is, the monetized value of lives saved and quality of life gained by reducing aflatoxin-induced hepatocellular carcinoma far exceeds the cost of either biocontrol or the post-harvest intervention package to achieve those health benefits. The estimated cost-effectiveness ratio (CER; gross domestic product multiplied by disability-adjusted life years saved per unit cost) for biocontrol in Nigerian maize ranged from 5.10 to 24.8; while the estimated CER for the post-harvest intervention package in Guinean groundnuts ranged from 0.21 to 2.08. Any intervention with a CER > 1 is considered by the World Health Organization (WHO) to be ‘very cost-effective’, while an intervention with a CER > 0.33 is considered ‘cost-effective’. Aside from cost-effectiveness, public health interventions must be readily accepted by the public, and must have financial and infrastructural support to be feasible in the parts of the world where they are most needed.

E. coliO157 from Sheep in Northeast Scotland: Prevalence, Concentration Shed, and Molecular Characterization by Multilocus Variable Tandem Repeat Analysis

We report the prevalence, concentrations, and strain diversity of Escherichia coli O157 shed by sheep fed on root crops during a winter period in northeast Scotland. E. coli O157 was isolated on 6 farms from 14 studied during January to March 2005. The individual sheep prevalence was 5.8% and concentration excreted was <10(2) colony-forming units/g for all but one fecal sample. Verocytotoxigenic E. coli O157, determined by polymerase chain reaction and verocell assay, was recovered from 27% of samples. Four farms had sheep shedding the same strain as determined by multiple-locus variable analysis and no within-farm diversity was observed. The low numbers shed and the high levels of atoxigenic strains indicate a lower risk to human health from these animals compared to many ruminants grazing pasture during summer months. These data will be valuable for quantitative risk assessments and provide preliminary information that feeding sheep on root crops may be a practical intervention to reduce E. coli O157 infection in animals and ultimately humans.

Classification of toxigenic and atoxigenic strains of Aspergillus flavus with hyperspectral imaging

Aspergillus flavus ( A. flavus) produces secondary metabolites, aflatoxins, that are harmful to both humans and animals. Because of stringent federal regulation requirements as well as the limitations of available detection methods, there is an urgent need for rapid, non-invasive and effective techniques such as hyperspectral imaging, for the detection of the toxigenic strains of A. flavus. Hyperspectral images of toxigenic and atoxigenic strains of A. flavus were classified. Principal component analysis (PCA) was applied for data decorrelation and dimensionality reduction. A Genetic Algorithm (GA) was implemented for the selection of principal components (PCs) based on Bhattacharya Distance (B-Distance). A Support Vector Machine (SVM) was successfully applied for the classification. Under halogen light sources, in average 83% of the toxigenic fungus pixels and 74% of the atoxigenic fungus pixels were correctly classified; while under UV light sources, 67% of the toxigenic fungus pixels and 85% of the atoxigenic fungus pixels were correctly classified. The pair-wise classification accuracies between toxigenic AF13 and each atoxigenic fungus species (AF38, AF283 and AF2038) were 80%, 91% and 95% under halogen light sources, and 75%, 97% and 99% under UV lights, respectively.

Molecular characterization of an atoxigenic Aspergillus flavus strain AF051

An atoxigenic Aspergillus flavus strain AF051 collected from a peanut field in Jiangsu province, P. R. China was characterized by analysis of aflatoxin gene cluster in this study. By using a thermal asymmetric interlaced PCR (TAIL-PCR) and conventional PCR techniques, an 89.59-kb deletion was found in the cluster, and this deletion was replaced by a 3.83-kb insert, which was located at 300-bp upstream ver1 gene and 2594-bp downstream a putative gluconolactone oxidase gene. Based on the DNA sequence at the breakpoint, a nested-PCR method was developed for the rapid and sensitive detection of AF051 strain in soil and peanut samples once the strain is used as a biological agent.

Distribution of Aspergillus section Flavi in soils of maize fields in three agroecological zones of Nigeria

Fungal communities in soils of Nigerian maize fields were examined to determine distributions of aflatoxin-producing fungi and to identify endemic atoxigenic strains of potential value as biological control agents for limiting aflatoxin contamination in West African crops. Over 1000 isolates belonging to Aspergillus section Flavi were collected from soil of 55 Nigerian maize fields located in three agroecological zones by dilution plating onto modified Rose Bengal agar. The most common member of Aspergillus section Flavi (85% of isolates) was the A. flavus L-strain followed by the unnamed taxon known as strain S BG (8%), A. tamarii (6%) and A. parasiticus (1%). Highest incidence of S BG was in Zaria district, and lowest was in Ogbomosho and Ado-Ekiti districts. Only 44% of 492 A. flavus isolates produced aflatoxins in liquid fermentation (limit of detection 5 ng g −1). Thirty-two percent of the A. flavus isolates produced >1 μg g −1 total aflatoxins but no A. flavus isolate produced G aflatoxins. When the agroecological zones were compared, significantly ( P < 0.05) greater proportions of aflatoxigenic A. flavus isolates were found in the Northern Guinea Savannah (61%) than in Southern Guinea Savannah (31%). The Derived Savannah was intermediate between the other two agroecological zones. Each of the regions had atoxigenic strains of potential value as biological control agents. All S BG and A. parasiticus isolates produced both B and G aflatoxins and greater than 300 μg g −1 total aflatoxins. S BG and A. parasiticus isolates were the greatest contributors to the aflatoxin-producing potential of fungal communities in regions where these isolates occurred.

BIOCONTROL OF AFLATOXINS IN FIGS

For several years, we have investigated the use of atoxigenic strains (strains not able to produce aflatoxins) of Aspergillus flavus as biocontrol agents to reduce aflatoxin contamination of figs in California, USA. The atoxigenic strain AF36 has been successful in reducing aflatoxin contamination of cottonseed in commercial cotton fields in Arizona. Approximately 6% of the A. flavus isolates obtained from commercial fig orchards in California belonged to the atoxigenic strain AF36. In 2003 and 2004, AF36 was applied in 'Calimyrna' fig orchard. In early summer, wheat seeds infected with AF36 were applied at the rate of 11.2 kg/ha to the orchard floor. Almost all A. flavus isolates obtained from noncaprified figs collected from the orchard floor belonged to the applied strain AF36, indicating that AF36 had colonized debris in the orchard. In late summer, leaf and soil samples were collected. The soil had a higher density of A. flavus/A. parasiticus in the areas under the drip lines where infected wheat had been placed (72.6 to 113.2 colony forming units (cfu)/g soil for areas treated with AF36 in 2003) than in the middles (5.5 to 11.9 cfu/g) or under the drip lines in the untreated areas (0.9 cfu/g). In 2003, all of the A. flavus isolates obtained from the soil under the drip lines in the treated areas belonged to AF36 compared to only 18.7% in the untreated areas. In contrast, in the middle the incidence of AF36 was approximately the same in the treated areas (21.5 and 33.7%) and in the untreated areas (20.8%). The density of A. flavus/A. parasiticus on leaves did not differ significantly between treatments, even though higher levels of AF36 were on leaves in the treated areas than in the untreated areas. Atoxigenic strains continue to show promise for the reduction of aflatoxin contamination of figs.

Use of Pyrosequencing to Quantify Incidence of a SpecificAspergillus flavusStrain Within Complex Fungal Communities Associated with Commercial Cotton Crops

Atoxigenic strains of Aspergillus flavus have been used as aflatoxin management tools on over 50,000 hectares of commercial crops since 2000. To assess treatment efficacy, atoxigenic strain incidence is routinely monitored by vegetative compatibility analyses (VCA) that require culturing, generation of auxotrophs, and complementation with tester mutants. Two pyrosequencing assays (PA) that require no culturing were developed for monitoring incidences of atoxigenic strains on ginned cottonseed. The assays, which quantify frequencies of characteristic single nucleotide polymorphisms (SNPs) in the aflR and pksA genes, were validated against standard VCA on cottonseed collected from commercial gins in South Texas, Arizona, and Southern California where the atoxigenic strain AF36 is used to manage aflatoxin contamination. Cottonseed washings were subjected to both VCA and PA. PA was performed directly on DNA isolated from particulates pelleted from the wash water by centrifugation. Addition of CaCl(2) and diatomaceous earth prior to pelleting increased the amount of DNA isolated. Accuracy and reproducibility of the PA were contrasted with those for the VCA that has been used for over a decade. Correlation coefficients between VCA and PA indicated good correspondence between the results from the two assays (r = 0.91 for aflR assay and r = 0.80 for pksA assay). PAs were highly variable for samples with low incidences of A. flavus due to variability in the initial polymerase chain reaction step. This held for both DNA isolated from cottonseed washes and for mixtures of purified DNA. For samples yielding low quantities of A. flavus DNA, averaging of results from 4 to 5 replicates was required to achieve acceptable correlations with VCA. Pyrosequencing has the potential to become a powerful tool for monitoring atoxigenic strains within complex A. flavus communities without limitations imposed by traditional culturing methods.

AFLATOXIN—RECOGNITION, UNDERSTANDING, AND CONTROL WITH PARTICULAR EMPHASIS ON THE ROLE OF THE AGRICULTURAL RESEARCH SERVICE

Aflatoxin, a metabolite of the fungi Aspergillus flavus and Aspergillus parasiticus, has been considered one of the world's most serious food safety problems since it was first identified in the 1960s following large numbers of turkey deaths in Britain. Imported peanut meal was targeted as the cause, leading to identification of mold hyphae, the causative fungus A. flavus, and isolation and identification of aflatoxins as the causative mycotoxins. The Agricultural Research Service (ARS) quickly joined the scientific effort in the 1960s to control aflatoxin and to assure food safety by toxicity testing and developing methodology for identification, quantification, and detoxification of aflatoxin. As National Program Leader for Food Safety at the ARS, I initiated a national aflatoxin research program to provide efficacious, workable preharvest controls to decrease losses and help assure human safety by preventing aflatoxin from occurring in susceptible commodities, peanut, corn, cottonseed, and tree nuts. A series of Aflatoxin Elimination Workshops was established to bring together the ARS scientists, university scientists, and industry representatives in a cooperative effort to control aflatoxin. The first success was methodology to reduce contamination of Arizona cottonseed by applying an atoxigenic strain of A. flavus, AF 36, to outcompete native toxigenic strains. This methodology is being extended to peanuts, corn, and tree nuts. Other lines of investigation include the discovery of natural compounds that prevent aflatoxin biosynthesis; understanding the role of oxidative stress in aflatoxin formation; and the isolation and identification of aflatoxin biosynthetic pathway synthesis and regulatory genes.

Distribution and toxigenicity of Aspergillus species isolated from maize kernels from three agro-ecological zones in Nigeria

Maize samples were collected during a survey in three agro-ecological zones in Nigeria to determine the distribution and aflatoxin-producing potential of members of Aspergillus section Flavi. The three agro-ecological zones were, Derived Savannah (DS) and Southern Guinea Savannah (SGS) in the humid south and North Guinea Savannah (NGS) in the drier north. Across agro-ecological zones, Aspergillus was the most predominant fungal genera identified followed by Fusarium with mean incidences of 70 and 24%, respectively. Among Aspergillus, A. flavus was the most predominant and L-strains constituted > 90% of the species identified, while the frequency of the unnamed taxon S BG was < 3%. The incidence of atoxigenic strains of A. flavus was higher in all the districts surveyed except in the Ogbomosho and Mokwa districts in DS and SGS zones, respectively, where frequency of toxigenic strains were significantly ( P < 0.05) higher than that of atoxigenic strains. The highest and lowest incidence of aflatoxin positive samples was recorded in the SGS (72%) and NGS (20%), respectively. Aflatoxin contamination in grain also followed a similar trend and the highest mean levels of B-aflatoxins were detected in maize samples obtained from Bida (612 ng g − 1 ) and Mokwa (169 ng g − 1 ) districts, respectively, in the SGS. Similarly, the highest concentrations of G-aflatoxins were detected in samples from Akwanga district in the SGS with a mean of 193 and 60 ng g − 1 , respectively. When agro-ecological zones were compared, B-aflatoxins were significantly ( P < 0.05) higher in SGS than in NGS, and intermediate in maize samples from the DS agro-ecological zone.

Ecology of aflatoxin producing fungi and biocontrol of aflatoxin contamination

Aflatoxins, highly toxic and carcinogenic compounds that frequently contaminate foods and feeds, are produced by several genera in the genusAspergillus. Aspergillus flavus, the most common species causing crop contamination, is a common inhabitant of the Sonoran desert of North America where it resides in complex communities composed of diverse individuals. This diversity reflects divergent adaptation to various ecological niches. SomeA. flavus isolates that are well adapted to plant associated niches do not produce aflatoxins yet have the capacity to competitively exclude aflatoxin producers. These atoxigenic strains can serve as biological control agents for management of aflatoxins in crops. Detailed knowledge of the ecology of aflatoxin-producing fungi may lead to novel practical methods for limiting contamination.

Genetic diversity within Aspergillus flavus strains isolated from peanut-cropped soils in Argentina

The genetic diversity of Aspergillus flavus populations isolated from the peanut-cropped soils in the peanut-growing region at Cordoba Province was evaluated by analysis of vegetative compatibility group (VCG). VCG s were determined through complementation assays between nitrate-nonutilizing (NNO) mutants. Fifty-six VCG s were identified from 100 isolates. Twenty-five VCG s contained two or more isolates and 31 VCG s contained only a single isolate. In general, there were significant differences among VCG s in aflatoxin and CPA production. One VCG group included a single atoxigenic strain since it was neither aflatoxin nor cyclopiazonic acid producer. This isolate could be useful as a biological control agent, since it was unable to form a stable heterokaryon in the complementation test with the other isolates. Seven A. flavus isolated from soil were atypical because they simultaneously produced aflatoxins B, G and CPA.

Volatile compound generation in dry fermented sausages by the surface inoculation of selected mould species

The effect of the inoculation of dry fermented sausage surface with an atoxigenic, proteolytic and lipolytic strain of Mucor racemosus, Penicillium aurantiogriseum and Penicillium camemberti on the volatile composition was studied. The analysis of the headspace volatile compounds using gas chromatography/mass spectrometry enabled the identification of 55 volatiles. The study showed that every mould species produced a different volatile profile which was also different from that of the control sausages. Compounds derived from amino acid catabolism, i.e. branched aldehydes and the corresponding alcohols, were produced in higher amounts in sausages inoculated with Penicillium spp. On the other hand, volatiles coming from the microbial esterification were related to sausages inoculated with M. racemosus. The development of the fungal mycelia on the sausage surface protected lipids from oxidation, thus giving rise to fewer lipid oxidation products in the inoculated sausages.

Population dynamics of Aspergillus flavus in the air of an intensively cultivated region of south‐west Arizona

Airborne propagules of Aspergillus flavus were quantified to investigate population dynamics of A. flavus in a region of south‐west Arizona prone to epidemics of aflatoxin contamination of cottonseed. Air was sampled continuously from May 1997 to March 1999 at two sites using Burkard cyclone samplers. One sampler was initially at the centre of 65 ha of cotton treated with an atoxigenic strain of A. flavus to manage aflatoxin contamination of cottonseed. The second sampler was 0·6 km from the treated field. Total fungal colony‐forming units (CFU) sampled ranged from 17 to 667 and from 9 to 1277 m−3 at the untreated and treated sites, respectively. Counts of A. flavus ranged from 0 to 406 m−3 and from 0 to 416 per m−3 at the untreated and treated sites, respectively. Aspergillus flavus comprised 1–46 and 1–51% of the total cultured fungi at the treated and untreated sites, respectively. Peaks in total fungal and A. flavus CFU coincided with boll maturation and cotton harvest (days 251–321). Autoregression analysis suggested that there was no difference in total fungal CFU between treated and untreated sites, but the analysis showed that the quantity of A. flavus decreased at the treated site. This is probably caused by changes in cropping making the conditions less conducive to growth and reproduction of A. flavus in the surrounding fields. The incidence of the S strain of A. flavus was highest between May and August. The L strain accounted for up to 100% of the A. flavus sampled in the other months, and autoregression analysis showed that the L strain accounted for a greater overall proportion of the A. flavus population at the treated site compared with the untreated site. Autoregression analysis also showed the vegetative compatibility group of the applied strain was a greater proportion of L‐strain A. flavus at the treated site (5–75%) than at the untreated site (0–65%), although this decreased with time. The quantity of A. flavus sampled at both treated and untreated sites was correlated with air and soil temperature. Large quantities of A. flavus occurred in the soil (up to 34 474 CFU g−1) of cotton fields and on cotton plant parts and debris (up to 272 461 CFU g−1) adjacent to the cyclone samplers. Aspergillus flavus is a major constituent of the airborne mycoflora associated with cotton fields in south‐west Arizona when temperature is conducive to fungal growth. Although application of atoxigenic A. flavus altered the proportion of A. flavus strains and vegetative compatibility groups in the aerial mycoflora, the total quantity of A. flavus remained similar to that in untreated fields. Dispersal of A. flavus between fields suggests that atoxigenic fungi will be most effective in area‐wide management programmes.

Aspergillus species from section Flavi isolated from soil at planting and harvest time in peanut‐growing regions of Argentina

AbstractAspergillus species from section Flavi were isolated from soil samples in three peanut‐growing regions of Córdoba Province, Argentina. The samples were collected during the planting and harvest periods. Both total fungal population and Aspergillus species from section Flavi showed no significant differences between planting and harvest time in two of the regions evaluated. Only in one region were there significant differences in cfu g−1 of total fungal population and Aspergillus species from section Flavi. A flavus was the dominant species isolated in all three localities during the planting and harvest periods. There were significant differences (p &lt; 0.05) in the ratio of toxigenic and atoxigenic strains dependent on the period and the region evaluated. In one region, higher frequencies of toxigenic A flavus and A parasiticus in soil were found and a high contamination level of aflatoxins was detected in peanut seeds. Copyright © 2003 Society of Chemical Industry