Potential of Atoxigenic Aspergillus flavus Vegetative Compatibility Groups Associated With Maize and Groundnut in Ghana as Biocontrol Agents for Aflatoxin Management

Abstract

Increasing knowledge of the deleterious health and economic impacts of aflatoxin in crop commodities has stimulated global interest in aflatoxin mitigation. Current evidence of the incidence of Aspergillus flavus isolates belonging to vegetative compatibility groups (VCGs) lacking the ability to produce aflatoxins (i.e., atoxigenic) in Ghana may lead to the development of an aflatoxin biocontrol strategy to mitigate crop aflatoxin content. In this study, 12 genetically diverse atoxigenic African A. flavus VCGs (AAVs) were identified from fungal communities associated with maize and groundnut grown in Ghana. Representative isolates of the 12 AAVs were assessed for their ability to inhibit aflatoxin contamination by an aflatoxin-producing isolate in laboratory assays. Then, the 12 isolates were evaluated for their potential as biocontrol agents for aflatoxin mitigation when included in three experimental products (each containing four atoxigenic isolates). The three experimental products were evaluated in 50 maize and 50 groundnut farmers’ fields across three agroecological zones (AEZs) in Ghana during the 2014 cropping season. In laboratory assays, the atoxigenic isolates reduced aflatoxin biosynthesis by 87–98% compared to grains inoculated with the aflatoxin-producing isolate alone. In field trials, the applied isolates moved to the crops and had higher (P < 0.05) frequencies than other A. flavus genotypes. In addition, although at lower frequencies, most atoxigenic genotypes were repeatedly found in untreated crops. Aflatoxin levels in treated crops were lower by 70–100% in groundnut and by 50–100% in maize (P < 0.05) than in untreated crops. Results from the current study indicate that combined use of appropriate, well-adapted isolates of atoxigenic AAVs as active ingredients of biocontrol products effectively displace aflatoxin producers and in so doing limit aflatoxin contamination. A member each of eight atoxigenic AAVs with superior competitive potential and wide adaptation across AEZs were selected for further field efficacy trials in Ghana. A major criterion for selection was the atoxigenic isolate’s ability to colonize soils and grains after release in crop field soils. Use of isolates belonging to atoxigenic AAVs in biocontrol management strategies has the potential to improve food safety, productivity, and income opportunities for smallholder farmers in Ghana.

Present Status and Perspective on the Future Use of Aflatoxin Biocontrol Products

“…The process of selecting atoxigenic isolates of A. flavus to use as biocontrol agents should follow a series of well-established steps ranging from selecting native and well-adapted isolates, determining the reasons for their inability to produce toxins (e.g., aflatoxins, cyclopiazonic acid [CPA]), and assessing their ability to out-compete aflatoxin producers under controlled and field conditions ( Figure 2). Another criterion for selecting the most competitive isolates in field conditions is to evaluate their abilities to spread from soil to crops [47]. Genetic diversity is large and aflatoxin production potential is highly variable within and among aflatoxin-producing species [7,42].…”

“…A shift from a single to a multiple-isolate product was made to exploit the repertoire of qualities that diverse atoxigenic isolates have in relationship to competitiveness and adaptation to diverse crops, environments, and agricultural practices [26,49,61]. Using isolates belonging to atoxigenic VCGs widely distributed and successful across environments and crops has proven to be effective in producing crops with low aflatoxin content [46,47,53]. In natural conditions, it has been noted that the dominance of single A. flavus VCGs over single and multiple areas, over multiple years, is transient [62].…”

“…This can result in selection of isolates that may not be the most appropriate and widely adapted to limit aflatoxin contamination. Area-wide adaptation of atoxigenic VCGs can be revealed by examining several thousand A. flavus isolates associated with multiple crops collected in several hundred locations across a target country/region [15,26,47]. Such studies require intensive microbiological, chemical, and molecular tests.…”

“…Field testing allows evaluating aflatoxin reduction abilities of candidate fungi under field conditions [50][51][52]54,58]. When selecting atoxigenic fungi for multi-isolate products, field evaluations are done by applying atoxigenic isolates individually (typically 12 isolates are tested) and then, as part of a candidate multi-isolate products [26,47,50]. An evaluation of this type allows detecting those isolates with the greatest ability to create a founding population in the soil and then to successfully move to the grains or other harvested crop product in the treated fields [47].…”

Aflatoxins: Production, Structure, Health Issues and Incidence in Southeast Asian and Sub-Saharan African Countries

“…As this technology proved to be an efficient biocontrol means to mitigate aflatoxin contamination in various crops, studies have been conducted in different countries and regions of the world to screen for proficient strains and well adapted to specific soils and AEZs. In Ghana, atoxigenic African A. flavus VCG (AAV) strains isolated from three different AEZ reduced aflatoxin contamination of maize and peanut by 87-98% in laboratory assays, and successfully displaced toxigenic A. flavus strains in field trials where crops obtained from treated grains contained 50-100% less aflatoxin at harvest than their untreated counterparts [212]. In Northern Italy, co-inoculation of maize ears with an endemic atoxigenic strain A. flavus A2085 of the VCG IT019 group and an aflatoxigenic strain (A2092) of the same species was reported to reduce the concentration of AFB1 by 93-98% compared with ears inoculated with the aflatoxigenic strain alone [213].…”