Biological control of Fusarium head blight of wheat and deoxynivalenol levels in grain via use of microbial antagonists

Drug repurposing strategy II: from approved drugs to agri-fungicide leads

Epidemic diseases of crops caused by fungi deeply affected the course of human history and processed a major restriction on social and economic development. However, with the enormous misuse of existing antimicrobial drugs, an increasing number of fungi have developed serious resistance to them, making the diseases caused by pathogenic fungi even more challenging to control. Drug repurposing is an attractive alternative, it requires less time and investment in the drug development process than traditional R&D strategies. In this work, we screened 600 existing commercially available drugs, some of which had previously unknown activity against pathogenic fungi. From the primary screen at a fixed concentration of 100 μg/mL, 120, 162, 167, 85, 102, and 82 drugs were found to be effective against Rhizoctonia solani, Sclerotinia sclerotiorum, Botrytis cinerea, Phytophthora capsici, Fusarium graminearum and Fusarium oxysporum, respectively. They were divided into nine groups lead compounds, including quinoline alkaloids, benzimidazoles/carbamate esters, azoles, isothiazoles, pyrimidines, pyridines, piperidines/piperazines, ionic liquids and miscellaneous group, and simple structure-activity relationship analysis was carried out. Comparison with fungicides to identify the most promising drugs or lead structures for the development of new antifungal agents in agriculture.

Developing Actinobacterial Endophytes as Biocontrol Products for Fusarium pseudograminearum in Wheat

Crown rot of wheat, caused by Fusarium pseudograminearum, results in millions of dollars of yield losses globally each year. Management strategies to control crown rot are limited and there are concerns about development of fungicide resistance so novel treatment strategies are desirable. A collection of endophytic Actinobacteria was screened for their ability to suppress the growth of F. pseudograminearum and the development of crown rot symptoms in wheat with the aim of identifying candidates that can be developed into biocontrol products. The ability of the Actinobacteria isolates to suppress the growth of three different F. pseudograminearum strains in vitro was assessed using agar-plate competition assays. Soil-free seedling assays were used to screen for suppression of development of early disease symptoms in the susceptible wheat (Triticum aestivum) cv. Tamaroi. Four of the isolates were tested in a glasshouse pot experiment to assess their ability to decrease disease symptoms and prevent yield losses in wheat cv. Tamaroi grown to maturity in an unsterilized soil. The screening of 53 isolates identified two Streptomyces isolates, MH71 and MH243, with very strong antifungal activity against F. pseudograminearum strains in agar-plate competition and seedling assays. In the glasshouse pot trial, plants treated with seed coatings of either MH71 or MH243 had > 24% lower disease severity than control plants infected with F. pseudograminearum. These two cultures show potential for development as biocontrol products because they are easy to culture, grow on relatively inexpensive media, produce highly durable spores and can be delivered to plants as a seed coat.

Deoxynivalenol (DON) Accumulation and Nutrient Recovery in Black Soldier Fly Larvae (Hermetia illucens) Fed Wheat Infected with Fusarium spp

Fusarium head blight (FHB) is one of the most significant causes of economic loss in cereal crops, resulting in a loss of $50–300 million for Canadian agriculture. The infected grain (containing Fusarium-damaged kernels (FDKs)) is often both lower in quality and kernel weight, and it may be unsuitable for human and animal consumption due to mycotoxin presence. However, it still contains a considerable amount of nutrients. A method to recover the nutrients without the mycotoxins should be beneficial for the agricultural economy. In this study, our objective was to examine recovery methods of the nutrients in relation to mycotoxin accumulation in the insect. The FDKs were fermented with Aspergillus oryzae and/or Lactobacillus plantarum (solid-state fermentation (SSF)). The SSF kernels were then provided to 50 young, black soldier fly larvae (BSFL) for 12 days. Weight gain, chemical composition, and mycotoxin bioaccumulation of BSFL and spent feed were evaluated. After 12 days of insect culture, the BSFL grew 5–6 times their initial weight. While the overall weights did not significantly vary, the proteins and lipids accumulated more in SSF FDK-fed insects. During the active growth period, the larval biomass contained deoxynivalenol (DON), a mycotoxin, at detectable levels; however, by day 12, when the larvae were in the pre-pupal stage, the amount of DON in the insect biomass was nearly negligible, i.e., BSFL did not accumulate DON. Thus, we conclude that the combination of BSFL and SSF can be employed to recover DON-free nutrients from FHB-infected grain to recover value from unmarketable grain.

Molecular Characterization, Fitness, and Mycotoxin Production of Fusarium asiaticum Strains Resistant to Fludioxonil

Fludioxonil is used in seedborne disease management of various fungal pathogens, including Fusarium asiaticum, the predominant causal agent of Fusarium head blight in China. In this study, we screened resistant strains from a large number of F. asiaticum strains collected from 2012 to 2016 and found that 4 of 1,000 field strains were highly resistant to fludioxonil. The 50% effective concentration values of the resistant strains and induced mutants ranged from 80 to >400 μg/ml. Compared with field-sensitive strains, all field-collected and laboratory-induced resistant strains exhibited fitness defects in traits including mycelial growth, conidial production, pathogenicity, and sensitivity to osmotic conditions. In the presence of fludioxonil, significantly higher glycerol accumulation was found in sensitive strains but not in resistant individuals. The fludioxonil-resistant strains produced lower amounts of glycerol in liquid culture and lower amounts of trichothecene mycotoxins in rice culture and inoculated wheat spikelets than the fludioxonil-sensitive strains. Sequence analyses of the key genes of the two-component histidine kinase signaling pathway showed various amino acid substitutions in the Os1, Os4, and Os5 genes between field-sensitive and resistant strains or mutants. The results of this study suggest a potential risk of fludioxonil resistance development and a possible influence of resistance mutations on fitness parameters and toxin production in F. asiaticum.

Poly-γ-glutamic acid productivity of Bacillus subtilis BsE1 has positive function in motility and biocontrol against Fusarium graminearum

In this study, we investigate the relationship between γ-PGA productivity and biocontrol capacity of Bacillus subtilis BsE1; one bacterial isolate displayed 62.14% biocontrol efficacy against Fusarium root rot. The γ-PGA yield assay, motility assay, wheat root colonization assay, and biological control assay were analysed in different γ-PGA yield mutants of BsE1. The pgsB (PGA-synthase-CapB gene) deleted mutant of BsE1 reduced γ-PGA yield and exhibited apparent decline of in vitro motile ability. Deletion of pgsB impaired colonizing capacity of BsE1 on wheat root in 30 days, also lowered biocontrol efficacies from 62.08% (wild type BsE1) to 14.22% in greenhouse experiment against Fusarium root rot. The knockout of pgdS and ggt (genes relate to two γ-PGA degrading enzymes) on BsE1, leads to a considerable improvement in polymer yield and biocontrol efficacy, which attains higher level compared with wild type BsE1. Compared with ΔpgsB mutant, defense genes related to reactive oxygen species (ROS) and phytoalexin expressed changes by notable levels on wheat roots treated with BsE1, demonstrating the functional role γ-PGA plays in biocontrol against Fusarium root rot. γ-PGA is not only important to the motile and plant root colonization ability of BsE1, but also essential to the biological control performed by BsE1 against Fusarium root rot. Our goal in this study is to reveals a new perspective of BCAs screening on bacterial isolates, without good performance during pre-assays of antagonism ability.

Fusarium and mycotoxin spectra in Swiss barley are affected by various cropping techniques

Fusarium head blight is one of the most important cereal diseases worldwide. Cereals differ in terms of the main occurring Fusarium species and the infection is influenced by various factors, such as weather and cropping measures. Little is known about Fusarium species in barley in Switzerland, hence harvest samples from growers were collected in 2013 and 2014, along with information on respective cropping factors. The incidence of different Fusarium species was obtained by using a seed health test and mycotoxins were quantified by LC-MS/MS. With these techniques, the most dominant species, F. graminearum, and the most prominent mycotoxin, deoxynivalenol (DON), were identified. Between the three main Swiss cropping systems, Organic, Extenso and Proof of ecological performance, we observed differences with the lowest incidence and toxin accumulation in organically cultivated barley. Hence, we hypothesise that this finding was based on an array of growing techniques within a given cropping system. We observed that barley samples from fields with maize as previous crop had a substantially higher F. graminearum incidence and elevated DON accumulation compared with other previous crops. Furthermore, the use of reduced tillage led to a higher disease incidence and toxin content compared with samples from ploughed fields. Further factors increasing Fusarium infection were high nitrogen fertilisation as well as the application of fungicides and growth regulators. Results from the current study can be used to develop optimised cropping systems that reduce the risks of mycotoxin contamination.

Thymol-based submicron emulsions exhibit antifungal activity against Fusarium graminearum and inhibit Fusarium head blight in wheat

AIMS

Fusarium graminearum is a very destructive fungal pathogen that leads to Fusarium head blight (FHB) in wheat, a disease which costs growers millions of dollars annually both in crop losses and in remediation efforts. Current countermeasures include the deployment of wheat varieties with some resistance to FHB in conjunction with timed fungicide treatments. In this article, we introduce a fungicide based on thymol, a naturally occurring plant phenolic derived from essential oils. To overcome the hydrophobicity of thymol, the thymol active was incorporated into a low-surfactant submicron emulsion with and without a carrier oil.

METHODS AND RESULTS

The minimum fungicidal concentration of F. graminearum was found to be both 0·02% for thymol emulsions with and without an oil component. Time-to-kill experiments showed that thymol emulsions were able to inactivate F. graminearum in as little as 10 s at concentrations above 0·06%. Spraying the thymol emulsions (~0·1% range) on the wheat variety Bobwhite demonstrated significant reductions in FHB infection rate (number of infected spikelets). However, with 0·5% thymol, the wheat heads exhibited premature senescence. Transmission and scanning electron micrographs suggest that the mechanism of antifungal action is membrane mediated, as conidia exposed to thymol showed complete organelle disorganization and evidence of lipid emulsification.

CONCLUSION

The collective experimental data suggest that thymol emulsions may be an effective naturally derived alternative to the current thymol treatments, and chemical fungicides in ameliorating FHB.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first thymol-derived nanoemulsion particles resuspended into water and not DMSO, exhibiting the same antibacterial/antifungal activity as previously described thymol and thyme oil treatments. This drastically reduces the environmental footprint thymol will leave if utilized as a fungicide treatment on field crops.

Histopathological Effects of Dietary Fusarium Graminearum on Rat Duodenum

Microscopic pathology of duodenum in rats exposed to Fusarium graminearum, a fungus infecting small-grain cereals, was investigated. Intestinal haemorrhage was observed macroscopically in one of the rats. Light microscopy demonstrated detachments between the surface epithelium and the lamina propria and severe interstitial oedema in the lamina propria in the test group. Electron microscopy identified epithelial absorptive cells with highly expanded endoplasmic reticulum tubules, abundant cytoplasmic vesicles containing electron-lucent materials, swollen mitochondria with spongiform appearance, and prominent cellular swelling. Other observations included opening of junctional complexes between epithelial cells lining the duodenum, highly enlarged intercellular spaces in duodenal epithelium, and numerous eosinophilic granulocytes and mast cells in the lamina propria. These findings indicate that dietary F. graminearum causes epithelial cell and connective tissue damage in rat duodenum. This is the first histopathological study showing that F. graminearum ingestion is associated with duodenal damage.

Phylogenomic analysis shows that Bacillus amyloliquefaciens subsp. plantarum is a later heterotypic synonym of Bacillus methylotrophicus

The rhizosphere-isolated bacteria belonging to the Bacillus amyloliquefaciens subsp. plantarum and Bacillus methylotrophicus clades are an important group of strains that are used as plant growth promoters and antagonists of plant pathogens. These properties have made these strains the focus of commercial interest. Here, we present the draft genome sequence of B. methylotrophicus KACC 13105(T) ( = CBMB205(T)). Comparative genomic analysis showed only minor differences between this strain and the genome of the B. amyloliquefaciens subsp. plantarum type strain, with the genomes sharing approximately 95% of the same genes. The results of morphological, physiological, chemotaxonomic and phylogenetic analyses indicate that the type strains of these two taxa are highly similar. In fact, our results show that the type strain of B. amyloliquefaciens subsp. plantarum FZB42(T) ( = DSM 23117(T) = BGSC 10A6(T)) does not cluster with other members of the B. amyloliquefaciens taxon. Instead, it clusters well within a clade of strains that are assigned to B. methylotrophicus, including the type strain of that species. Therefore, we propose that the subspecies B. amyloliquefaciens subsp. plantarum should be reclassified as a later heterotypic synonym of B. methylotrophicus.

Antagonistic action of Bacillus subtilis strain SG6 on Fusarium graminearum

Fusarium graminearum causes Fusarium head blight (FHB), a devastating disease that leads to extensive yield and quality loss of wheat and barley. Bacteria isolated from wheat kernels and plant anthers were screened for antagonistic activity against F. graminearum. Based on its in vitro effectiveness, strain SG6 was selected for characterization and identified as Bacillus subtilis. B. subtilis SG6 exhibited a high antifungal effect on the mycelium growth, sporulation and DON production of F. graminearum with the inhibition rate of 87.9%, 95.6% and 100%, respectively. In order to gain insight into biological control effect in situ, we applied B. subtilis SG6 at anthesis through the soft dough stage of kernel development in field test. It was revealed that B. subtilis SG6 significantly reduced disease incidence (DI), FHB index and DON (P ≤ 0.05). Further, ultrastructural examination shows that B. subtilis SG6 strain induced stripping of F. graminearum hyphal surface by destroying the cellular structure. When hypha cell wall was damaged, the organelles and cytoplasm inside cell would exude, leading to cell death. The antifungal activity of SG6 could be associated with the coproduction of chitinase, fengycins and surfactins.

Cyclic lipopeptide profile of three Bacillus subtilis strains; antagonists of Fusarium head blight

The objective of the study was to identify the lipopetides associated with three Bacillus subtilis strains. The strains are antagonists of Gibberella zeae, and have been shown to be effective in reducing Fusarium head blight in wheat. The lipopeptide profile of three B. subtilis strains (AS43.3, AS43.4, and OH131.1) was determined using mass spectroscopy. Strains AS43.3 and AS43.4 produced the anti-fungal lipopeptides from the iturin and fengycin family during the stationary growth phase. All three strains produced the lipopeptide surfactin at different growth times. Strain OH131.1 only produced surfactin under these conditions. The antifungal activity of the culture supernatant and individual lipopeptides was determined by the inhibition of G. zeae. Cell-free supernatant from strains AS43.3 and AS43.4 demonstrated strong antibiosis of G. zeae, while strain OH131.1 had no antibiosis activity. These results suggest a different mechanism of antagonism for strain OH131.1, relative to AS43.3 and AS43.4.

Concurrent selection for microbial suppression of Fusarium graminearum, Fusarium head blight and deoxynivalenol in wheat

AIM

Identify biological agents that can both control Fusarium head blight (FHB) and reduce deoxynivalenol (DON) production.

METHODS AND RESULTS

Concurrent screening methods were used to progressively select soil and food micro-organisms for the ability to suppress Fusarium graminearum, FHB and DON production. The micro-organisms were assessed using up to five assays including: a co-culture and dual-culture assay, an indirect impedance assay, a wheat floret assay, and two assays assessing DON production. Paenibacillus polymyxa W1-14-3 and C1-8-b gave the greatest inhibition of F. graminearum and reduction of DON production in greenhouse evaluations. Compared to a control treatment, they reduced disease severity by 56.5 and 55.4%, F. graminearum colonization of wheat heads by 58.8 and 62.4%, DON production by 84.8 and 89.4%, and increased 100-kernel weights by 56.6 and 66.9%, respectively.

CONCLUSIONS

The concurrent selection has resulted in promising antagonists that may possess multiple modes of action, and have the ability to colonize wheat heads in controlled environments.

SIGNIFICANCE AND IMPACT OF THE STUDY

A novel concurrent screening method was developed for selection of biocontrol agents for FHB. Two isolates of P. polymyxa were selected and identified. Their potential use as biocontrol agents for FHB is highlighted in this study.

Isolation of maize soil and rhizosphere bacteria with antagonistic activity against Aspergillus flavus and Fusarium verticillioides

Bacterial isolates from Mississippi maize field soil and maize rhizosphere samples were evaluated for their potential as biological control agents against Aspergillus flavus and Fusarium verticillioides. Isolated strains were screened for antagonistic activities in liquid coculture against A. flavus and on agar media against A. flavus and F. verticillioides. We identified 221 strains that inhibited growth of both fungi. These bacteria were further differentiated by their production of extracellular enzymes that hydrolyzed chitin and yeast cell walls and by production of antifungal metabolites. Based on molecular and nutritional identification of the bacterial strains, the most prevalent genera isolated from rhizosphere samples were Burkholderia and Pseudomonas, and the most prevalent genera isolated from nonrhizosphere soil were Pseudomonas and Bacillus. Less prevalent genera included Stenotrophomonas, Agrobacterium, Variovorax, Wautersia, and several genera of coryneform and enteric bacteria. In quantitative coculture assays, strains of P. chlororaphis and P. fluorescens consistently inhibited growth of A. flavus and F. verticillioides in different media. These results demonstrate the potential for developing individual biocontrol agents for simultaneous control of the mycotoxigenic A. flavus and F. verticillioides.

Crystal Structure of the Major Malassezia sympodialis Allergen Mala s 1 Reveals a β-Propeller Fold: A Novel Fold Among Allergens

Atopic eczema (AE) is a chronic inflammatory disease in which genetic predisposition and environmental factors such as microorganisms contribute to the symptoms. The yeast Malassezia Sympodialis, part of the normal human cutaneous flora, can act as an allergen eliciting specific IgE and T-cell reactivity in patients with AE. The major M. sympodialis allergen Mala s 1 is localized mainly in the yeast cell wall and exposed on the cell surface. Interestingly, Mala s 1 does not exhibit any significant sequence homology to known proteins. Here we present the crystal structure of Mala s 1 determined by single-wavelength anomalous dispersion techniques using selenomethionine-substituted Mala s 1. Mala s 1 folds into a 6-fold beta-propeller, a novel fold among allergens. The putative active site of Mala s 1 overlaps structurally to putative active sites in potential homologues, Q4P4P8 and Tri 14, from the plant parasites Ustilago maydis and Gibberella zeae, respectively. This resemblance suggests that Mala s 1 and the parasite proteins may have similar functions. In addition, we show that Mala s 1 binds to the phosphoinositides (PI) PI(3)P, PI(4)P, and PI(5)P, lipids possibly playing a role in the localization of Mala s 1 to the cell surface. The crystal structure of Mala s 1 will provide insights into the role of this major allergen in the host-microbe interactions and induction of an allergic response in AE.

Evaluation of Bacterial Antagonists for Biological Control of Broccoli Head Rot Caused by Pseudomonas fluorescens

ABSTRACT Pectolytic strains of Pseudomonas fluorescens are opportunistic pathogens of broccoli, causing head rot in temperate regions of the world. In this study, we investigated the potential of two bacterial isolates, P. fluorescens m6418 and Bacillus sp. A24, for biological control of broccoli head rot caused by P. fluorescens 5064, isolated from diseased broccoli in Scotland, UK. P. fluorescens m6418, a Tn5 mutant of wild-type 5064, is nonpathogenic and overproduces an extracellular metabolite with strong antimicrobial activity. In this study, we identified the anti-microbial metabolite produced by strain m6418 as pyrrolnitrin. P. fluorescens m6418 had significant inhibitory effects against strain 5064 both in culture and on broccoli leaves. In an excised broccoli head pathogenicity test, strain m6418, when coinoculated with P. fluorescens 5064, reduced disease by 41%. Bacillus sp. A24 produces an enzyme that can degrade N-acyl homoserine lactones, signaling molecules employed by bacteria for quorum sensing. Bacillus sp. A24 was capable of out-competing P. fluorescens 5064 when grown together in culture, and could degrade the quorum sensing signal of P. fluorescens 5064 (and thereby attenuate its virulence gene production). However, Bacillus sp. A24 had only a limited biocontrol effect on P. fluorescens 5064 in the excised broccoli head assay.

Influence of agricultural practices on fusarium infection of cereals and subsequent contamination of grain by trichothecene mycotoxins

Fusarium head blight (FHB) of small grain cereals and ear rot in maize are significant diseases across the world. Infection can not only result in reduced yield as a result of shrunken grains but also result in reduced milling and malting quality and the contamination of grains with mycotoxins. Mycotoxins are hazardous to animal and human health. Therefore, guidelines and legislation are in place, or under consideration, in most countries to protect consumers and animal welfare. As fusarium mycotoxins are produced within the growing crop, it is important to understand how agricultural practices affect mycotoxin contamination of grain. Such information could then be used to determine guidelines on "Good Agricultural Practice" (GAP) to minimise the mycotoxin contamination of cereal products. Evidence is provided to show the importance of choice of cultivar, crop rotation, soil cultivation, fertiliser and the chemical and biological control of insects, weeds and fungi.

Kidney damage by dietary Fusarium graminearum in rats: a microscopic study

Fusarium graminearum is a fungus frequently isolated from cereal grants. In this study, the histopathological effects of dietary F. graminearum on rat kidneys were examined. Treated rats and controls were fed F. graminearum-inoculated and non-inoculated rice, respectively. After 14 days, all the rats were killed and their kidneys were removed and examined using light microscopy. The kidneys of the rats in the treatment group were characterized by the following histopathological findings: patch-like mononuclear cell infiltrations in the cortex; prominent necrosis of proximal tubules; debris areas containing mononuclear cells and degraded renal tubules; shrunken and densely eosinophilic apoptotic cells in the proximal tubule epithelium; karyorrhexis and mitotic figures in the epithelium of both proximal and distal tubules; and many apoptotic bodies in the areas between the renal tubules. It was concluded that dietary F. graminearum caused inflammation and tubular damage in the rat kidney. This is the first microscopic study showing that F. graminearum ingestion is associated with renal damage.

Microscopic pathology of the liver in rats fed a Fusarium graminearum-inoculated diet

Fusarium graminearum is a fungus frequently isolated from cereal grains. This study investigates the histopathological effects of dietary F. graminearum on rat liver. Treatment and control group rats were fed F. graminearum-inoculated and non-inoculated rice, respectively. After 14 days, all rats were sacrificed, and their livers analysed by electron and light microscopy. Electron microscopy of treatment group livers identified hepatocytes with well-developed smooth endoplasmic reticulum, swollen mitochondria, lipid accumulation, numerous vesicles containing electron-lucent materials and increased lysosomes. Many Kupffer's cells containing apoptotic bodies were also seen. Light microscopy identified hepatocytes from the treatment group with: cytoplasmic and nuclear pleomorphism; foci of necrosis; mononuclear cell infiltration; and presence of apoptotic bodies. These changes were absent in control rat livers indicating that dietary F. graminearum causes inflammation and parenchymal damage in the rat liver. This is the first histopathological study showing the association between F. graminearum and liver damage.

Mycotoxigenic Fusarium and deoxynivalenol production repress chitinase gene expression in the biocontrol agent Trichoderma atroviride P1

Mycotoxin contamination associated with head blight of wheat and other grains caused by Fusarium culmorum and F. graminearum is a chronic threat to crop, human, and animal health throughout the world. One of the most important toxins in terms of human exposure is deoxynivalenol (DON) (formerly called vomitoxin), an inhibitor of protein synthesis with a broad spectrum of toxigenicity against animals. Certain Fusarium toxins have additional antimicrobial activity, and the phytotoxin fusaric acid has recently been shown to modulate fungus-bacterium interactions that affect plant health (Duffy and Défago, Phytopathology 87:1250-1257, 1997). The potential impact of DON on Fusarium competition with other microorganisms has not been described previously. Any competitive advantage conferred by DON would complicate efforts to control Fusarium during its saprophytic growth on crop residues that are left after harvest and constitute the primary inoculum reservoir for outbreaks in subsequent plantings. We examined the effect of the DON mycotoxin on ecological interactions between pathogenic Fusarium and Trichoderma atroviride strain P1, a competitor fungus with biocontrol activity against a wide range of plant diseases. Expression of the Trichoderma chitinase genes, ech42 and nag1, which contribute to biocontrol activity, was monitored in vitro and on crop residues of two maize cultivars by using goxA reporter gene fusions. We found that DON-producing F. culmorum and F. graminearum strains repressed expression of nag1-gox. DON-negative wild-type Fusarium strains and a DON-negative mutant with an insertional disruption in the tricothecene biosynthetic gene, tri5, had no effect on antagonist gene expression. The role of DON as the principal repressor above other pathogen factors was confirmed. Exposure of Trichoderma to synthetic DON or to a non-DON-producing Fusarium mutant resulted in the same level of nag1-gox repression as the level observed with DON-producing FUSARIUM: DON repression was specific for nag1-gox and had no effect, either positive or negative, on expression of another key chitinase gene, ech42. This is the first demonstration that a target pathogen down-regulates genes in a fungal biocontrol agent, and our results provide evidence that mycotoxins have a novel ecological function as factors in Fusarium competitiveness.

United States Department of Agriculture-Agricultural Research Service research on pre-harvest prevention of mycotoxins and mycotoxigenic fungi in US crops

Mycotoxins (ie toxins produced by molds) are fungal metabolites that can contaminate foods and feeds and cause toxic effects in higher organisms that consume the contaminated commodities. Therefore, mycotoxin contamination of foods and feeds results is a serious food safety issue and affects the competitiveness of US agriculture in both domestic and export markets. This article highlights research accomplished by Agricultural Research Service (ARS) laboratories on control of pre-harvest toxin contamination by using biocontrol, host-plant resistance enhancement and integrated management systems. Emphasis is placed on the most economically relevant mycotoxins, namely aflatoxins produced by Aspergillus flavus, Link, trichothecenes produced by various Fusarium spp and fumonisins produced by F verticillioides. Significant inroads have been made in establishing various control strategies such as development of atoxigenic biocontrol fungi that can outcompete their closely related, toxigenic cousins in field environments, thus reducing levels of mycotoxins in the crops. Potential biochemical and genetic resistance markers have been identified in crops, particularly in corn, which are being utilized as selectable markers in breeding for resistance to aflatoxin contamination. Prototypes of genetically engineered crops have been developed which: (1) contain genes for resistance to the phytotoxic effects of certain trichothecenes, thereby helping reduce fungal virulence, or (2) contain genes encoding fungal growth inhibitors for reducing fungal infection. Gene clusters housing the genes governing formation of trichothecenes, fumonisins and aflatoxins have been elucidated and are being targeted in strategies to interrupt the biosynthesis of these mycotoxins. Ultimately, a combination of strategies using biocompetitive fungi and enhancement of host-plant resistance may be needed to adequately prevent mycotoxin contamination in the field. To achieve this, plants may be developed that resist fungal infection and/or reduce the toxic effects of the mycotoxins themselves, or interrupt mycotoxin biosynthesis. This research effort could potentially save affected agricultural industries hundreds of millions of dollars during years of serious mycotoxin outbreaks.

Greenhouse and Field Evaluation of Biological Control of Fusarium Head Blight on Durum Wheat

Fusarium head blight (FHB) is a devastating disease that causes extensive yield and quality losses to wheat and barley. In durum wheat, the pathogen-produced toxin deoxynivalenol (DON) is retained in semolina at ˜50%, and the causal agent of FHB, Gibberella zeae, has a strong adverse effect on pasta color. Two bacteria and two yeast strains with known efficacy against G. zeae on hexaploid wheats were produced in liquid culture and assayed on two cultivars of durum wheat in greenhouse bioassays. All antagonists reduced FHB severity on cultivar Renville, and three of the four reduced severity on cultivar Ben, with Bacillus subtilis strain AS 43.3 decreasing FHB severity by as much as 90%. In separate greenhouse bioassays, the car-bon:nitrogen ratio of the medium used to produce antagonists did not consistently influence antagonist efficacy. All antagonist/production medium combinations but one were effective in reducing disease on both durum cultivars. Of six antagonists tested at field sites, Cryptococcus sp. OH 71.4 and C. nodaensis OH 182.9 reduced disease severity by as much as 57% in Peoria, IL, while Cryptococcus sp. OH 181.1 reduced disease severity by as much as 59% in a trial at Langdon, ND. Antagonists did not influence the DON content of grain in the Peoria trial. Relative performance indices for four antagonists calculated from greenhouse and field results on the two durum cultivars demonstrated that the bioassay location, but not the cultivar of durum, influenced the relative performance of antagonists. Yeast antagonists OH 71.4, OH 181.1, and OH 182.9 appear to have the highest potential for contributing to the reduction of FHB on du-rum wheat in the field.