Maize Pathogen Research Articles

Antagonistic activity of Trichoderma harzianum and Trichoderma viride strains against some fusarial pathogens causing stalk rot disease of maize, in vitro

ObjectivesA high incidence of stalk rot disease of maize causes huge economic losses as well as deleterious effects on the environment and human health resulting from fungicide use. Against this background, the current study was established to evaluate the antagonistic efficacy of Trichoderma viride and Trichoderma harzianum strains against Fusarium proliferatum and Fusarium verticillioides strains, the most common causative agents of stalk rot disease of maize. MethodsDual culture assay was performed to determine the antagonistic efficacy of Trichoderma strains against some fusarial pathogens of maize. Mycoparasitic relationships of the antagonistic fungal strains against fungal pathogens were investigated using a slide culture technique. Furthermore, a food poisoning technique was performed to detect the antimycotic efficacy of Trichoderma culture filtrates against fusarial pathogens of maize. Antifungal activity of organic solvent extracts of T. harzianum and T. viride was evaluated using the disc diffusion method. GC–MS analysis was used to detect the active components of these extracts. Results and conclusionTrichoderma viride showed antagonistic activity against F. proliferatum and F. verticillioides with mycelial inhibition rates of 80.17% and 70.46%, while T. harzianum exhibited rates of 68.38% and 60.64%, respectively. The culture filtrates of T. viride and T. harzianum strains exhibited antifungal activity against F. verticillioides strain with suppressive rates of 56.7% and 32.2%, while the mycelial inhibition rates against F. proliferatum strain were 44.09% and 23.50%, respectively. Mycoparasitic action of T. harzianum strain against fusarial strains was detected, while no mycoparasitism was observed between T. viride and fungal pathogens. The fungicidal concentration of carbendazim fungicide against F. proliferatum was 2.00 ppm, while F. verticillioides strain exhibited resistance to carbendazim fungicide. Moreover, the acetonic extracts of T. viride and T. harzianum strains exerted the highest antifungal potency against F. proliferatum strain, recording minimum inhibitory concentrations of 0.25 and 0.50 mg/ml, respectively. The main bioactive constituents of the acetonic extracts of T. viride and T. harzianum strains were palmitic acid (22.87%) and acetic acid (21.36%), respectively. In conclusion, the antagonistic strains could be a potential source of novel biological fungicides, especially against carbendazim-resistant F. verticillioides strain, avoiding side effects of chemical fungicides.

First Report of Leaf Blight Caused by Limonomyces roseipellis on Maize (Zea mays L) in China.

Maize (Zea mays L.) is one of the major crops in China. In July 2020, leaf blight was observed on approximately 18% of maize plants at the 852 Farm, Heilongjiang province. Symptoms appeared as yellow necrotic lesions on leaf tips and margins, which later expanded to the entire. The disease was first observed on the lower leaves of the plants and then progressed up the plant. Thirty symptomatic leaves were collected in 2020, and tissue samples between healthy and necrotic area (4 × 2 mm) were surface disinfected with 1% NaOCl for 3 min, 70% ethanol for 10 s, and washed three times with sterile water. Disinfected tissues were placed on potato dextrose agar (PDA) amended with streptomycin sulfate (50 mg/liter) and incubated at 25°C for 1 week. Cultures were purified using the hyphal-tip technique for morphological and molecular analyses. Morphological characteristics were observed on 1-week-old PDA cultures grown at 25°C. Mycelium changed from cream colored to pale pink or reddish, and the back of plate turned pink with time. Hyphae were hyaline, 2 to 7 μm wide, with clamps at primary septa. Hyaline basidiospores were sphere to ellipsoid, and ranged from 8 to 10 × 6 to 9 μm. Based on the presence of clamp connections and morphological features, the fungus was preliminarily identified as Limonomyces roseipellis (Stalpers et al. 1982). To confirm the identity of L. roseipellis, primers ITS1/ITS4 (White et al. 1990), MS1/MS2 (White et al. 1990), and LR0R/LR5 (Vilgalys and Hester 1990) were used to amplify the internal transcribed spacer (ITS) region, the partial mitochondrial small subunit rDNA (mtSSU) and nuclear large subunit rDNA (nuLSU), respectively. These sequences were deposited in GenBank (GenBank accession no.s MW067756, MW322806 and MW386178). The ITS sequence had 99.55% nucleotide identity (660 bp/668 bp) with L. roseipellis isolate EF82 (GenBank accession no. MK918632). The mtSSU sequence was 99.69% identical (634 bp/636 bp) to that of L. roseipellis strain SY-LQG101 (Genbank accession no. KF824718). The nuLSU sequence was 99.14% identical (924 bp/932 bp) to that of L. roseipellis (GenBank accession no. EU622844). A single basidiospore was isolated and cultured on PDA for pathogenicity testing. To fulfill Koch's postulates, ten healthy, surface-disinfected maize plants grown in pots (four to five leaves stage) were sprayed with basidiospore suspension (1×106 spores/ml); another ten healthy surface-disinfected maize plants sprayed with distilled water to serve as controls. Plants were sealed in plastic bags immediately after inoculation and maintained at 90% relative humidity in a mist chamber for 24 h at 25°C with a 12-h light cycle (Nicoli et al. 2016). Plants were moved and maintained in the greenhouse and observed for disease development. The experiment was conducted twice. Leaf blight symptoms appeared on all inoculated plants 3 to 5 days postinoculation and were consistent with symptoms observed in the field. No disease symptoms were observed on control plants. The pathogen was reisolated from diseased plants, and species identification was confirmed by the morphological and molecular method described. L. roseipellis has been reported to infect Cynodon dactylon, Lolium perenne and Festuca arundinacea, respectively. To our knowledge, this is the first report of the identification of L. roseipellis as a pathogen of maize in China, and this report will assist with monitoring distribution of the disease to assist with developing management recommendations.

Mapping and Validation of Major Quantitative Trait Loci for Resistance to Northern Corn Leaf Blight Along With the Determination of the Relationship Between Resistances to Multiple Foliar Pathogens of Maize (Zea mays L.).

Among various foliar diseases affecting maize yields worldwide, northern corn leaf blight (NCLB) is economically important. The genetics of resistance was worked out to be quantitative in nature thereby suggesting the need for the detection of quantitative trait loci (QTL) to initiate effective marker-aided breeding strategies. From the cross CML153 (susceptible) × SKV50 (resistant), 344 F2:3 progenies were derived and screened for their reaction to NCLB during the rainy season of 2013 and 2014. The identification of QTL affecting resistance to NCLB was carried out using the genetic linkage map constructed with 194 polymorphic SNPs and the disease data recorded on F2:3 progeny families. Three QTL for NCLB resistance were detected on chromosomes 2, 5, and 8 with the QTL qNCLB-8-2 explaining the highest phenotypic variation of 16.34% followed by qNCLB-5 with 10.24%. QTL for resistance to sorghum downy mildew (SDM) and southern corn rust (SCR) were also identified from one season phenotypic data, and the co-location of QTL for resistance to three foliar diseases was investigated. QTL present in chromosome bins 8.03, 5.03, 5.04, and 3.04 for resistance to NCLB, SDM, and SCR were co-localized, indicating their usefulness for the pyramiding of quantitative resistance to multiple foliar pathogens. Marker-assisted selection was practiced in the crosses CM212 × SKV50, HKI162 × SKV50, and CML153 × SKV50 employing markers linked to major QTL on chromosomes 8, 2, and 10 for NCLB, SDM, and SCR resistance, respectively. The populations were advanced to F6 stage to derive multiple disease-resistant inbred lines. Out of the 125 lines developed, 77 lines were tested for their combining ability and 39 inbred lines exhibited high general combining ability with an acceptable level of resistance to major diseases.

Induction of defense-related enzymes and enhanced disease resistance in maize against Fusarium verticillioides by seed treatment with Jacaranda mimosifolia formulations

Fusarium verticillioides is an important fungal pathogen of maize, causing stalk rot and severely affecting crop production. The aim of this study was to characterize the protective effects of formulations based on Jacaranda mimosifolia leaf extracts against F. verticillioides in maize. We compared different seed treatments comprising J. mimosifolia extracts, chemical fungicide (mefenoxam) and salicylic acid to modulate the defense system of maize host plants. Both aqueous and methanolic leaf extracts of J. mimosifolia (1.2% w/v) resulted in 96–97% inhibition of mycelial growth of F. verticillioides. While a full-dose (1.2%) extract of J. mimosifolia provided significant protective effects on maize plants compared to the inoculated control, a half-dose (0.6% w/v) application of J. mimosifolia in combination with half-strength mefenoxam was the most effective treatment in reducing stalk rot disease in pot and field experiments. The same seed treatment significantly upregulated the expression of genes in the leaves encoding chitinase, glucanase, lipid transfer protein, and pathogenesis-related proteins PR-1, PR-5 and PR-10, 72 h after inoculation. This treatment also induced the activities of peroxidase, polyphenol oxidase, protease, acid invertase, chitinase and phenylalanine ammonia lyase. We conclude that seed pre-treatment with J. mimosifolia extract with half-strength chemical mefenoxam is a promising approach for the management of stalk rot in maize.

Two regulators of G-protein signaling (RGS) proteins FlbA1 and FlbA2 differentially regulate fumonisin B1 biosynthesis in Fusarium verticillioides.

Fumonisins are a group of mycotoxins produced by maize pathogen Fusarium verticillioides that pose health concerns to humans and animals. Yet we still lack a clear understanding of the mechanism of fumonisins regulation during pathogenesis. The heterotrimeric G protein complex, which consists of canonical subunits and various regulators of G-protein signaling (RGS) proteins, plays an important role in transducing signals under environmental stress. Earlier studies demonstrated that Gα and Gβ subunits are positive regulators of fumonisin B1 (FB1) biosynthesis and that two RGS genes, FvFlbA1 and FvFlbA2, were highly upregulated in Gβ deletion mutant ∆Fvgbb1. Notably, FvFlbA2 has a negative role in FB1 regulation. While many fungi contain a single copy of FlbA, F. verticillioides harbors two putative FvFlbA paralogs, FvFlbA1 and FvFlbA2. In this study, we further characterized functional roles of FvFlbA1 and FvFlbA2. While ∆FvflbA1 deletion mutant exhibited no significant defects, ∆FvflbA2 and ∆FvflbA2/A1 mutants showed thinner aerial hyphal growth while promoting FB1 production. FvFlbA2 is required for proper expression of key conidia regulation genes, including putative FvBRLA, FvWETA, and FvABAA, while suppressing FUM21, FUM1, and FUM8 expression. Split luciferase assays determined that FvFlbA paralogs interact with key heterotrimeric G protein components, which in turn will lead altered G-protein-mediated signaling pathways that regulate FB1 production and asexual development in F. verticillioides.

Fusarium species from tropical grasses in Brazil and description of two new taxa

Fusarium isolates were obtained from asymptomatic seeds of wild grasses collected in six regions of Brazil. Eleven phylogenetic species were identified among 41 isolates based on sequences of EF-1α. These are members of the F. fujikuroi (FFSC, n = 24), F. incarnatum-equiseti (FIESC, n = 13), and F. chlamydosporum (FCSC, n = 5) species complexes that encompass known plant pathogens, mycotoxigenic species, and endophytes. Phylogenetic analyses based on EF-1α, RPB2, and TUB revealed two new species, F. caapi and F. brachiariae, that belong to the African clade of the FFSC and share main morphological features of F. mundagurra and F. nygamai. Another encountered isolate formed a singleton phylogenetic lineage within the FIESC. This survey shows that naturally occurring and cultivated grasses not only harbor a high diversity of known species, which are pathogens of maize, sorghum, rice, and sugarcane, but also novel Fusarium species.

Maize spermosphere bacterial endophytes and their biotic and abiotic stress tolerance traits

In the present study, 12 bacterial endophytes (C1 to C12) were isolated from three different genotypes of maize seed (COH6). Seed endophytes are considered as pioneers of plant microbiome and enhance the plant defense response at an early stage of crop growth and later stages of seedling development. The seed-associated endophytes exhibit the antagonistic property against the fungal pathogen (Fusarium oxysporum and Macrophomina phaseolina) of maize. Among 12 isolates, C11 suppressed the mycelial growth of both the tested pathogens. Furthermore, the selected isolates were subjected to in vitro abiotic stress tolerance such as salt, drought and temperature stress. The isolates C3, C5, C7, and C12 showed increased tolerance to drought, salt and temperature stress and might assist the host plant in alleviating the ill-effects of abiotic stress on maize. On a whole, microbial isolates associated with the seeds could be a better candidate in biotic and abiotic stress tolerance and would develop those isolates as consortia for obtaining desirable growth in maize.

Uticaj različitih kombinacija izvora ugljenika i azota u podlozi za proizvodnju biokontrolnog agensa Trichoderma harzianum

The increasing usage of chemicals for plant protection in recent years has become a serious problem. One of the possible solutions is use of beneficial microorganisms instead of synthetic fungicides, which will contribute to the protection of the environment and human health. Since the fungi Aspergillus flavus and Fusarium graminearum are the most important pathogens that cause maize diseases and produce mycotoxins, the potential of Trichoderma harzianum for biocontrol of both phytopathogens was examined in this paper. The aim of this paper was to study the influence of different carbon and nitrogen combinations in the medium for T. harzianum production. T. harzianum was cultivated in Erlenmeyer flasks and the effect of cultivation broth against selected maize pathogens was tested using well diffusion method. The results of this study showed that the combination of different carbon and nitrogen sources in the T. harzianum cultivation medium statistically significantly affects the production of Trichoderma cultivation broth effective on two tested phytopathogens. Dextrose as a carbon source and soybean flour as a nitrogen source proved to be the best combination in the medium for production of T. harzianum cultivation broth effective on A. flavus and F. graminearum. Maximal inhibition zone diameters of 31 mm and 56.33 mm were registered in those medium formulations for A. flavus and F. graminearum, respectively. These researches represent an important step for further research in which a medium of low market value would be selected. This would reduce the price of the production process but also the final product.

Biochemical characterization of two chitinases from Bacillus cereus sensu lato B25 with antifungal activity against Fusarium verticillioides P03.

Bacterial chitinases are a subject of intense scientific research due to their biotechnological applications, particularly their use as biological pesticides against phytopathogenic fungi as a green alternative to avoid the use of synthetic pesticides. Bacillus cereus sensu lato B25 is a rhizospheric bacterium that is a proven antagonist of Fusarium verticillioides, a major fungal pathogen of maize. This bacterium produces two chitinases that degrade the fungal cell wall and inhibit its growth. In this work, we used a heterologous expression system to purify both enzymes to investigate their biochemical traits in terms of Km, Vmax, optimal pH and temperature. ChiA and ChiB work as exochitinases, but ChiB exhibited a dual substrate activity and it is also an endochitinase. In this work, the direct addition of these chitinases inhibited fungal conidial germination and therefore they may play a major role in the antagonism against F. verticillioides.

Divergence and Gene Flow Between Fusarium subglutinans and F. temperatum Isolated from Maize in Argentina.

Fusarium subglutinans and F. temperatum are two important fungal pathogens of maize whose distinctness as separate species has been difficult to assess. We isolated strains of these species from commercial and native maize varieties in Argentina and sequenced >28,000 loci to estimate genetic variation in the sample. Our objectives were to measure genetic divergence between the species, infer demographic parameters related to their split, and describe the population structure of the sample. When analyzed together, over 30% of each species' polymorphic sites (>2,500 sites) segregate as polymorphisms in the other. Demographic modeling confirmed the species split predated maize domestication, but subsequent between-species gene flow has occurred, with gene flow from F. subglutinans into F. temperatum greater than gene flow in the reverse direction. In F. subglutinans, little evidence exists for substructure or recent selective sweeps, but there is evidence for limited sexual reproduction. In F. temperatum, there is clear evidence for population substructure and signals of abundant recent selective sweeps, with sexual reproduction probably less common than in F. subglutinans. Both genetic variation and the relative number of polymorphisms shared between species increase near the telomeres of all 12 chromosomes, where genes related to plant-pathogen interactions often are located. Our results suggest that species boundaries between closely related Fusarium species can be semipermeable and merit further study. Such semipermeability could facilitate unanticipated genetic exchange between species and enable quicker permanent responses to changes in the agro-ecosystem, e.g., pathogen-resistant host varieties, new chemical and biological control agents, and agronomic practices.

Panicum maximum as a reservoir of a potential maize pathogen

Endophytic fungi are capable of infecting plants without inducing the development of any observable symptoms. Certain fungal species can also coexist as latent pathogens in their specific hosts. Several species of Fusarium have been reported to form an association with grasses as endophytes or latent pathogens, some of which are also mycotoxin producers. In this study, we aimed to address whether forage grasses could act as potential reservoirs of species that are pathogenic to maize (Zea mays). To this end, we performed morphological and phylogenetic analyses of the elongation factor 1-α (EF-1α) and the second largest subunit of RNA polymerase II (RPB2) genes and characterized three fungal strains isolated from Panicum maximum (cv. Mombaca) as a new phylogenetic species, Fusarium gigantea, within the Fusarium fujikuroi species complex. Moreover, the results of the pathogenicity test, such as the toothpick inoculation assay, revealed that these species caused the stalk rot disease in maize when maintained under greenhouse conditions. This finding highlights P. maximum as a potential reservoir of this pathogen and its role as a threat to maize cultivation.

Maize endophytic microbial-communities revealed by removing PCR and 16S rRNA sequencing and their synthetic applications to suppress maize banded leaf and sheath blight

Endophytic microbial-communities have specific beneficial functions and are considered key drivers for host plant health. The removing-PCR (R-PCR) is a simple culture-independent cost-effective method to identify endophytic microbial-communities. Microbial communities from maize plant grown in different soil types were identified and characterized via the R-PCR and 16S rRNA sequencing. Culture-dependent microbial community identified through 16S rRNA gene sequencing, further these bacterial communities screened for antagonistic assay against Rhizoctonia solani WH1, in vitro compatibility tests, plant-growth-promoting traits and BIOLOG identification. After that, synthetic-communities (SycomA and SycomB) were prepared by mixing different compatible bacterial-strains to use as an inoculant to suppress pathogens of maize. We identified 167 bacterial operational taxonomic units (OTUs) and unexpected 8 fungal OTUs through the R-PCR, whereas, 95 bacterial OTUs via 16S rRNA sequencing from maize leaves and roots. SycomA and SycomB treatments suppressed the disease level and promoted growth attributes more effectively as compare to the single bacterial-strain and control treatments. This study establishes an efficient approach to isolate, identify and characterize diverse endophytic microbial-community assembly in maize leaves and roots, to successfully apply particular microbes to improve crop growth in soils affected by soil-borne-pathogens.

Transmission of Pantoea ananatis, the causal agent of leaf spot disease of maize (Zea mays), by western corn rootworm (Diabrotica virgiferavirgifera LeConte)

The aim of this study was to determine the role of Diabrotica virgifera virgifera (western corn rootworm, WCR) as a potential insect vector of the pathogenic bacterium Pantoea ananatis. The experiment included a greenhouse test to investigate if WCR was able to transfer the pathogenic bacteria from infected to healthy maize plants. Adult WCR specimens collected from maize fields near Rzeszów were used in the experiment. The plant materials were sweetcorn plants of the Waza variety. Pure cultures of previously verified P. ananatis strain M241 were the source of the inoculum. Insects caught under natural conditions were incubated in an isolator containing pathogen-negative plants. Randomly selected insects were then examined for the presence of bacterial pathogens of maize in their digestive tract. Pathogen-negative insects were used in the next stage of the experiment still carried out in isolators, in which the insects foraged on maize seedlings previously infected with P. ananatis. The control group consisted of healthy, uninfected insects and plants. After the incubation period, the presence of bacterial pathogens in the gastrointestinal tract of the WCR specimens was confirmed. Subsequent insects that acquired P. ananatis were bred on pathogen-negative maize plants. After incubation, the presence of pathogenic bacteria in the body of the examined WCR beetles was confirmed, and the presence of symptoms of the bacterial disease was noted on maize plants, fulfilling Koch's postulates and indicating that the WCR is a vector of P. ananatis on maize plants. This is the first report on the vectorization of P. ananatis by D. virgifera globally.

Engineering Smut Resistance in Maize by Site-Directed Mutagenesis of LIPOXYGENASE 3.

Biotic stresses caused by microbial pathogens impair crop yield and quality if not restricted by expensive and often ecologically problematic pesticides. For a sustainable agriculture of tomorrow, breeding or engineering of pathogen-resistant crop varieties is therefore a major cornerstone. Maize is one of the four most important cereal crops in the world. The biotrophic fungal pathogen Ustilago maydis causes galls on all aerial parts of the maize plant. Biotrophic pathogens like U. maydis co-evolved with their host plant and depend during their life cycle on successful manipulation of the host’s cellular machinery. Therefore, removing or altering plant susceptibility genes is an effective and usually durable way to obtain resistance in plants. Transcriptional time course experiments in U. maydis-infected maize revealed numerous maize genes being upregulated upon establishment of biotrophy. Among these genes is the maize LIPOXYGENASE 3 (LOX3) previously shown to be a susceptibility factor for other fungal genera as well. Aiming to engineer durable resistance in maize against U. maydis and possibly other pathogens, we took a Cas endonuclease technology approach to generate loss of function mutations in LOX3. lox3 maize mutant plants react with an enhanced PAMP-triggered ROS burst implicating an enhanced defense response. Based on visual assessment of disease symptoms and quantification of relative fungal biomass, homozygous lox3 mutant plants exposed to U. maydis show significantly decreased susceptibility. U. maydis infection assays using a transposon mutant lox3 maize line further substantiated that LOX3 is a susceptibility factor for this important maize pathogen.

Intercontinental trials reveal stable QTL for Northern corn leaf blight resistance in Europe and in Brazil

Key messageNCLB is the most devastating leaf disease in European maize, and the introduction of Brazilian resistance donors can efficiently increase the resistance levels of European maize germplasm.Northern corn leaf blight (NCLB) is one of the most devastating leaf pathogens in maize (Zea mays L.). Maize cultivars need to be equipped with broad and stable NCLB resistance to cope with production intensification and climate change. Brazilian germplasm is a great source to increase low NCLB resistance levels in European materials, but little is known about their effect in European environments. To investigate the usefulness of Brazilian germplasm as NCLB resistance donors, we conducted multi-parent QTL mapping, evaluated the potential of marker-assisted selection as well as genome-wide selection of 742 F1-derived DH lines. The line per se performance was evaluated in one location in Brazil and six location-by-year combinations (= environments) in Europe, while testcrosses were assessed in two locations in Brazil and further 10 environments in Europe. Jointly, we identified 17 QTL for NCLB resistance explaining 3.57–30.98% of the genotypic variance each. Two of these QTL were detected in both Brazilian and European environments indicating the stability of these QTL in contrasting ecosystems. We observed moderate to high genomic prediction accuracies between 0.58 and 0.83 depending on population and continent. Collectively, our study illustrates the potential use of tropical resistance sources to increase NCLB resistance level in applied European maize breeding programs.

Fungal Mycoflora Associated with Busseola fusca Frass in Maize Plants

Busseola fusca and Fusarium verticillioides are considered the most important pest and pathogen, respectively, of maize in South Africa. Busseola fusca tunnels into stems and ears of maize plants, whereas F. verticillioides causes Fusarium ear rot and deposits fumonisins in maize kernels. Frass (excreta) deposited by B. fusca larvae in maize stems and ears during feeding could potentially be contaminated with pathogens such as F. verticillioides, thereby indirectly and passively contributing to plant diseases. The mycoflora present in frass of B. fusca larvae was thus investigated in this study, and the dissemination of fumonisin-producing Fusarium spp. by B. fusca larvae in maize stems determined. Busseola fusca frass was collected from plants with visible insect damage in three maize-growing districts in South Africa. The mycoflora in the frass was isolated and identified morphologically to genus level, followed by the sequencing of target genes for species identification. Species of Acremonium, Aspergillus, Fusarium, Mucor, Rhizopus and Talaromyces were associated with the B. fusca frass. The role of B. fusca larvae in disseminating fumonisin-producing Fusarium spp. was then determined in greenhouse and field trials. Maize whorls were inoculated with F. verticillioides MRC826 spores 4 weeks after plant emergence, and infested with B. fusca larvae 2 days later. The stems were split open after 3 weeks and frass collected from feeding channels, thereafter, target DNA of fumonisin-producing Fusarium spp. was quantified using real-time PCR. Target DNA of fumonisin-producing Fusarium spp. was significantly higher in frass collected from greenhouse plants inoculated with F. verticillioides than in frass collected from the uninoculated control, indicating that the inoculation was successful in the absence of soil-borne inoculum. Nevertheless, the field trial showed no significant differences in target DNA in frass from inoculated and non-inoculated plants. This is possibly due to natural F. verticillioides infection of maize plants in the field. This study further indicated that B. fusca frass also contained maize pathogens such as Aspergillus spp. and Fusarium spp., which could cause ear rot diseases. The occurrence of Acremonium zeae in frass has potential implications for the biological control of F. verticillioides, as the fungus produces pyrrocidines A and B antibiotics which are known to be antagonistic to F. verticillioides. The occurrence of Aspergillus niger in frass requires further investigation in order to clarify its role in fumonisin contamination of maize in South Africa.

Discovery of broad-spectrum fungicides that block septin-dependent infection processes of pathogenic fungi.

Many pathogenic fungi depend on the development of specialized infection structures called appressoria to invade their hosts and cause disease. Impairing the function of fungal infection structures therefore provides a potential means by which diseases could be prevented. In spite of this extraordinary potential, however, relatively few anti-penetrant drugs have been developed to control fungal diseases, of either plants or animals. In the present study, we report the identification of compounds that act specifically to prevent fungal infection. We found that the organization of septin GTPases, which are essential for appressorium-mediated infection in the rice blast fungus Magnaporthe oryzae, requires very-long-chain fatty acids (VLCFAs), which act as mediators of septin organization at membrane interfaces. VLCFAs promote septin recruitment to curved plasma membranes and depletion of VLCFAs prevents septin assembly and host penetration by M. oryzae. We observed that VLCFA biosynthesis inhibitors not only prevent rice blast disease, but also show effective, broad-spectrum fungicidal activity against a wide range of fungal pathogens of maize, wheat and locusts, without affecting their respective hosts. Our findings reveal a mechanism underlying septin-mediated infection structure formation in fungi and provide a class of fungicides to control diverse diseases of plants and animals.

The insertion of a mitochondrial selfish element into the nuclear genome and its consequences.

Homing endonucleases (HE) are enzymes capable of cutting DNA at highly specific target sequences, the repair of the generated double‐strand break resulting in the insertion of the HE‐encoding gene (“homing” mechanism). HEs are present in all three domains of life and viruses; in eukaryotes, they are mostly found in the genomes of mitochondria and chloroplasts, as well as nuclear ribosomal RNAs. We here report the case of a HE that accidentally integrated into a telomeric region of the nuclear genome of the fungal maize pathogen Ustilago maydis. We show that the gene has a mitochondrial origin, but its original copy is absent from the U. maydis mitochondrial genome, suggesting a subsequent loss or a horizontal transfer from a different species. The telomeric HE underwent mutations in its active site and lost its original start codon. A potential other start codon was retained downstream, but we did not detect any significant transcription of the newly created open reading frame, suggesting that the inserted gene is not functional. Besides, the insertion site is located in a putative RecQ helicase gene, truncating the C‐terminal domain of the protein. The truncated helicase is expressed during infection of the host, together with other homologous telomeric helicases. This unusual mutational event altered two genes: The integrated HE gene subsequently lost its homing activity, while its insertion created a truncated version of an existing gene, possibly altering its function. As the insertion is absent in other field isolates, suggesting that it is recent, the U. maydis 521 reference strain offers a snapshot of this singular mutational event.

Uncovering the biofumigant capacity of allyl isothiocyanate from several Brassicaceae crops against Fusarium pathogens in maize

Driven by environmental concerns, chemical fumigants are no longer allowed in many countries. Therefore, other strategies for reducing fungal inoculum in soils and on crop debris are being explored. In the present study, several Brassicaceae crops were screened for their potential to control Fusarium gramineaum and Fusarium poae mycelial growth in an in vitro inverted Petri dish experiment. Volatile production was measured using gas chromatography-mass spectrometry headspace analysis. A selection of cultivars from each crop species was further investigated using a pot experiment with maize. Ethiopian mustard (Brassica carinata) and brown mustard (Brassica juncea) released volatile allyl isothiocyanate (AITC) and a higher concentration of AITC was correlated with a better fungal growth reduction in the in vitro screening. Brown mustard cultivar Etamine completely inhibited growth of both Fusarium spp. Pure AITC in a solution with methanol resulted in a sigmoid dose-response curve for both Fusarium spp. tested. Fusarium poae appeared to be more tolerant to AITC than F. graminearum. A pot experiment revealed that the incorporation of brown mustard plant material could alleviate the clear negative effect of F. graminearum infection on maize growth. The present study demonstrated the correlation between the fungistatic effect of biofumigation crops on Fusarium spp. and their production of volatile AITC in vitro, without the addition of exogenous enzymes, and confirmed the biofumigation potential of brown mustard in a pot experiment with maize. These results may help farmers when selecting a green manure crop suitable for biofumigation. © 2020 Society of Chemical Industry.

The Cysteine-Rich Repeat Protein TaCRR1 Participates in Defense against Both Rhizoctonia cerealis and Bipolaris sorokiniana in Wheat.

The domain of unknown function 26 (DUF26), harboring a conserved cysteine-rich motif (C-X8-C-X2-C), is unique to land plants. Several cysteine-rich repeat proteins (CRRs), belonging to DUF26-containing proteins, have been implicated in the defense against fungal pathogens in ginkgo, cotton, and maize. However, little is known about the functional roles of CRRs in the important staple crop wheat (Triticum aestivum). In this study, we identified a wheat CRR-encoding gene TaCRR1 through transcriptomic analysis, and dissected the defense role of TaCRR1 against the soil-borne fungi Rhizoctonia cerealis and Bipolaris sorokiniana, causal pathogens of destructive wheat diseases. TaCRR1 transcription was up-regulated in wheat towards B. Sorokiniana or R. cerealis infection. The deduced TaCRR1 protein contained a signal peptide and two DUF26 domains. Heterologously-expressed TaCRR1 protein markedly inhibited the mycelia growth of B. sorokiniana and R. cerealis. Furthermore, the silencing of TaCRR1 both impaired host resistance to B. sorokiniana and R. cerealis and repressed the expression of several pathogenesis-related genes in wheat. These results suggest that the TaCRR1 positively participated in wheat defense against both B. sorokiniana and R. cerealis through its antifungal activity and modulating expression of pathogenesis-related genes. Thus, TaCRR1 is a candidate gene for improving wheat resistance to B. sorokiniana and R. cerealis.