An experimental approach for estimating the genomic selection advantage for Fusarium head blight and Septoria tritici blotch in winter wheat

The genomic selection advantage for Fusarium head blight is promising but failed for Septoria tritici blotch. Selection of new breeding parents based on predictions must be treated with caution. Genomic selection (GS) is an approach that uses whole-genome marker data to estimate breeding values of untested genotypes and holds the potential to improve the genetic gain in breeding programs by shortening the breeding cycle and increasing the selection intensity. However, reported realized gain from genomic selection is limited to few experiments. In this study, a training population of 1120 winter wheat lines derived from 14 bi-parental families was genotyped with genome-wide single nucleotide polymorphism markers and phenotyped for Fusarium head blight (FHB) and Septoria tritici blotch (STB) severity, plant height and heading date. We used weighted ridge regression best linear unbiased prediction to calculate genomic estimated breeding values (GEBVs) of 2500 genotypes. Based on GEBVs, we selected the most resistant individuals as well as a random sample and tested them in a multi-location field trial. We computed moderate coefficients of correlation between observed and predicted trait values for FHB severity, plant height and heading date and achieved a genomic selection advantage of 10.62 percentage points for FHB resistance compared to the randomly chosen subpopulation. Genomic selection failed for the improvement of STB resistance with a genomic selection advantage of only 2.14 percentage points. Our results also indicate that the selection of new breeding parents based on GEBVs must be treated with caution. Taken together, the implementation of GS for FHB resistance, plant height and heading date seems to be promising. For traits with very strong genotype × environment variance, like STB resistance, GS appears to be still challenging.

Characterization of Fusarium solani Populations Associated with Spanish Strawberry Crops

Fusarium solani is an emerging pathogen reported on Spanish strawberry crops both in nurseries and in fruit production fields, causing wilt and root rot. Pathogenicity, morphocultural characteristics, and sensitivity to biocides of 103 F. solani isolates recovered from symptomatic strawberry plants and soils from both Spanish strawberry areas were determined. The differences of isolates within and between nurseries and field crops in relation to these parameters were analyzed. Considerable variability in morphological and pathogenic characteristics was observed among the isolates in both areas. The majority of isolates were not pathogenic (62%), and only 38 F. solani isolates (37.62%) caused disease on strawberry plants under controlled conditions; 52.63% of pathogenic isolates induced low severity symptoms. Almost 70% of pathogenic isolates caused stunting on plants. The morphological characters that best explain the F. solani variability (86.85%) were colony color and the presence of macroconidia on culture medium. The sensitivity to the fumigants tested was similar between the isolates from nurseries and fruit production fields, showing greater sensitivity to the field doses of dazomet and chloropicrin. However, the isolates were less sensitive to metam sodium and poorly sensitive to 1,3-dichloropropene. This work can contribute to the advancement of sustainable production of strawberry.

Transcriptome analysis of virulence-differentiated Fusarium oxysporum f. sp. cucumerinum isolates during cucumber colonisation reveals pathogenicity profiles

BACKGROUND

Cucumber Fusarium wilt, caused by Fusarium oxysporum f. sp. cucumerinum (Foc), is one of the most notorious diseases in cucumber production. Our previous research showed the virulence of Foc significantly increases over consecutive rounds of infection in a resistant cultivar. To understand the virulence variation of Foc under host pressure, the mildly virulent strain foc-3b (WT) and its virulence-enhanced variant Ra-4 (InVir) were selected and their transcriptome profiles in infected cucumber roots were analyzed at 24 h after inoculation (hai) and 120 hai.

RESULTS

A series of differentially expressed genes (DEGs) potentially involved in fungal pathogenicity and pathogenicity variation were identified and prove mainly involved in metabolic, transport, oxidation-reduction, cell wall degradation, macromolecules modification, and stress and defense. Among these DEGs, 190 up- and 360 down-regulated genes were expressed in both strains, indicating their importance in Foc infection. Besides, 286 and 366 DEGs showed up-regulated expression, while 492 and 214 showed down-regulated expression in InVir at 24 and 120 hai, respectively. These DEGs may be involved in increased virulence. Notably, transposases were more active in InVir than WT, indicating transposons may contribute to adaptive evolution.

CONCLUSIONS

By a comparative transcriptome analysis of the mildly and highly virulent strains of Foc during infection of cucumber, a series of DEGs were identified that may be associated with virulence. Hence, this study provides new insight into the transcriptomic profile underlying pathogenicity and virulence differentiation of Foc.

Mutation of a histidine-rich calcium-binding-protein gene in wheat confers resistance to Fusarium head blight

Head or ear blight, mainly caused by Fusarium species, can devastate almost all staple cereal crops (particularly wheat), resulting in great economic loss and imposing health threats on both human beings and livestock^1-3. However, achievement in breeding for highly resistant cultivars is still not satisfactory. Here, we isolated the major-effect wheat quantitative trait locus, Qfhs.njau-3B, which confers head blight resistance, and showed that it is the same as the previously designated Fhb1. Fhb1 results from a rare deletion involving the 3' exon of the histidine-rich calcium-binding-protein gene on chromosome 3BS. Both wheat and Arabidopsis transformed with the Fhb1 sequence showed enhanced resistance to Fusarium graminearum spread. The translation products of this gene's homologs among plants are well conserved and might be essential for plant growth and development. Fhb1 could be useful not only for curbing Fusarium head blight in grain crops but also for improving other plants vulnerable to Fusarium species.

Effect of Planting Date and Inoculum Density on Severity of Fusarium Wilt of Lettuce in California

Seven lettuce cultivars were transplanted into a field infested with Fusarium oxysporum f. sp. lactucae (causal agent of Fusarium wilt of lettuce) in August and September of 2014, 2015, and 2016. For moderately susceptible cultivars, 1- or 2-week differences in planting date had a significant effect on severity of Fusarium wilt. In growth chamber experiments, cultivars of moderate susceptibility were maintained in a growth chamber under cool conditions (23/18°C), and a subset of plants was transferred to a warm chamber (33/23°C) for 1 week, at weekly intervals after transplanting. Plants exposed to high temperatures at 2 and 3 weeks after transplanting (WAT) had more severe symptoms of Fusarium wilt than those exposed at 4 WAT. In October 2015, April 2016, August 2016, and August 2017, moderately susceptible cultivars were planted into field soil with an inoculum density gradient. Moderately susceptible cultivars were little affected by Fusarium wilt where inoculum densities of F. oxysporum f. sp. lactucae were <125 per gram of soil, even in warm planting windows. Adjusting planting dates to avoid high temperatures during a critical stage of growth and maintaining low inoculum density in soil can contribute to management of Fusarium wilt in moderately susceptible lettuce cultivars.

A deletion mutation in TaHRC confers Fhb1 resistance to Fusarium head blight in wheat

Fusarium head blight (FHB), which is mainly caused by Fusarium graminearum, is a destructive wheat disease that threatens global wheat production. Fhb1, a quantitative trait locus discovered in Chinese germplasm, provides the most stable and the largest effect on FHB resistance in wheat. Here we show that TaHRC, a gene that encodes a putative histidine-rich calcium-binding protein, is the key determinant of Fhb1-mediated resistance to FHB. We demonstrate that TaHRC encodes a nuclear protein conferring FHB susceptibility and that a deletion spanning the start codon of this gene results in FHB resistance. Identical sequences of the TaHRC-R allele in diverse accessions indicate that Fhb1 had a single origin, and phylogenetic and haplotype analyses suggest that the TaHRC-R allele most likely originated from a line carrying the Dahongpao haplotype. This discovery opens a new avenue to improve FHB resistance in wheat, and possibly in other cereal crops, by manipulating TaHRC sequence through bioengineering approaches.

Characterization of LuWRKY36, a flax transcription factor promoting secoisolariciresinol biosynthesis in response to Fusarium oxysporum elicitors in Linum usitatissimum L. hairy roots

The involvement of a WRKY transcription factor in the regulation of lignan biosynthesis in flax using a hairy root system is described. Secoisolariciresinol is the main flax lignan synthesized by action of LuPLR1 (pinoresinol-lariciresinol reductase 1). LuPLR1 gene promoter deletion experiments have revealed a promoter region containing W boxes potentially responsible for the response to Fusarium oxysporum. W boxes are bound by WRKY transcription factors that play a role in the response to stress. A candidate WRKY transcription factor, LuWRKY36, was isolated from both abscisic acid and Fusarium elicitor-treated flax cell cDNA libraries. This transcription factors contains two WRKY DNA-binding domains and is a homolog of AtWRKY33. Different approaches confirmed LuWRKY36 binding to a W box located in the LuPLR1 promoter occurring through a unique direct interaction mediated by its N-terminal WRKY domain. Our results propose that the positive regulator action of LuWRKY36 on the LuPLR1 gene regulation and lignan biosynthesis in response to biotic stress is positively mediated by abscisic acid and inhibited by ethylene. Additionally, we demonstrate a differential Fusarium elicitor response in susceptible and resistant flax cultivars, seen as a faster and stronger LuPLR1 gene expression response accompanied with higher secoisolariciresinol accumulation in HR of the resistant cultivar.

Antifungal potential of Lauraceae rhizobacteria from a tropical montane cloud forest against Fusarium spp

The occurrence of pests and diseases can affect plant health and productivity in ecosystems that are already at risk, such as tropical montane cloud forests. The use of naturally occurring microorganisms is a promising alternative to mitigate forest tree fungal pathogens. The objectives of this study were to isolate rhizobacteria associated with five Lauraceae species from a Mexican tropical montane cloud forest and to evaluate their antifungal activity against Fusarium solani and F. oxysporum. Fifty-six rhizobacterial isolates were assessed for mycelial growth inhibition of Fusarium spp. through dual culture assays. Thirty-three isolates significantly reduced the growth of F. solani, while 21 isolates inhibited that of F. oxysporum. The nine bacterial isolates that inhibited fungal growth by more than 20% were identified through 16S rDNA gene sequence analysis; they belonged to the genera Streptomyces, Arthrobacter, Pseudomonas, and Staphylococcus. The volatile organic compounds (VOC) produced by these nine isolates were evaluated for antifungal activity. Six isolates (Streptomyces sp., Arthrobacter sp., Pseudomonas sp., and Staphylococcus spp.) successfully inhibited F. solani mycelial growth by up to 37% through VOC emission, while only the isolate INECOL-21 (Pseudomonas sp.) inhibited F. oxysporum. This work provides information on the microbiota of Mexican Lauraceae and is one of the few studies identifying forest tree-associated microbes with inhibitory activity against tree pathogens.

Genome-Wide Association Mapping of Resistance to Fusarium Head Blight Spread and Deoxynivalenol Accumulation in Chinese Elite Wheat Germplasm

Improving resistance to Fusarium head blight (FHB) in wheat is crucial in the integrated management of the disease and prevention of deoxynivalenol (DON) contamination in grains. To identify novel sources of resistance, a genome-wide association study (GWAS) was performed using a panel of 213 accessions of elite wheat germplasm of China. The panel was evaluated for FHB severity in four environments and DON content in grains in two environments. High correlations across environments and high heritability were observed for FHB severity and DON content in grains. The panel was also genotyped with the 90K Illumina iSelect single nucleotide polymorphism (SNP) array and 11,461 SNP markers were obtained. The GWAS revealed a total of six and three loci significantly associated with resistance to fungal spread and DON accumulation in at least two environments, respectively. QFHB-2BL.1 tagged by IWB52433 and QFHB-3A tagged by IWB50548 were responsible for resistance to both fungal spread and DON accumulation. In summary, this study provided an overview of FHB resistance resources in elite Chinese wheat germplasm and identified novel resistance loci that could be used for wheat improvement.

Treatment With Wheat Root Exudates and Soil Microorganisms From Wheat/Watermelon Companion Cropping Can Induce Watermelon Disease Resistance Against Fusarium oxysporum f. sp. niveum

Companion cropping with wheat (Triticum aestivum L.) can enhance watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] wilt disease resistance against Fusarium oxysporum f. sp. niveum. However, the mechanism of resistance induction remains unknown. In this study, the effects of microbial community dynamics and the interactions between wheat and watermelon plants, particularly the effect of wheat root exudates on watermelon resistance against F. oxysporum f. sp. niveum, were examined using a plant-soil feedback trial and plant tissue culture approach. The plant-soil feedback trial showed that treating watermelon with soil from wheat/watermelon companion cropping decreased watermelon wilt disease incidence and severity, increased lignin biosynthesis- and defense-related gene expression, and increased β-1,3-glucanase activity in watermelon roots. Furthermore, soil microbes can contribute to increasing disease resistance in watermelon plants. Tissue culture experiments showed that both exogenous addition of wheat root exudates and companion cropping with wheat increased host defense gene expression, lignin and total phenols, and increased β-1,3-glucanase activity in watermelon roots. In conclusion, both root exudates from wheat and the related soil microorganisms in a wheat/watermelon companion cropping system played critical roles in enhancing resistance to watermelon wilt disease induced by F. oxysporum f. sp. niveum.

CaMPK9 increases the stability of CaWRKY40 transcription factor which triggers defense response in chickpea upon Fusarium oxysporum f. sp. ciceri Race1 infection

Physical interaction and phosphorylation by CaMPK9 protects the degradation of CaWRKY40 that induces resistance response in chickpea to Fusarium wilt disease by modulating the transcription of defense responsive genes. WRKY transcription factors (TFs) are the global regulators of plant defense signaling that modulate immune responses in host plants by regulating transcription of downstream target genes upon challenged by pathogens. However, very little is known about immune responsive role of Cicer arietinum L. (Ca) WRKY TFs particularly. Using two contrasting chickpea genotypes with respect to resistance against Fusarium oxysporum f. sp. ciceri Race1 (Foc1), we demonstrate transcript accumulation of different CaWRKYs under multiple stresses and establish that CaWRKY40 triggers defense. CaWRKY40 overexpressing chickpea mounts resistance to Foc1 by positively modulating the defense related gene expression. EMSA, ChIP assay and real-time PCR analyses suggest CaWRKY40 binds at the promoters and positively regulates transcription of CaDefensin and CaWRKY33. Further studies revealed that mitogen Activated Protein Kinase9 (CaMPK9) phosphorylates CaWRKY40 by directly interacting with its two canonical serine residues. Interestingly, CaMPK9 is unable to interact with CaWRKY40 when the relevant two serine residues were replaced by alanine. Overexpression of serine mutated WRKY40 isoform in chickpea fails to provide resistance against Foc1. Mutated WRKY40^{Ser.224/225 to AA} overexpressing chickpea resumes its ability to confer resistance against Foc1 after application of 26S proteasomal inhibitor MG132, suggests that phosphorylation is essential to protect CaWRKY40 from proteasomal degradation. CaMPK9 silencing also led to susceptibility in chickpea to Foc1. Altogether, our results elucidate positive regulatory roles of CaMPK9 and CaWRKY40 in modulating defense response in chickpea upon Foc1 infection.

Secretome Profiling Reveals Virulence-Associated Proteins of Fusarium proliferatum during Interaction with Banana Fruit

Secreted proteins are vital for the pathogenicity of many fungi through manipulating their hosts for efficient colonization. Fusarium proliferatum is a phytopathogenic fungus infecting many crops, vegetables, and fruit, including banana fruit. To access the proteins involved in pathogen–host interaction, we used label-free quantitative proteomics technology to comparatively analyze the secretomes of F. proliferatum cultured with and without banana peel in Czapek’s broth medium. By analyzing the secretomes of F. proliferatum, we have identified 105 proteins with 40 exclusively secreted and 65 increased in abundance in response to a banana peel. These proteins were involved in the promotion of invasion of banana fruit, and they were mainly categorized into virulence factors, cell wall degradation, metabolic process, response to stress, regulation, and another unknown biological process. The expressions of corresponding genes confirmed the existence of these secreted proteins in the banana peel. Furthermore, expression pattern suggested variable roles for these genes at different infection stages. This study expanded the current database of F. proliferatum secreted proteins which might be involved in the infection strategy of this fungus. Additionally, this study warranted the further attention of some secreted proteins that might initiate infection of F. proliferatum on banana fruit.

Cucurbit Rootstocks Resistant to Fusarium oxysporum f. sp. niveum Remain Resistant When Coinfected by Meloidogyne incognita in the Field

Interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata) rootstocks used to graft watermelon (Citrullus lanatus var. lanatus) are resistant to Fusarium oxysporum f. sp. niveum, the fungus that causes Fusarium wilt of watermelon, but they are susceptible to Meloidogyne incognita, the southern root knot nematode. A new citron (Citrullus amarus) rootstock cultivar Carolina Strongback is resistant to F. oxysporum f. sp. niveum and M. incognita. The objective of this study was to determine if an interaction between M. incognita and F. oxysporum f. sp. niveum race 2 occurred on grafted or nongrafted triploid watermelon susceptible to F. oxysporum f. sp. niveum race 2. In 2016 and 2018, plants of nongrafted cultivar Fascination and Fascination grafted onto Carolina Strongback and interspecific hybrid squash cultivar Carnivor were inoculated or not inoculated with M. incognita before transplanting into field plots infested or not infested with F. oxysporum f. sp. niveum race 2. Incidence of Fusarium wilt and area under the disease progress curve did not differ when hosts were inoculated with F. oxysporum f. sp. niveum alone or F. oxysporum f. sp. niveum and M. incognita together. Fusarium wilt was greater on nongrafted watermelon (78% mean incidence) than on both grafted rootstocks and lower on Carnivor (1% incidence) than on Carolina Strongback (12% incidence; P ≤ 0.01). Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. At the end of the season, Carnivor had a greater percentage of the root system galled than the other two hosts, whereas galling did not differ on Fascination and Carolina Strongback. F. oxysporum f. sp. niveum reduced marketable weight of nongrafted Fascination with and without coinoculation with M. incognita. M. incognita reduced marketable weight of Fascination grafted onto Carnivor compared with noninoculated, nongrafted Fascination. In conclusion, cucurbit rootstocks that are susceptible and resistant to M. incognita retain resistance to F. oxysporum f. sp. niveum when they are coinfected with M. incognita.

Relationship Between Invasive Brown Marmorated Stink Bug (Halyomorpha halys) and Fumonisin Contamination of Field Corn in the Mid-Atlantic U.S

Brown marmorated stink bug (Halyomorpha halys Stål) is an invasive agricultural pest that causes severe damage to many crops. To determine potential associations between H. halys feeding damage, Fusarium infection, and mycotoxin contamination in field corn, a field survey was conducted in eight counties in Virginia. Results indicated an association between H. halys feeding damage and fumonisin contamination. Subsequent field experiments in Delaware, Maryland, and Virginia examined the ability of H. halys to increase Fusarium verticillioides (Sacc.) Nirenberg infection and fumonisin concentrations in corn. At the milk stage, H. halys (0 or 4 adults) and Fusarium (with or without F. verticillioides inoculum) treatments were applied to bagged ears in a two by two factorial randomized complete block design with 12 replicates. H. halys treatments increased levels of feeding damage (P < 0.0001) and Fusarium infection (P = 0.0380). Interaction between H. halys and Fusarium treatments influenced severity of infection (P = 0.0018) and fumonisin concentrations (P = 0.0360). Results suggest H. halys has the ability to increase both Fusarium infection and fumonisin concentrations in field corn. Further studies are needed to understand mechanisms by which H. halys increases fumonisin and to develop management strategies to mitigate impacts of H. halys on field corn in the region.

Transmission of Fusarium oxysporum f. sp. fragariae Through Stolons in Strawberry Plants

Fusarium wilt of strawberry, caused by the soilborne pathogen Fusarium oxysporum f. sp. fragariae, is a growing threat to the strawberry industry worldwide. Symptoms of the disease typically include stunting, wilting, crown discoloration, and eventual plant death. When Fusarium wilt was discovered in California, the disease was not known to occur anywhere else in North America. Long distance movement of the pathogen would most likely occur through transport of infected plants, which seems plausible if strawberry plants can sustain infections without showing symptoms of disease. The results of this study document that F. oxysporum f. sp. fragariae can move through stolons of infected mother plants and colonize first-generation daughter plants. The pathogen can also move through stolons from first to second-generation daughter plants. Daughter plants of both generations were always symptomless. The pathogen was recovered from both roots and petioles of infected daughter plants. Similar results were obtained for two cultivars known to be susceptible to Fusarium wilt, Albion and Monterey. Transmission through stolons from mother to daughter plants also occurred in the resistant cultivar, San Andreas, but less frequently than in Albion and Monterey.

Hybrid Genome Assembly of a Major Quantitative Disease Resistance Locus in Soybean Toward Fusarium graminearum

Schwabe [teleomorph: Gibberella zeae (Schweintiz) Petch] has been identified as a pathogen of soybean [ (L.) Merr.] causing seed, seedling damping-off and root rot in North America. A major quantitative disease resistance locus (QDRL) that contributed 38.5% of the phenotypic variance toward in soybean was previously identified through mapping of a recombinant inbred line (RIL) population derived from a cross between 'Wyandot' and PI 567301B. This major QDRL mapped to chromosome 8 to a predicted 305 kb region harboring 36 genes. This locus maps near the locus for soybean cyst nematode (SCN) and the locus contributing to seed coat color. Long-read sequencing of the region was completed and variations in gene sequence and gene order compared with the 'Williams 82' reference were identified. Molecular markers were developed for genes within this region and mapped in the original population, slightly narrowing the region of interest. Analyses of the hybrid genome reassembly using three previously published bacterial artificial chromosome (BAC) sequences (BAC56G2, BAC104J7, and BAC77G7-a) combined with RNA-sequencing narrowed the region making candidate gene identification possible. The markers within this region may be used for marker-assisted selection (MAS). There were 10 differentially expressed genes between resistant and susceptible lines, with four of these candidates also located within the genomic interval defined by the flanking markers. These genes included an actin-related protein 2/3 complex subunit, an unknown protein, a hypothetical protein, and a chalcone synthase 3.

Control of Wheat Fusarium Head Blight by Heat-Stable Antifungal Factor (HSAF) from Lysobacter enzymogenes

Heat-stable antifungal factor (HSAF), which belongs to the polycyclic tetramate macrolactam family, was isolated from Lysobacter enzymogenes fermentations and exhibited inhibitory activities against a wide range of fungal pathogens. In this study, the antifungal activity of HSAF against Fusarium graminearum in vitro and in vivo was investigated. A total of 50% of mycelial growth of F. graminearum was suppressed with 4.1 μg/ml of HSAF (EC₅₀ value). HSAF treatment resulted in abnormal morphology of the hyphae, such as curling, apical swelling, and depolarized growth. Furthermore, HSAF adequately inhibited conidial germination and conidiation of F. graminearum with an inhibition rate of 100% when 1 and 6 μg/ml of HSAF were applied, respectively. HSAF caused ultrastructural changes of F. graminearum, including cell wall thickening and plasmolysis. Moreover, the application of HSAF significantly controlled Fusarium head blight in wheat caused by F. graminearum in the field. Overall, these results indicate that HSAF has potential for development as a fungicide against F. graminearum.

Characterization of Soybean STAY-GREEN Genes in Susceptibility to Foliar Chlorosis of Sudden Death Syndrome

Genetic mappings for soybean sudden death syndrome foliar chlorosis suggested that STAY-GREEN genes with loss-of-susceptibility mechanism may have different breeding merits for disease resistance.

Soybean Sudden Death Syndrome Causal Agent Fusarium brasiliense Present in Michigan

Sudden death syndrome (SDS), caused by members of Fusarium solani species complex (FSSC) clade 2, is a major and economically important disease in soybean worldwide. The primary causal agent of SDS isolated to date in North America has been F. virguliforme. In 2014 and 2016, SDS symptoms were found in two soybean fields located on the same farm in Michigan. Seventy Fusarium strains were isolated from roots of the SDS-symptomatic soybeans in two fields. Phylogenetic analysis of partial sequences of elongation factor-1α, the nuclear ribosomal DNA intergenic spacer region, and the RNA polymerase II beta subunit revealed that the primary FSSC species isolated was F. brasiliense (58 and 36% in each field) and the remaining Fusarium strains were identified as F. cuneirostrum, F. phaseoli, an undescribed Fusarium sp. from FSSC clade 2, and strains in FSSC clade 5 and FSSC clade 11. Molecular identification was supported with morphological analysis and a pathogenicity assay. The soybean seedling pathogenicity assay indicated that F. brasiliense was capable of causing typical foliar SDS symptoms. Both root rot and foliar disease severity were variable by strain, just as they are in F. virguliforme. Both FSSC 5 and FSSC 11 strains were also capable of causing root rot, but SDS foliar symptoms were not detected. To our knowledge, this is the first report of F. brasiliense causing SDS in soybean in the United States and the first report of F. cuneirostrum, F. phaseoli, an as-yet-unnamed Fusarium sp., and strains in FSSC clade 5 and FSSC clade 11 associated with or causing root rot of soybean in Michigan.

Induction of ethylene inhibits development of soybean sudden death syndrome by inducing defense-related genes and reducing Fusarium virguliforme growth

Ethylene is a gaseous hormone that regulates plant responses to biotic and abiotic stresses. To investigate the importance of ethylene in soybean resistance to Fusarium virguliforme (Fv), the causal agent of sudden death syndrome (SDS), soybean cultivars Williams 82 (SDS-susceptible) and MN1606 (SDS-resistant) were treated 24 h before and 24h after Fv inoculation with either ethephon (ethylene inducer), cobalt chloride (ethylene biosynthesis inhibitor), or 1-MCP (ethylene perception inhibitor). Inoculated plants were grown for 21 days at 24°C in the greenhouse and then evaluated for SDS severity and expression of soybean defense genes. In both cultivars, plants treated with ethephon showed lower SDS foliar severity compared to the other treatments, whereas those treated with cobalt chloride or 1-MCP showed the same or higher SDS foliar severity compared to the water-treated control. Ethephon application resulted in activation of genes involved in ethylene biosynthesis, such as ethylene synthase (ACS) and ethylene oxidase (ACO), and genes involved in soybean defense response, such as pathogenesis-related protein (PR), basic peroxidase (IPER), chalcone synthase (CHS), and defense-associated transcription factors. Cobalt chloride and 1-MCP treatments had little or no effect on the expression of these genes. In addition, ethephon had a direct inhibitory effect on in-vitro growth of Fv on PDA media. Our results suggest that ethephon application inhibits SDS development directly by slowing Fv growth and/or by inducing soybean ethylene signaling and the expression of defense related genes.

Genetic factors affecting Fusarium head blight resistance improvement from introgression of exotic Sumai 3 alleles (including Fhb1, Fhb2, and Fhb5) in hard red spring wheat

BACKGROUND

Fusarium head blight resistance genes, Fhb1 (for Type-II resistance), Fhb2 (Type-II), and Fhb5 (Type-I plus some Type-II), which originate from Sumai 3, are among the most important that confer resistance in hexaploid wheat. Near-isogenic lines (NILs), in the CDC Alsask (susceptible; n = 32) and CDC Go (moderately susceptible; n = 38) backgrounds, carrying these genes in all possible combinations were developed using flanking microsatellite markers and evaluated for their response to FHB and deoxynivalenol (DON) accumulation in eight environments. NILs were haplotyped with wheat 90 K iSelect assay to elucidate the genomic composition and confirm alleles' presence. Other than evaluating the effects of three major genes in common genetic background, the study elucidated the epistatic gene interactions as they influence FHB measurements; identified loci other than Fhb1, Fhb2, and Fhb5, in both recurrent and donor parents and examined annotated proteins in gene intervals.

RESULTS

Genotyping using 81,857 single nucleotide polymorphism (SNP) markers revealed polymorphism on all chromosomes and that the NILs carried < 3% of alleles from the resistant donor. Significant improvement in field resistance (Type-I + Type-II) resulted only among the CDC Alsask NILs, not the CDC Go NILs. The phenotypic response of NILs carrying combinations of Sumai 3 derived genes suggested non-additive responses and Fhb5 was as good as Fhb1 in conferring field resistance in both populations. In addition to Fhb1, Fhb2, and Fhb5, four to five resistance improving alleles in both populations were identified and three of five in CDC Go were contributed by the susceptible parent. The introgressed chromosome regions carried genes encoding disease resistance proteins, protein kinases, nucleotide-binding and leucine rich repeats' domains. Complex epistatic gene-gene interactions among marker loci (including Fhb1, Fhb2, Fhb5) explained > 20% of the phenotypic variation in FHB measurements.

CONCLUSIONS

Immediate Sumai 3 derivatives carry a number of resistance improving minor effect alleles, other than Fhb1, Fhb2, Fhb5. Results verified that marker-assisted selection is possible for the introgression of exotic FHB resistance genes, however, the genetic background of the recipient line and epistatic interactions can have a strong influence on expression and penetrance of any given gene.

Amylose-Free ("waxy") Wheat Colonization by Fusarium spp. and Response to Fusarium Head Blight

Hexaploid waxy wheat (Triticum aestivum L.) has null mutations in Wx genes and grain lacking amylose with increased digestibility and usability for specialty foods. The waxy cultivar Mattern is susceptible to Fusarium head blight (FHB) caused by Fusarium graminearum species complex, which produces the mycotoxin deoxynivalenol (DON). In experiment 1, conducted during low natural FHB, grain from waxy breeding lines, Mattern, and wild-type breeding lines and cultivars were assessed for Fusarium infection and DON concentration. Nine Fusarium species and species complexes were detected from internally infected (disinfested) grain; F. graminearum infections were not different between waxy and wild-type. Surface- and internally infected grain (nondisinfested) had greater numbers of Fusarium isolates across waxy versus wild-type, but F. graminearum-like infections were similar; however, DON levels were higher in waxy. In experiment 2, conducted during a timely epidemic, disease severity, Fusarium-damaged kernels (FDK), and DON were assessed for waxy breeding lines, Mattern, and wild-type cultivars. Disease severity and FDK were not significantly different from wild-type, but DON was higher in waxy than wild-type lines. Across both experiments, waxy breeding lines, Plant Introductions 677876 and 677877, responded similarly to FHB as moderately resistant wild-type cultivar Overland, showing promise for breeding advanced waxy cultivars with reduced FHB susceptibility.

Targeting the pattern-triggered immunity pathway to enhance resistance to Fusarium graminearum

Fusarium head blight (FHB) is a disease of the floral tissues of wheat and barley for which highly resistant varieties are not available. Thus, there is a need to identify genes/mechanisms that can be targeted for the control of this devastating disease. Fusarium graminearum is the primary causal agent of FHB in North America. In addition, it also causes Fusarium seedling blight. Fusarium graminearum can also cause disease in the model plant Arabidopsis thaliana. The Arabidopsis-F. graminearum pathosystem has facilitated the identification of targets for the control of disease caused by this fungus. Here, we show that resistance against F. graminearum can be enhanced by flg22, a bacterial microbe-associated molecular pattern (MAMP). flg22-induced resistance in Arabidopsis requires its cognate pattern recognition receptor (PRR) FLS2, and is accompanied by the up-regulation of WRKY29. The expression of WRKY29, which is associated with pattern-triggered immunity (PTI), is also induced in response to F. graminearum infection. Furthermore, WRKY29 is required for basal resistance as well as flg22-induced resistance to F. graminearum. Moreover, constitutive expression of WRKY29 in Arabidopsis enhances disease resistance. The PTI pathway is also activated in response to F. graminearum infection of wheat. Furthermore, flg22 application and ectopic expression of WRKY29 enhance FHB resistance in wheat. Thus, we conclude that the PTI pathway provides a target for the control of FHB in wheat. We further show that the ectopic expression of WRKY29 in wheat results in shorter stature and early heading time, traits that are important to wheat breeding.

QTL Mapping Identifies Novel Source of Resistance to Fusarium Wilt Race 1 in Citrullus amarus

Fusarium wilt race 1, caused by the soilborne fungus Fusarium oxysporum Schlechtend.: Fr. f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans (Fon), is a major disease of watermelon (Citrullus lanatus) in the United States and throughout the world. Although Fusarium wilt race 1 resistance has been incorporated into several watermelon cultivars, identification of additional genetic sources of resistance is crucial if a durable and sustainable level of resistance is to be continued over the years. We conducted a genetic mapping study to identify quantitative trait loci (QTLs) associated with resistance to Fon race 1 in segregating populations (F_2:3 and recombinant inbred lines) of Citrullus amarus (citron melon) derived from the Fon race 1 resistant and susceptible parents USVL246-FR2 and USVL114, respectively. A major QTL (qFon1-9) associated with resistance to Fon race 1 was identified on chromosome 9 of USVL246-FR2. This discovery provides a novel genetic source of resistance to Fusarium wilt race 1 in watermelon and, thus, an additional host-resistance option for watermelon breeders to further the effort to mitigate this serious phytopathogen.

QTLs for Resistance to Fusarium Ear Rot in a Multiparent Advanced Generation Intercross (MAGIC) Maize Population

Alternative approaches to linkage and association mapping using inbred panels may allow further insights into loci involved in resistance to Fusarium ear rot and lead to the discovery of suitable markers for breeding programs. Here, the suitability of a maize multiparent advanced-generation intercross population for detecting quantitative trait loci (QTLs) associated with Fusarium ear rot resistance was evaluated and found to be valuable in uncovering genomic regions containing resistance-associated loci in temperate materials. In total, 13 putative minor QTLs were located over all of the chromosomes, except chromosome 5, and frequencies of favorable alleles for resistance to Fusarium ear rot were, in general, high. These findings corroborated the quantitative characteristic of resistance to Fusarium ear rot in which many loci have small additive effects. Present and previous results indicate that crucial regions such as 210 to 220 Mb in chromosome 3 and 166 to 173 Mb in chromosome 7 (B73-RefGen-v2) contain QTLs for Fusarium ear rot resistance and fumonisin content.

Tomato tolerance and pest control following fumigation with different ratios of dimethyl disulfide and chloropicrin

BACKGROUND

The phaseout of methyl bromide (MeBr) continues to stimulate research into the use of other soil fumigants for controlling soil-borne diseases and weeds. This research evaluated tomato (Solanum lycopersicum L.) tolerance, weed emergence and the recovery of Fusarium oxysporum f.sp. lycopersici (FOL) inoculum following fumigation with various combination ratios of dimethyl disulfide plus chloropicrin (DMDS + Pic).

RESULTS

On its own, DMDS did not effectively control purple nutsedge (Cyperus rotundus L.) compared with DMDS + Pic. Control of C. rotundus and fusarium wilt increased with Pic based on weed emergence throughout the growing season and FOL inoculum recovery from soil. In all three growing seasons, 159 kg ha^-1 DMDS + 379 kg ha^-1 Pic provided season-long control of C. rotundus.

CONCLUSION

This research confirms that formulating DMDS + Pic containing a high percentage of Pic offers an effective alternative to MeBr for tomato production. © 2018 Society of Chemical Industry.

Detection of Fusarium oxysporum f. sp. fragariae from Infected Strawberry Plants

Isolates of the Fusarium oxysporum species complex have been characterized as plant pathogens that commonly cause vascular wilt, stunting, and yellowing of the leaves in a variety of hosts. F. oxysporum species complex isolates have been grouped into formae speciales based on their ability to cause disease on a specific host. F. oxysporum f. sp. fragariae is the causal agent of Fusarium wilt of strawberry and has become a threat to production as fumigation practices have changed in California. F. oxysporum f. sp. fragariae is polyphyletic and limited genetic markers are available for its detection. In this study, next-generation sequencing and comparative genomics were used to identify a unique genetic locus that can detect all of the somatic compatibility groups of F. oxysporum f. sp. fragariae identified in California. This locus was used to develop a TaqMan quantitative polymerase chain reaction assay and an isothermal recombinase polymerase amplification (RPA) assay that have very high sensitivity and specificity for more than 180 different isolates of the pathogen tested. RPA assay results from multiple field samples were validated with pathogenicity tests of recovered isolates.

The Protective Effect of Trichoderma asperellum on Tomato Plants against Fusarium oxysporum and Botrytis cinerea Diseases Involves Inhibition of Reactive Oxygen Species Production

Trichoderma species are fungi widely employed as plant-growth-promoting agents and for biological control. Several commercial and laboratory-made solid formulations for mass production of Trichoderma have been reported. In this study, we evaluated a solid kaolin-based formulation to promote the absortion/retention of Trichoderma asperellum in the substrate for growing tomato plants. The unique implementation of this solid formulation resulted in an increased growth of the tomato plants, both in roots and shoots after 40 days of its application. Plants were challenged with two fungal pathogens, Fusarium oxysporum and Botrytis cinerea, and pretreatment with T. asperellum resulted in less severe wilting and stunting symptoms than non-treated plants. Treatment with T. asperellum formulation inhibited Reactive Oxygen Species (ROS) production in response to the pathogens in comparison to plants that were only challenged with both pathogens. These results suggest that decrease in ROS levels contribute to the protective effects exerted by T. asperellum in tomato.

Investigation and genome-wide association study for Fusarium crown rot resistance in Chinese common wheat

BACKGROUND

Fusarium crown rot (FCR) is a severe and chronic disease in common wheat and is able to cause serious yield loss and health problems to human and livestock.

RESULTS

Here, 234 Chinese wheat cultivars were evaluated in four greenhouse experiments for FCR resistance and genome-wide association studies (GWAS) were performed using the wheat 660 K genotyping assay. The results indicated that most cultivars evaluated showed FCR disease index (DI) of 40-60, while some cultivars showed stably good FCR resistance (DI < 30). GWAS identified 286 SNPs to be significantly associated with FCR resistance, of which 266, 6 and 8 were distributed on chromosomes 6A, 6B and 6D, respectively. The significant SNPs on 6A were located in a 7.0-Mb region containing 51 annotated genes. On the other hand, QTL mapping using a bi-parental population derived from UC1110 and PI610750 detected three QTLs on chromosomes 6A (explaining 7.77-10.17% of phenotypic variation), 2D (7.15-9.29%) and 2A (5.24-6.92%). The 6A QTL in the UC1110/PI610750 population falls into the same chromosomal region as those detected from GWAS, demonstrating its importance in Chinese materials for FCR resistance.

CONCLUSION

This study could provide useful information for utilization of FCR-resistant wheat germplasm and further understanding of molecular and genetics basis of FCR resistance in common wheat.

The Effect of pH on Spore Germination, Growth, and Infection of Strawberry Roots by Fusarium oxysporum f. sp. fragariae, Cause of Fusarium wilt of Strawberry

Previous work has shown that raising the pH of acidic soil to near neutrality can reduce the incidence of Fusarium wilt. The basis for this effect has not been established. The present study assessed effects of pH on spore germination, growth, and infection of strawberry roots by Fusarium oxysporum f. sp. fragariae, the cause of Fusarium wilt of strawberry. There was not a significant effect of pH (5 versus 7) on the rate of spore germination at either 20 or 25°C for any of the three tested isolates (one representative of each clonal lineage of F. oxysporum f. sp. fragariae found in California). Likewise, pH did not have a significant effect on fungal growth at 20°C. At 25°C, two isolates grew faster at pH 7 than at pH 5. Growth of the third isolate was unaffected by the difference in pH. For the strawberry cultivar Albion, the frequency of root infection was significantly higher for plants grown in acidified soil (near pH 5) than for plants grown in soil near neutrality. The higher frequency of root infection in acidified soil was associated with a lower level of microbial activity, as measured by hydrolysis of fluorescein diacetate.

Variation in abundance of predicted resistance genes in the Brassica oleracea pangenome

Brassica oleracea is an important agricultural species encompassing many vegetable crops including cabbage, cauliflower, broccoli and kale; however, it can be susceptible to a variety of fungal diseases such as clubroot, blackleg, leaf spot and downy mildew. Resistance to these diseases is meditated by specific disease resistance genes analogs (RGAs) which are differently distributed across B. oleracea lines. The sequenced reference cultivar does not contain all B. oleracea genes due to gene presence/absence variation between individuals, which makes it necessary to search for RGA candidates in the B. oleracea pangenome. Here we present a comparative analysis of RGA candidates in the pangenome of B. oleracea. We show that the presence of RGA candidates differs between lines and suggests that in B. oleracea, SNPs and presence/absence variation drive RGA diversity using separate mechanisms. We identified 59 RGA candidates linked to Sclerotinia, clubroot, and Fusarium wilt resistance QTL, and these findings have implications for crop breeding in B. oleracea, which may also be applicable in other crops species.

Exploring and exploiting the genetic variation of Fusarium head blight resistance for genomic-assisted breeding in the elite durum wheat gene pool

KEY MESSAGE

Genomic selection had a higher selection response for FHB resistance than phenotypic selection, while association mapping identified major QTL on chromosome 3B unaffected by plant height and flowering date. Fusarium head blight (FHB) is one of the most destructive diseases of durum wheat. Hence, minimizing losses in yield, quality and avoiding contamination with mycotoxins are of pivotal importance, as durum wheat is mostly used for human consumption. While growing resistant varieties is the most promising approach for controlling this fungal disease, FHB resistance breeding in durum wheat is hampered by the limited variation in the elite gene pool and difficulties in efficiently combining the numerous small-effect resistance quantitative trait loci (QTL) in the same line. We evaluated an international collection of 228 genotyped durum wheat cultivars for FHB resistance over 3 years to investigate the genetic architecture and potential of genomic-assisted breeding for FHB resistance in durum wheat. Plant height was strongly positively correlated with FHB resistance and led to co-localization of plant height and resistance QTL. Nevertheless, a major QTL on chromosome 3B independent of plant height was identified in the same chromosomal interval as reported for the prominent hexaploid resistance QTL Fhb1, though haplotype analysis highlighted the distinctiveness of both QTL. Comparison between phenotypic and genomic selection for FHB resistance revealed a superior prediction ability of the former. However, simulated selection experiments resulted in higher selection responses when using genomic breeding values for early generation selection. An earlier identification of the most promising lines and crossing parents was feasible with a genomic selection index, which suggested a much faster short-term population improvement than previously possible in durum wheat, complementing long-term strategies with exotic resistance donors.

Effects of Atmospheric CO2 Level on the Metabolic Response of Resistant and Susceptible Wheat to Fusarium graminearum Infection

Rising atmospheric CO₂ concentrations and associated climate changes are thought to have contributed to the steady increase of Fusarium head blight (FHB) on wheat. However, our understanding of precisely how elevated CO₂ influences the defense response of wheat against Fusarium graminearum remains limited. In this study, we evaluated the metabolic profiles of susceptible (Norm) and moderately resistant (Alsen) spring wheat in response to whole-head inoculation with two deoxynivalenol (DON)-producing F. graminearum isolates (DON⁺), isolates 9F1 and Gz3639, and a DON-deficient (DON^-) isolate (Gzt40) at ambient (400 ppm) and elevated (800 ppm) CO₂ concentrations. The effects of elevated CO₂ were dependent on both the Fusarium strain and the wheat variety, but metabolic differences in the host can explain the observed changes in F. graminearum biomass and DON accumulation. The complexity of abiotic and biotic stress interactions makes it difficult to determine if the observed metabolic changes in wheat are a result of CO₂-induced changes in the host, the pathogen, or a combination of both. However, the effects of elevated CO₂ were not dependent on DON production. Finally, we identified several metabolic biomarkers for wheat that can reliably predict FHB resistance or susceptibility, even as atmospheric CO₂ levels rise.

Accuracy of within- and among-family genomic prediction for Fusarium head blight and Septoria tritici blotch in winter wheat

Genomic selection is an approach that uses whole-genome marker data to predict breeding values of genotypes and holds the potential to improve the genetic gain in breeding programs. In this study, two winter wheat populations (DS1 and DS2) consisting of 438 and 585 lines derived from six and eight bi-parental families, respectively, were genotyped with genome-wide single nucleotide polymorphism markers and phenotyped for Fusarium head blight and Septoria tritici blotch severity, plant height and heading date. We used ridge regression-best linear unbiased prediction to investigate the potential of genomic selection under different selection scenarios: prediction across each winter wheat population, within- and among-family prediction in each population, and prediction from DS1 to DS2 and vice versa. Moreover, we compared a full random model to a model incorporating quantitative trait loci (QTL) as fixed effects. The prediction accuracies obtained by cross-validation within populations were moderate to high for all traits. Accuracies for individual families were in general lower and varied with population size and genetic architecture of the trait. In the among-family prediction scenario, highest accuracies were achieved by predicting from one half-sib family to another, while accuracies were lowest between unrelated families. Our results further demonstrate that the prediction accuracy can be considerably increased by a fixed effect model approach when major QTL are present. Taken together, the implementation of genomic selection for Fusarium head blight and Septoria tritici blotch resistance seems to be promising, but the composition of the training population is of utmost importance.

Genetic analysis of cob resistance to F. verticillioides: another step towards the protection of maize from ear rot

We lay the foundation for further research on maize resistance to Fusarium verticillioides cob rot by identifying a candidate resistance gene. Fusarium verticillioides ear rot is the most common type of maize ear rot in the Huanghuaihai Plain of China. Ear rot resistance includes cob and kernel resistance. Most of the current literature concentrates on kernel resistance, and genetic studies on cob resistance are scarce. We aimed on identifying the QTLs responsible for F. verticillioides cob rot (FCR) resistance. Twenty-eight genes associated with 48 single nucleotide polymorphisms (SNPs) were identified (P < 10^-4) to correlate with FCR resistance using a whole-genome association study. The major quantitative trait locus, qRcfv2, for FCR resistance was identified on chromosome 2 through linkage mapping and was validated in near-isogenic line populations. Two candidate genes associated with two SNPs were detected in the qRcfv2 region with a lower threshold (P < 10^-3). Through real-time fluorescence quantitative PCR, one candidate gene was found to have no expression in the cob but the other was expressed in response to F. verticillioides. These results lay a foundation for research on the resistance mechanisms of cob and provide resources for marker-assisted selection.

Confirmation of Fusarium root rot resistance QTL Fsp-Ps 2.1 of pea under controlled conditions

BACKGROUND

Dry pea production has increased substantially in North America over the last few decades. With this expansion, significant yield losses have been attributed to an escalation in Fusarium root rots in pea fields. Among the most significant rot rotting pathogenic fungal species, Fusarium solani fsp. pisi (Fsp) is one of the main causal agents of root rot of pea. High levels of partial resistance to Fsp has been identified in plant genetic resources. Genetic resistance offers one of the best solutions to control this root rotting fungus. A recombinant inbred population segregating for high levels of partial resistance, previously single nucleotide polymorphism (SNP) genotyped using genotyping-by-sequencing, was phenotyped for disease reaction in replicated and repeated greenhouse trials. Composite interval mapping was deployed to identify resistance-associated quantitative trait loci (QTL).

RESULTS

Three QTL were identified using three disease reaction criteria: root disease severity, ratios of diseased vs. healthy shoot heights and dry plant weights under controlled conditions using pure cultures of Fusarium solani fsp. pisi. One QTL Fsp-Ps 2.1 explains 44.4-53.4% of the variance with a narrow confidence interval of 1.2 cM. The second and third QTL Fsp-Ps3.2 and Fsp-Ps3.3 are closely linked and explain only 3.6-4.6% of the variance. All of the alleles are contributed by the resistant parent PI 180693.

CONCLUSION

With the confirmation of Fsp-Ps 2.1 now in two RIL populations, SNPs associated with this region make a good target for marker-assisted selection in pea breeding programs to obtain high levels of partial resistance to Fusarium root rot caused by Fusarium solani fsp. pisi.

The Auxin-Regulated Protein ZmAuxRP1 Coordinates the Balance between Root Growth and Stalk Rot Disease Resistance in Maize

To optimize fitness, plants must efficiently allocate their resources between growth and defense. Although phytohormone crosstalk has emerged as a major player in balancing growth and defense, the genetic basis by which plants manage this balance remains elusive. We previously identified a quantitative disease-resistance locus, qRfg2, in maize (Zea mays) that protects against the fungal disease Gibberella stalk rot. Here, through map-based cloning, we demonstrate that the causal gene at qRfg2 is ZmAuxRP1, which encodes a plastid stroma-localized auxin-regulated protein. ZmAuxRP1 responded quickly to pathogen challenge with a rapid yet transient reduction in expression that led to arrested root growth but enhanced resistance to Gibberella stalk rot and Fusarium ear rot. ZmAuxRP1 was shown to promote the biosynthesis of indole-3-acetic acid (IAA), while suppressing the formation of benzoxazinoid defense compounds. ZmAuxRP1 presumably acts as a resource regulator modulating indole-3-glycerol phosphate and/or indole flux at the branch point between the IAA and benzoxazinoid biosynthetic pathways. The concerted interplay between IAA and benzoxazinoids can regulate the growth-defense balance in a timely and efficient manner to optimize plant fitness.

Ethylene signaling regulates natural variation in the abundance of antifungal acetylated diferuloylsucroses and Fusarium graminearum resistance in maize seedling roots

The production and regulation of defensive specialized metabolites play a central role in pathogen resistance in maize (Zea mays) and other plants. Therefore, identification of genes involved in plant specialized metabolism can contribute to improved disease resistance. We used comparative metabolomics to identify previously unknown antifungal metabolites in maize seedling roots, and investigated the genetic and physiological mechanisms underlying their natural variation using quantitative trait locus mapping and comparative transcriptomics approaches. Two maize metabolites, smilaside A (3,6-diferuloyl-3',6'-diacetylsucrose) and smiglaside C (3,6-diferuloyl-2',3',6'-triacetylsucrose), were identified that could contribute to maize resistance against Fusarium graminearum and other fungal pathogens. Elevated expression of an ethylene signaling gene, ETHYLENE INSENSITIVE 2 (ZmEIN2), co-segregated with a decreased smilaside A : smiglaside C ratio. Pharmacological and genetic manipulation of ethylene availability and sensitivity in vivo indicated that, whereas ethylene was required for the production of both metabolites, the smilaside A : smiglaside C ratio was negatively regulated by ethylene sensitivity. This ratio, rather than the absolute abundance of these two metabolites, was important for maize seedling root defense against F. graminearum. Ethylene signaling regulates the relative abundance of the two F. graminearum-resistance-related metabolites and affects resistance against F. graminearum in maize seedling roots.

Influence of different biological control agents and compost on total and nitrification-driven microbial communities at rhizosphere and soil level in a lettuce - Fusarium oxysporum f. sp. lactucae pathosystem

AIMS

The response of rhizosphere and bulk soil indigenous microbial communities focusing on nitrifiers was evaluated after the application of different biological control agents (BCAs; Bacillus, Trichoderma, Pseudomonas) and compost in controlling lettuce Fusarium wilt.

METHODS AND RESULTS

Experiments were conducted 'in situ' over two lettuce cropping seasons. Total fungal, bacterial and archaeal populations and the nitrifiers were analysed using quantitative polymerase chain reaction method. The pathogen, Fusarium oxysporum forma specialis lactucae (FOL), Bacillus, Trichoderma and Pseudomonas and three antifungal genes (chiA, 2,4-diacetylphloroglucinol - phlD and HCN synthase - hcnAB genes) were also assessed. Quantitative data were corroborated with disease severity (DS), potential nitrification activity and soil chemical parameters. The application of BCAs and compost resulted in the disease reduction by as much as 69%, confirmed by significant negative correlations between Bacillus subtilis, Trichoderma and Pseudomonas sp. abundances and DS. The FOL presence in the untreated control resulted in the nitrifiers niche differentiation.

CONCLUSIONS

The used treatments were efficient against Fusarium wilt and did not influence negatively the nontarget microbial communities.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of BCAs and compost appears as an effective and safe strategy to implement sustainable agricultural practices.

Potential of Novel Sequence Type of Burkholderia cenocepacia for Biological Control of Root Rot of Maize (Zea mays L.) Caused by Fusarium temperatum

In this study, two Burkholderia strains, strain KNU17BI2 and strain KNU17BI3, were isolated from maize rhizospheric soil, South Korea. The 16S rRNA gene and multilocus sequence analysis and typing (MLSA-MLST) were used for the identification of the studied strains. Strain KNU17BI2, which belonged to Burkholderia cenocepacia, was of a novel sequence type (ST) designated ST-1538, while strain KNU17BI3 had a similar allelic profile with the seven loci of Burkholderia contaminans strain LMG 23361. The strains were evaluated in vitro for their specific plant growth promoting (PGP) traits, such as zinc solubilization, phosphate solubilization, ammonia production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, indole acetic acid (IAA) production, siderophore, and hydrolytic enzyme activity. Interestingly, the strains exhibited a positive effect on all of the tested parameters. The strains also showed broad-spectrum antifungal activity against economically important phytopathogens in the dual culture assay. Furthermore, the strains were evaluated under greenhouse conditions for their in vivo effect to promote plant growth and to suppress the root rot of maize that is caused by Fusarium temperatum on four Korean maize cultivars. The results of the greenhouse study revealed that both of the strains were promising to significantly suppress fusarium root rot and enhance plant growth promotion on the four maize cultivars. This study, for the first time, reported in vitro antifungal potential of B. cenocepacia of novel ST against economically important plant pathogens viz., F. temperatum, Fusarium graminearum, Fusarium moniliforme, Fusarium oxysporum f.sp. melonis, Fusarium subglutinans, Phytophthora drechsleri, and Stemphylium lycopersici. This is also the first report of zinc solubilization by B. cenocepacia. Moreover, the present research work reports, for the first time, about the potential of B. cenocepacia and B. contaminans to control the root rot of maize that is caused by F. temperatum. Therefore, we recommend further studies to precisely identify the bioactive chemical compounds behind such activities that would be novel sources of natural products for biological control and plant growth promotion of different crops.

Genome wide association study identifies novel single nucleotide polymorphic loci and candidate genes involved in soybean sudden death syndrome resistance

Fusarium virguliforme is a soil borne root pathogen that causes sudden death syndrome (SDS) in soybean [Glycine max (L.) Merrill]. Once the fungus invades the root xylem tissues, the pathogen secretes toxins that cause chlorosis and necrosis in foliar tissues leading to defoliation, flower and pod drop and eventually death of plants. Resistance to F. virguliforme in soybean is partial and governed by over 80 quantitative trait loci (QTL). We have conducted genome-wide association study (GWAS) for a group of 254 plant introductions lines using a panel of approximately 30,000 SNPs and identified 19 single nucleotide polymorphic loci (SNPL) that are associated with 14 genomic regions encoding foliar SDS and eight SNPL associated with seven genomic regions for root rot resistance. Of the identified 27 SNPL, six SNPL for foliar SDS resistance and two SNPL for root rot resistance co-mapped to previously identified QTL for SDS resistance. This study identified 13 SNPL associated with eight novel genomic regions containing foliar SDS resistance genes and six SNPL with five novel regions for root-rot resistance. This study identified five genes carrying nonsynonymous mutations: (i) three of which mapped to previously identified QTL for foliar SDS resistance and (ii) two mapped to two novel regions containing root rot resistance genes. Of the three genes mapped to QTL for foliar SDS resistance genes, two encode LRR-receptors and third one encodes a novel protein with unknown function. Of the two genes governing root rot resistance, Glyma.01g222900.1 encodes a soybean-specific LEA protein and Glyma.10g058700.1 encodes a heparan-alpha-glucosaminide N-acetyltransferase. In the LEA protein, a conserved serine residue was substituted with asparagine; and in the heparan-alpha-glucosaminide N-acetyltransferase, a conserved histidine residue was substituted with an arginine residue. Such changes are expected to alter functions of these two proteins regulated through phosphorylation. The five genes with nonsynonymous mutations could be considered candidate SDS resistance genes and should be suitable molecular markers for breeding SDS resistance in soybean. The study also reports desirable plant introduction lines and novel genomic regions for enhancing SDS resistance in soybean.

Molecular characterization of carbendazim resistance of Fusarium species complex that causes sugarcane pokkah boeng disease

BACKGROUND

Pokkah boeng is one of the most serious and devastating diseases of sugarcane and causes significant loss in cane yield and sugar content. Although carbendazim is widely used to prevent fungal diseases, the molecular basis of Fusarium species complex (FSC) resistance to carbendazim remains unknown.

RESULTS

The EC50 (fungicide concentration that inhibits 50% of mycelial growth) values of carbendazim for 35 FSC isolates collected in cane growing regions of China were ranged from 0.5097 to 0.6941 μg mL- 1 of active ingredient (a.i.), in an average of 0.5957 μg a.i. mL- 1. Among carbendazim-induced mutant strains, SJ51M (F. verticillioides) had a CTG rather than CAG codon (Q134L) at position 134 of the FVER_09254 gene, whereas in the mutant strain HC30M (F. proliferatum) codon ACA at position 351 of the FPRO_07779 gene was replaced by ATA (T351I). Gene expression profiling analysis was performed for SJ51M and its corresponding wild type strain SJ51, with and without carbendazim treatment. The gene expression patterns in SJ51 and SJ51M changed greatly as evidenced by the detection of 850 differentially expressed genes (DEGs). Functional categorization indicated that genes associated with oxidation-reduction process, ATP binding, integral component of membrane, transmembrane transport and response to stress showed the largest expression changes between SJ51M and SJ51. The expression levels of many genes involved in fungicide resistance, such as detoxification enzymes, drug efflux transporters and response to stress, were up-regulated in SJ51M compared to SJ51 with and without carbendazim treatment.

CONCLUSION

FSC was sensitive to carbendazim and had the potential for rapid development of carbendazim resistance. The transcriptome data provided insight into the molecular pathways involved in FSC carbendazim resistance.

Fusarium oxysporum f. sp. lycopersici C2H2 transcription factor FolCzf1 is required for conidiation, fusaric acid production, and early host infection

The soil-borne, asexual fungus Fusarium oxysporum f.sp. lycopersici (Fol) is a causal agent of tomato wilt disease. The infection process of Fol comprises root recognition, adhesion, penetration, colonization of the root cortex and hyphal proliferation within the xylem vessels, which are under the regulation of virulence-involved transcription factors (TFs). In this study, we identified a gene, designated FolCZF1, which encodes a C₂H₂ TF in Fol. The homologs of FolCzf1 are also known to affect pathogenicity in F. graminearum and Magnaporthe oryzae on wheat and rice, respectively. We learned that FolCZF1 transcript level is upregulated in conidia and early host infection stage, which led us to hypothesize that FolCzf1 is associated with early host infection in Fol. The FolCZF1 deletion mutant (ΔFolCZF1) exhibited defects in growth rate, conidiation, conidia morphology and a complete loss of virulence on tomato root. Further microscopic observation showed that ΔFolCZF1 can penetrate the root but the primary infection hypha cannot extend its colonization inside the host tissue, suggesting that FolCzf1 TF plays an important role in early infection. Fusaric acid, a secondary metabolite produced by Fusarium species, is suggested as a virulence factor in many crop diseases. We found that FolCzf1 plays a critical role in fusaric acid production by regulating the expression of fusaric acid biosynthesis genes. In summary, FolCzf1 is required for conidiation, secondary metabolism, and early host infection in Fol, and we propose that homologs of FolCzf1 are required for early parasitic growth in other plant pathogenic filamentous fungi.

Effects of validamycin in controlling Fusarium head blight caused by Fusarium graminearum: Inhibition of DON biosynthesis and induction of host resistance

Validamycin, known to interfere with fungal energy metabolism by inhibiting trehalase, has been extensively used to control plant diseases caused by Rhizoctonia spp. However, the effect of validamycin on controlling Fusarium graminearum has not been previously reported. In this study, when applied to F. graminearum in vitro, validamycin inhibited the synthesis of deoxynivalenol (DON), which is a mycotoxin and virulence factor, by decreasing trehalase activity and the production of glucose and pyruvate, which are precursors of DON biosynthesis. Because FgNTH encodes the main trehalase in F. graminearum, these effects were nullified in the FgNTH deletion mutant ΔFgNTH but restored in the complemented strain ΔFgNTHC. In addition, validamycin also increased the expression of pathogenesis-related genes (PRs) PR1, PR2, and PR5 in wheat, inducing resistance responses of wheat against F. graminearum. Therefore, validamycin exhibits dual efficacies on controlling Fusarium head blight (FHB) caused by F. graminearum: inhibition of DON biosynthesis and induction of host resistance. In addition, field trials further confirmed that validamycin increased FHB control and reduced DON contamination in grain. Control of FHB and DON contamination by validamycin increased when the antibiotic was applied with the triazole fungicide metconazole. Overall, this study is a successful case from foundational research to applied research, providing useful information for wheat protection programs against toxigenic fungi responsible for FHB and the consequent mycotoxin accumulation in grains.

Effect of Wheat Cultivar on the Concentration of Fusarium Mycotoxins in Wheat Stems

Effective control of Fusarium-mycotoxin accumulation in grain affected by Fusarium head blight (FHB) (caused by Fusarium graminearum) begins with selecting moderately resistant wheat cultivars; however, little is known about how this resistance affects mycotoxin levels in the stem. A study was conducted from 2011 to 2014 in a mist-irrigated FHB nursery in Urbana, IL to determine whether the FHB resistance class of a cultivar (very susceptible, susceptible, moderately susceptible, and moderately resistant) affects the concentration of Fusarium mycotoxins in the stem. FHB incidence, FHB severity, and Fusarium-damaged kernel ratings were collected and used to calculate FHB index; incidence, severity, and kernel damage (ISK) index; and deoxynivalenol (DON), incidence, severity, and kernel damage (DISK) index. Grain was assayed for levels of DON, and the bottom 25 cm of plant stems was collected from each plot and assayed for DON, 3-acetyl-deoxynivalenol (3ADON), and 15-acetyl-deoxynivalenol (15ADON). Significant differences in DON concentration in the grain were detected among cultivars (P = 0.0001) and for the concentration of all DON (P = 0.003), 3ADON (P = 0.03), and 15ADON (P < 0.0001) in the stem. Significant differences among resistance classes were observed for FHB index value (P < 0.0001), ISK index (P = 0.006), and DISK index (P = 0.004). In all years of this study, the concentration of DON in the grain and the concentrations of all mycotoxins in the stem were consistently lower in the moderately resistant cultivars. All three indices were poor indicators of mycotoxin concentrations in the stem. Overall, the selection of a moderately resistant cultivar provides effective control of DON accumulation in the grain and mycotoxin accumulation in the stem.

Effects of the dinitroaniline fungicide fluazinam on Fusarium fujikuroi and rice

Fusarium fujikuroi is the primary causal agent of rice bakanae disease. Fluazinam is a protective dinitroaniline fungicide which could interrupt the fungal cell's energy production. Little is known about the effects of fluazinam on F. fujikuroi. In this study, baseline sensitivity of F. fujikuroi to fluazinam was determined using 103 isolates collected from diseased young rice of different fields in Shaoxing of Zhejiang Province and Huaian of Jiangsu Province of China in 2016. The EC₅₀ values of fluazinam on inhibiting mycelial growth against 103 isolates of F. fujikuroi ranged from 0.0621 to 0.5446 μg/mL with the average value of 0.2038 ± 0.0099 μg/mL (mean ± standard error). The EC₅₀ values of fluazinam on suppressing conidium germination against 103 isolates of F. fujikuroi ranged from 0.1006 to 0.9763 μg/mL with the mean value of 0.3552 ± 0.0181 μg/mL. Treated with fluazinam, hyphae of F. fujikuroi were contorted, offshoot of top mycelia increased, conidial production descreased significantly and exopolysaccharide (EPS) content did not change significantly while peroxidase (POD) activity significantly decreased. Meanwhile, cell membrane permeability increased after treated with fluazinam. The analysis of cell ultrastructure indicated that fluazinam could damage the membrane structure of F. fujikuroi and cause a large number of vacuoles formed. In addition, fluazinam did not affect germination rate, plant height and fresh weight of rice, which indicated that fluazinam was safe to rice. All the results indicated that fluazinam had strong antifungal activity against F. fujikuroi and a potential application in controlling rice bakanae disease. These results will provide useful information for management of rice bakanae disease caused by F. fujikuroi and further increase our understanding about the mode of action of fluazinam against F. fujikuroi and other phytopathogens.

Epigenetic and transcriptional control of chickpea WRKY40 promoter activity under Fusarium stress and its heterologous expression in Arabidopsis leads to enhanced resistance against bacterial pathogen

Promoters of many defense related genes are enriched with W-box elements serving as binding sites for plant specific WRKY transcription factors. In this study, expression of WRKY40 transcription factor was analyzed in two contrasting susceptible (JG62) and resistant (WR315) genotypes of chickpea infected with Foc1. The resistant plants showed up-regulation of WRKY40 under Fusarium stress, whereas in susceptible plants WRKY40 expression was absent. Additionally, global changes in the histone modification patterns were studied in above two chickpea genotypes by immunoblotting and real-time PCR analyses under control and Fusarium infected conditions. Notably, region specific Histone 3 lysine 9 acetylation, a positive marker of transcription gets enriched at WRKY40 promoter during resistant interaction with Foc1. H3K9 Ac is less enriched at WRKY40 promoter in Foc1 infected susceptible plants. WRKY40 promoter activity was induced by jasmonic acid and pathogen treatment, while salicylic acid failed to stimulate such activity. Moreover, WRKY40 was found to bind to its own promoter and auto-regulates its activity. The present study also showed that heterologous over-expression of chickpea WRKY40 triggers defense response in Arabidopsis against Pseudomonas syringae. Overall, we present epigenetic and transcriptional control of WRKY40 in chickpea under Fusarium stress and its immunomodulatory role is tested in Arabidopsis.

Physical interaction between nuclear accumulated CC-NB-ARC-LRR protein and WRKY64 promotes EDS1 dependent Fusarium wilt resistance in chickpea

Fusarium wilt is one of the most serious diseases affecting chickpea (Cicer arietinum L.). Here, we identified a putative Resistance Gene Analog (CaRGA) from chickpea, encoding a coiled-coil (CC) nucleotide-binding oligomerization domain (NB-ARC) containing leucine-rich repeat (LRR) protein (CC-NLR protein) that confers resistance against Fusarium oxysporum f. sp. ciceri race1 (Foc1). Over-expression and silencing of CaRGA in chickpea resulted in enhanced resistance and hyper-susceptibility, respectively against Foc1. Furthermore, defense response to Foc1 depends on CC-NLR interaction with WRKY64 transcription factor. CaRGA mediated wilt resistance largely compromised when WRKY64 was silenced. We also determined in planta intramolecular interactions and self-association of chickpea CC-NLR protein. The study shows CC domain suppressing auto-activation of the full-length CC-NLR protein in the absence of pathogen through self-inhibitory intramolecular interaction with NB-ARC domain, which is attenuated by self-interactions to LRR domain. Chickpea CC-NLR protein forms homocomplexes and then interacts with WRKY64. CC-NLR protein further phosphorylates WRKY64 thereby, ubiquitination and proteasome mediated degradation are protected. Phosphorylated WRKY64 with increased stability binds to EDS1 promoter and stimulates its transcription that induces in planta ectopic cell-death. The detailed analysis of CC-NLR and WRKY interactions provide a better understanding of the immune regulation by NLR proteins under biotic stresses.

The saprophytic fungus Fusarium solani increases the insecticidal efficacy of the entomopathogenic nematode Steinernema diaprepesi

In two field surveys, high proportions of Galleria mellonella L. (Lepidoptera: Pyralidae) sentinel larval cadavers were infected by Fusarium solani without evidence of concomitant entomopathogenic nematode (EPN) or entomopathogenic fungus (EPF) reproduction. Because F. solani is not considered entomopathogenic, the survey suggested the possibility that F. solani competes with EPNs. We tested the hypotheses that F. solani attracts the EPN, Steinernema diaprepesi, to facilitate infection of Diaprepes root weevils (Diaprepes abbreviatus L.) and thereafter competes with the nematode in the insect cadaver. In two-choice olfactometer assays where one side was treated with F. solani mycelia and conidia, juvenile S. diaprepesi were attracted to the fungus, in either raw soil, or in autoclaved soil in the presence or absence of insects. However, this attraction was attenuated as the habitat became more complex, by using raw soil in combination with insect larvae. Fusarium oxysporum did not recruit the nematode. When soil microcosms were tested with F. solani conidia and S. diaprepesi, the concomitant infection increased the mortality of the insect (P = 0.02) to 83%, compared to 58% and 0% mortality when nematodes or fungi were individually applied, respectively. Concomitant inoculation also increased the number of cadavers that supported nematode reproduction and increased the population density of fungus in soil. The number of IJs entering the host insect was not affected by F. solani. These results support the possibility that F. solani can facilitate the insecticidal efficiency of S. diaprepesi in order to exploit the resources in the cadaver.

Root Exudates of Stressed Plants Stimulate and Attract Trichoderma Soil Fungi

Plant roots release complex mixtures of bioactive molecules, including compounds that affect the activity and modify the composition of the rhizosphere microbiome. In this work, we investigated the initial phase of the interaction between tomato and an effective biocontrol strain of Trichoderma harzianum (T22). We found that root exudates (RE), obtained from plants grown in a split-root system and exposed to various biotic and abiotic stress factors (wounding, salt, pathogen attack), were able to stimulate the growth and act as chemoattractants of the biocontrol fungus. On the other hand, some of the treatments did not result in an enhanced chemotropism on Fusarium oxysporum f. sp. lycopersici, indicating a mechanism that may be selective for nonpathogenic microbes. The involvement of peroxidases and oxylipins, both known to be released by roots in response to stress, was demonstrated by using RE fractions containing these molecules or their commercial purified analogs, testing the effect of an inhibitor, and characterizing the complex pattern of these metabolites released by tomato roots both locally and systemically.