Morphological and molecular characterization of Fusarium verticillioides from rotten banana imported into Japan

Fusarium mindanaoense sp. nov., a New Fusarium Wilt Pathogen of Cavendish Banana from the Philippines Belonging to the F. fujikuroi Species Complex

The pathogen causing Fusarium wilt in banana is reported to be Fusarium oxysporum f. sp. cubense (FOC). In 2019, wilt symptoms in banana plants (cultivar: Cavendish) in the Philippines were detected, i.e., the yellowing of the leaves and discoloration of the pseudostem and vascular tissue. The fungus isolated from the vascular tissue was found to be pathogenic to Cavendish bananas and was identified as a new species, F. mindanaoense, belonging to the F. fujikuroi species complex (FFSC); species classification was assessed using molecular phylogenetic analyses based on the tef1, tub2, cmdA, rpb1, and rpb2 genes and morphological analyses. A reciprocal blast search using genomic data revealed that this fungus exclusively included the Secreted in Xylem 6 (SIX6) gene among the SIX homologs related to pathogenicity; it exhibited a highly conserved amino acid sequence compared with that of species in the FFSC, but not with that of FOC. This was the first report of Fusarium wilt in Cavendish bananas caused by a species of the genus Fusarium other than those in the F. oxysporum species complex.

Fusarium verticillioides of maize plant: Potentials of propitious phytomicrobiome as biocontrol agents

Disease outbreaks have been recorded due to exposure to Fusarium verticillioides and fumonisin, a mycotoxin produced by this fungus. F. verticillioides is a fungal pathogen of maize that causes infections, such as wilting and rotting, while contact with its fumonisin derivative manifests in the form of mild to severe illnesses in humans and animals. Maize infection by F. verticillioides causes loss or reduction in expected crop yield, thereby influencing households and nations' economies. While several efforts have been made to control the pathogenic fungus and its occurrence in the environment, it remains a challenge in agriculture, particularly in maize production. Several microorganisms which are plant-associated, especially those associated with the rhizosphere niche have been noted to possess antagonistic effects against F. verticillioides. They can inhibit the pathogen and tackle its debilitating effects on plants. Hence this study reviews the use of rhizosphere-associated biocontrol agents, such as Bacillus spp., Pseudomonas, Enterobacter, and Microbacterium oleivorans which forms part of the phytomicrobiome in other to prevent and control this toxicogenic fungus. These microorganisms were found to not only be effective in controlling its occurrence on maize plants but are environmentally safe and promote crop yield.

Fusarium musae from Diseased Bananas and Human Patients: Susceptibility to Fungicides Used in Clinical and Agricultural Settings

Fusarium musae belongs to the Fusarium fujikuroi species complex. It causes crown rot disease in banana but also keratitis and skin infections as well as systemic infections in immunocompromised patients. Antifungal treatments in clinical and agricultural settings rely mostly on molecules belonging to the azole class. Given the potential risk of pathogen spread from food to clinical settings, the goal of the work was to define the level of susceptibility to different azoles of a worldwide population of F. musae. Eight fungicides used in agriculture and five antifungals used in clinical settings (4 azoles and amphotericin B) were tested using the CLSI (Clinical and Laboratory Standards Institute) protocol methodology on 19 F. musae strains collected from both infected patients and bananas. The level of susceptibility to the different active molecules was not dependent on the source of isolation with the exception of fenbuconazole and difenoconazole which had a higher efficiency on banana-isolated strains. Minimal inhibitory concentrations (MICs) of the different molecules ranged from 0.12–0.25 mg/L for prochloraz to more than 16 mg/L for tetraconazole and fenbuconazole. Compared to the F. verticillioides, F. musae MICs were higher suggesting the importance of monitoring the potential future spread of this species also in clinical settings.

Fusarium musae infected banana fruits as potential source of human fusariosis: May occur more frequently than we might think and hypotheses about infection

The banana fruit infecting fungus Fusarium musae was originally known as a distinct population within Fusarium verticillioides. However, recently, Fusarium musae was installed as a separate species and the first cases of human infection associated with Fusarium musae were found. In this article, we report an additional survey indicating that human pathogenic Fusarium musae infections may occur more frequently than we might think. Moreover, we evaluate the hypotheses on how infection can be acquired. A first hypothesis is that banana fruits act as carriers of Fusarium musae spores and thereby be the source of human infection with Fusarium musae. Acquisition is likely to be caused through contact with Fusarium musae contaminated banana fruits, either being imported or after traveling of the patient to a banana-producing country. An alternative hypothesis is that Fusarium musae is not only present on banana fruits, but also on other plant hosts or environmental sources.

Toxigenic profiles and trinucleotide repeat diversity of Fusarium species isolated from banana fruits

Infesting Fusarium species isolated from banana fruit samples were identified and quantified by morphological, mycotoxicological and molecular tools. A total of 19 Fusarium isolates were obtained: F. semitectum was most predominant (26%), followed by F. proliferatum (16%), F. circinatum (16%), F. chlamydosporum (10.5%), F. solani (10.5%), F. oxysporum (10.5%) and F. thapsinum (5%). Fumonisin B1, deoxynivalenol and zearalenone contents were assayed by high-performance liquid chromatography (HPLC). Seventeen isolates, belonging to F. chlamydosporum, F. circinatum, F. semitectum, F. solani, F. thapsinum, F. proliferatum and Fusarium spp., produced mycotoxins when cultured on rice medium. Fumonisin was produced by all of the studied Fusarium isolates, except F. oxysporum, at a concentration of over 1 μg/mL. F. citrinium isolates 4 and 5 and F. solani isolate 3 were the most potent producers of deoxynivalenol. We compared the 19 Fusarium isolates based on the bands amplified by 10 microsatellite primers. Of these, seven primers, (TCC)5, (TGG)5, (GTA)5, (ATG)5, (TAC)5, (TGC)5 and (TGT)5, yielded a high number of bands and different mean number of alleles. The similarity level between isolates was calculated using a simple matching coefficient. Dendrograms were constructed by the unweighted pair-group method with arithmetical averages (UPGMA). Two main clusters were observed. The interspecific genetic similarity between Fusarium spp. isolates was between 40% and 58% and the intraspecific similarity from 58% to 100%, indicating a high degree of genetic diversity in the tested isolates. Some unexpected genetic similarities were observed among the isolates, indicating non-agreement between morphological and molecular identification of the isolates.

Banana infecting fungus, Fusarium musae, is also an opportunistic human pathogen: are bananas potential carriers and source of fusariosis?

During re-identification of Fusarium strains in the BCCM™/IHEM fungal collection by multilocus sequence-analysis we observed that five strains, previously identified as Fusarium verticillioides, were Fusarium musae, a species described in 2011 from banana fruits. Four strains were isolated from blood samples or biopsies of immune-suppressed patients and one was isolated from the clinical environment, all originating from different hospitals in Belgium or France, 2001-2008. The F. musae identity of our isolates was confirmed by phylogenetic analysis using reference sequences of type material. Absence of the gene cluster necessary for fumonisin biosynthesis, characteristic to F. musae, was also the case for our isolates. In vitro antifungal susceptibility testing revealed no important differences in their susceptibility compared to clinical F. verticillioides strains and terbinafine was the most effective drug. Additional clinical F. musae strains were searched by performing BLAST queries in GenBank. Eight strains were found, of which six were keratitis cases from the U.S. multistate contact lens-associated outbreak in 2005 and 2006. The two other strains were also from the U.S., causing either a skin infection or sinusitis. This report is the first to describe F. musae as causative agent of superficial and opportunistic, disseminated infections in humans. Imported bananas might act as carriers of F. musae spores and be a potential source of infection with F. musae in humans. An alternative hypothesis is that the natural distribution of F. musae is geographically a lot broader than originally suspected and F. musae is present on different plant hosts.

Characterization of Fusarium secorum, a new species causing Fusarium yellowing decline of sugar beet in north central USA

This study characterized a novel sugar beet (Beta vulgaris L.) pathogen from the Red River Valley in north central USA, which was formally named Fusarium secorum. Molecular phylogenetic analyses of three loci (translation elongation factor1α, calmodulin, mitochondrial small subunit) and phenotypic data strongly supported the inclusion of F. secorum in the Fusarium fujikuroi species complex (FFSC). Phylogenetic analyses identified F. secorum as a sister taxon of F. acutatum and a member of the African subclade of the FFSC. Fusarium secorum produced circinate hyphae sometimes bearing microconidia and abundant corkscrew-shaped hyphae in culture. To assess mycotoxin production potential, 45 typical secondary metabolites were tested in F. secorum rice cultures, but only beauvericin was produced in detectable amounts by each isolate. Results of pathogenicity experiments revealed that F. secorum isolates are able to induce half- and full-leaf yellowing foliar symptoms and vascular necrosis in roots and petioles of sugar beet. Inoculation with F. acutatum did not result in any disease symptoms. The sugar beet disease caused by F. secorum is named Fusarium yellowing decline. Since Fusarium yellowing decline incidence has been increasing in the Red River Valley, disease management options are discussed.

Gibberella musae (Fusarium musae) sp. nov., a recently discovered species from banana is sister to F. verticillioides

Several strains of Fusarium isolated from banana were identified previously as F. verticillioides (Sacc.) Nirenberg but described as unable to produce fumonisin. Here we report biochemical and morphological evidence, as well as multilocus phylogenetic analyses based on elongation factor (EF-1α), calmodulin, β-tubulin, and the second largest subunit of RNA polymerase II (RPB2) sequences, indicating that these isolates represent a unique lineage in the Gibberella fujikuroi species complex related to but distinct from F. verticillioides. Together with previous results of molecular studies, as well as with results of metabolite analyses, crossing experiments, pathogenicity tests and morphological characterization, these new data indicate that these strains isolated from banana represent a new species, Gibberella musae Van Hove et al. sp. nov. (anamorph: Fusarium musae Van Hove et al. sp. nov.), which is described herein.

Transformation-mediated complementation of a FUM gene cluster deletion in Fusarium verticillioides restores both fumonisin production and pathogenicity on maize seedlings

The filamentous ascomycete Fusarium verticillioides is a pathogen of maize and produces the fumonisin mycotoxins. However, a distinct population of F. verticillioides is pathogenic on banana and does not produce fumonisins. Fumonisin-producing strains from maize cause leaf lesions, developmental abnormalities, stunting, and sometimes death of maize seedlings, whereas fumonisin-nonproducing banana strains do not. A Southern analysis of banana strains did not detect genes in the fumonisin biosynthetic gene (FUM) cluster but did detect genes flanking the cluster. Nucleotide sequence analysis of the genomic region carrying the flanking genes revealed that the FUM cluster was absent in banana strains except for portions of FUM21 and FUM19, which are the terminal genes at each end of the cluster. Polymerase chain reaction analysis confirmed the absence of the cluster in all banana strains examined. Cotransformation of a banana strain with two overlapping cosmids, which together contain the entire FUM cluster, yielded fumonisin-producing transformants that were pathogenic on maize seedlings. Conversely, maize strains that possess the FUM cluster but do not produce fumonisins because of mutations in FUM1, a polyketide synthase gene, were not pathogenic on maize seedlings. Together, the data indicate that fumonisin production may have been lost by deletion of the FUM cluster in the banana population of F. verticillioides but that fumonisin production could be restored by molecular genetic complementation. The results also indicate that fumonisin production by F. verticillioides is required for development of foliar disease symptoms on maize seedlings.

Fumonisin disruption of ceramide biosynthesis in maize roots and the effects on plant development and Fusarium verticillioides-induced seedling disease

The fungus Fusarium verticillioides infects maize and produces fumonisins, inhibitors of ceramide synthase. Seeds of the cultivar Silver Queen were inoculated with fumonisin-producing or non-fumonisin-producing strains of F. verticillioides. Leaf lesion incidence and severity of effects on root and stalk growth were significantly correlated with fumonisin in roots and disruption of sphingolipid metabolism in roots. Uninoculated seeds grown in soil watered with solutions of fumonisin B1 exhibited above-ground symptoms indicative of F. verticillioides-induced seedling disease and dose-dependent reduction in root mass that was inversely correlated with fumonisin B1, sphingoid bases, and sphingoid base 1-phosphates in roots. There was also evidence of an adaptive response to disrupted sphingolipid metabolism in both the virulence and watering assays, suggesting induction of pathways responsible for metabolism of sphingoid base 1-phosphates after prolonged exposure. The results suggest that fumonisin, and its effects on sphingolipids, could contribute to all aspects of F. verticillioides maize seedling disease.

Natural variation of ascospore and conidial germination by Fusarium verticillioides and other Fusarium species

Fusarium verticillioides and other Fusarium species were examined for their spore germination phenotypes. In general, germinating spores of F. verticillioides formed germ tubes that immediately penetrated into agar. Such invasive germination was the predominant growth phenotype among 22 examined field isolates of F. verticillioides from a broad range hosts and locations. However, two of the field isolates were unique in that they formed conidial germ tubes and hyphae that grew along the surface of agar before penetration eventually occurred. Conidia of 22 other Fusarium species were assessed for their germination phenotypes, and only some strains of F. annulatum, F. fujikuroi, F. globosum, F. nygamai, and F. pseudoanthophilum had the surface germination phenotype (21% of the strains assessed). Sexual crosses and segregation analyses involving one of the F. verticillioides surface germination strains, NRRL 25059, indicated a single locus, designated SIG1 (surface vs. invasive germination), controlled the germ tube growth phenotypes exhibited by both conidia and ascospores. Perfect correlation was observed between an ascospore germination phenotype and the germination phenotype of the conidia produced from the resulting ascospore-derived colony. Recombination data suggested SIG1 was linked ( approximately 7% recombination frequency) to FPH1, a recently described locus necessary for enteroblastic conidiogenesis. Corn seedling blight assays indicated surface germinating strains of F. verticillioides were less virulent than invasively germinating strains. Assays also indicated pathogenicity segregated independently of the FPH1 locus. Invasive germination is proposed as the dominant form of spore germination among Fusarium species. Furthermore, conidia were not necessary for corn seedling disease development, but invasive germination may have enhanced the virulence of conidiating strains.