Tropical Race Research Articles

NOVEL SPECIFIC PRIMERS FOR THE SPECIFIC DETECTION OF FUSARIUM OXYSPORUM F. SP. CUBENSE BASED ON SYBR GREEN REAL-TIME PCR

Fusarium oxysporum f. sp. cubense (Foc) is the causal agent of Fusarium wilt of banana worldwide. Foc is transmitted to another banana plantation via an infected banana sucker. A specific and sensitive detection assay could be used for disease-free propagule screening to prevent disease dispersal effectively. In this study, SYBR green-based real-time PCR assay was developed based on novel specific primers targeting the large subunit of RNA polymerase II (RPB1) gene. The partial RPB1 gene was amplified and sequenced from Foc isolate FOC1708 and SK3-2. Regions specific for Foc were identified and used for designing real-time PCR primers. The specificity of the designed primers was evaluated on genomic DNA from Foc isolates and non-target microorganisms. The results showed that the designed primers were specific to only Foc isolates from race 1 and tropical race 4 (TR4). The detection efficiency of the designed primers was compared to other published primers through optimized SYBR green-based real-time PCR assay and nested PCR. The new primers could detect the Foc genomic DNA at a minimum of 5 pg and target pathogen in a symptomless banana sucker. The specificity and sensitivity of the new primers were comparable to the published real-time PCR primers and the nested PCR assay. This developed assay with these novel primers can aid the disease quarantine for effective prevention and control of the Fusarium wilt of banana.

A Polyphasic Approach Reveals Novel Genotypes and Updates the Genetic Structure of the Banana Fusarium Wilt Pathogen.

Fusarium oxysporum f. sp. cubense (Foc) is a soil-borne fungus that causes Fusarium wilt, a destructive plant disease that has resulted in devastating economic losses to banana production worldwide. The fungus has a complex evolutionary history and taxonomic repute and consists of three pathogenic races and at least 24 vegetative compatibility groups (VCGs). Surveys conducted in Asia, Africa, the Sultanate of Oman and Mauritius encountered isolates of F. oxysporum pathogenic to banana that were not compatible to any of the known Foc VCGs. Genetic relatedness between the undescribed and known Foc VCGs were determined using a multi-gene phylogeny and diversity array technology (DArT) sequencing. The presence of putative effector genes, the secreted in xylem (SIX) genes, were also determined. Fourteen novel Foc VCGs and 17 single-member VCGs were identified. The multi-gene tree was congruent with the DArT-seq phylogeny and divided the novel VCGs into three clades. Clustering analysis of the DArT-seq data supported the separation of Foc isolates into eight distinct clusters, with the suite of SIX genes mostly conserved within these clusters. Results from this study indicates that Foc is more diverse than hitherto assumed.

Isolation and Molecular Identification of a Siderophore Producing Bacterium and its Antagonistic Effect against Fusarium oxysporum f. sp. cubense Tropical Race 4

Bananas are one of the world’s most consumed fruits. Developing countries in the Global South depend on bananas for food security and livelihoods. Still, the banana industry also drives a multinational trade worth billions of US dollars. In addition, banana plants also hold cultural and religious significances in many Asian countries. However, banana production faces several challenges, and one of the major issues is the Fusarium wilt disease caused by the fungus Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4). This disease threatens numerous banana cultivars, including the Cavendish, the most traded banana cultivar. Therefore, the objective of this study was to find effective measures to control the spread of this disease through antagonistic soil bacteria. This study isolated 14 fluorescent, siderophore-producing bacteria with in vitro inhibition rates of 21.73-50.38% against Foc TR4 from the soil surrounding banana plants. Most of the isolates were identified as Pseudomonas spp. via 16S rRNA sequencing, and phylogenetic analysis based on 16s rRNA sequences showed clustering of isolates congruent with the results of similarity searches. Three isolates that exhibited relatively higher antagonistic activity against Fusarium oxysporum f. sp. cubense compared to other isolates were JBAA132 (50.38%), K2B131 (46.28%), and KTP231 (45.38%). Isolate JBAA132 displayed differences in biochemical characteristics compared to its closest match (Pseudomonas aeruginosa type strain DSM50071). It also demonstrated nitrogen-fixing and phosphate solubilising properties common to plant growth promoters. Therefore, isolate JBAA132 may be further explored as a potential biocontrol agent in curbing the spread of Foc TR4.

Polymeric diallyl quaternary ammonium salts for inhibiting banana Fusarium wilt

Banana Fusarium wilt is a typical soil-borne pathogen disease caused by Fusarium oxysporum f. sp. cubense (Foc) and is difficult to control because Foc conidia can survive for many years in soil. Thus, the inhibition of Foc conidia in soil would be an effective and possible way to control banana Fusarium wilt. In this work, three kinds of poly(N,N-diene propyl quaternary ammonium salts) (PDPQASs), namely, poly(methyl butyl diallyl ammonium chloride) (PM), poly(methyl benzyl diallyl ammonium chloride) (PB) and poly(dimethyl diallyl ammonium chloride) (PD), were synthesized, and the antifungal activities on Foc4, the adsorption and leaching properties in soil, and the biotoxicities on silkworms, zebrafish and mice were evaluated. It was found that the three kinds of PDPQAS showed obvious and stronger inhibiting effects on Foc4 conidia, and they were essentially nontoxic to silkworms and mice. Their toxicities were much lower than that of benzalkonium chloride (BC, a typical commercial small molecular QAS) to zebrafish, in which PB showed the best comprehensive properties in antifungal activities and biotoxicities. Benefitting from the stronger polymer-soil interaction, PB could absorb in soil more easily and hardly be leached from the soil, which helped PB stay in the soil more stably and effectively inhibited Foc4 conidia in the long term, giving PB a potential application in the control of banana Fusarium wilt.

Analysis of soil bacterial communities and physicochemical properties associated with Fusarium wilt disease of banana in Malaysia

Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 (TR4) is a soil-borne disease that infects bananas, causing severe economic losses worldwide. To reveal the relationship between bacterial populations and FW, the bacterial communities of healthy and TR4-infected rhizosphere and bulk soils were compared using 16S rRNA gene sequencing. Soil physicochemical properties associated with FW were also analyzed. We found the community structure of bacteria in the healthy and TR4 infected rhizosphere was significantly different compared to bulk soil within the same farm. The rhizosphere soils of infected plants exhibited higher richness and diversity than healthy plant with significant abundance of Proteobacteria. In the healthy rhizosphere soil, beneficial bacteria such as Burkholderia and Streptomyces spp. were more abundant. Compared to the infected rhizosphere soil, healthy rhizosphere soil was associated with RNA metabolism and transporters pathways and a high level of magnesium and cation exchange capacity. Overall, we reported changes in the key taxa of rhizospheric bacterial communities and soil physicochemical properties of healthy and FW-infected plants, suggesting their potential role as indicators for plant health.

Inducing banana Fusarium wilt disease suppression through soil microbiome reshaping by pineapple–banana rotation combined with biofertilizer application

Abstract. Crop rotation and biofertilizer application have historically been employed as efficient management strategies for soil-borne disease suppression through soil microbiome manipulation. However, how this occurs and to what extent the combination of methods affects the microbiota reconstruction of diseased soil is unknown. In this study, pineapple–banana rotation combined with biofertilizer application was used to suppress banana Fusarium wilt disease, and the effects on both bacterial and fungal communities were investigated using the MiSeq Illumina sequencing platform. Our results showed that pineapple–banana rotation significantly reduced Fusarium wilt disease incidence and the application of biofertilizer caused additional suppression. Bacterial and fungal communities thrived using rotation combined with biofertilizer application: taxonomic and phylogenetic α diversity of both bacteria and fungi increased along with disease suppression. Between the two strategies, biofertilizer application predominantly affected both bacterial and fungal community composition compared to rotation. Burkholderia genus may have been attributed to the general wilt suppression for its change in network structure and high relative importance in linear models. Our results indicated that pineapple–banana rotation combined with biofertilizer application has strong potential for the sustainable management of banana Fusarium wilt disease.

Investigating the Responses of Microbial Communities to Banana Fusarium Wilt in Suppressive and Conducive Soils Based on Soil Particle-Size Differentiation

The microbiota plays a primary role in inhibiting plant pathogens in the soils. However, the correlation between soil particles and local microbial communities has not been fully confirmed. In this study, we contrasted the different assemblages of microbial communities between suppressive and conducive soils via the differentiation of soil particle-size fractions (PSFs). We further extracted the direct and indirect interactive associations among the soil biotic and abiotic factors by using samples from two continuous banana cropping systems. Notable differences were shown in PSF composition, biological traits (microbial communities and enzyme patterns) and physiochemical parameters between suppressive and conducive soils among the different soil fractions. For example, compared with conducive soils, suppressive soils have higher nutrient contents, fungal abundance and diversity and enzyme activities, and the extent of these differences was explored for fractions of different sizes. Moreover, the microbial taxonomic composition strongly varied between disease-suppressive and disease-conducive soils. For instance, there were significant differences in the relative abundance among key microbiology communities, such as Actinobacteria, Firmicutes, Bacteroidetes, Proteobacteria and Ascomycota, especially for antagonistic microorganisms (i.e., Streptomyces, Pseudomonas, Trichoderma, etc.) across various soil fractions. In addition, structural equation modeling (SEM) showed that the complex associations among soil PSFs, physiochemical parameters and microbial communities were mediated by multiple pathways, which then drive the soil enzyme activities and may further influence the suppressiveness of the soil. These results demonstrate that the resident microbial communities in specific soil particles may play a crucial role in the development of soil suppressiveness against banana Fusarium wilt disease.

Vacuolar Processing Enzymes Modulating Susceptibility Response to Fusarium oxysporum f. sp. cubense Tropical Race 4 Infections in Banana.

Fusarium oxysporum f. sp. cubense tropical race 4 (FocTR4) is a destructive necrotrophic fungal pathogen afflicting global banana production. Infection process involves the activation of programmed cell death (PCD). In this study, seven Musa acuminata vacuolar processing enzyme (MaVPE1–MaVPE7) genes associated with PCD were successfully identified. Phylogenetic analysis and tissue-specific expression categorized these MaVPEs into the seed and vegetative types. FocTR4 infection induced the majority of MaVPE expressions in the susceptible cultivar “Berangan” as compared to the resistant cultivar “Jari Buaya.” Consistently, upon FocTR4 infection, high caspase-1 activity was detected in the susceptible cultivar, while low level of caspase-1 activity was recorded in the resistant cultivar. Furthermore, inhibition of MaVPE activities via caspase-1 inhibitor in the susceptible cultivar reduced tonoplast rupture, decreased lesion formation, and enhanced stress tolerance against FocTR4 infection. Additionally, the Arabidopsis VPE-null mutant exhibited higher tolerance to FocTR4 infection, indicated by reduced sporulation rate, low levels of H2O2 content, and high levels of cell viability. Comparative proteomic profiling analysis revealed increase in the abundance of cysteine proteinase in the inoculated susceptible cultivar, as opposed to cysteine proteinase inhibitors in the resistant cultivar. In conclusion, the increase in vacuolar processing enzyme (VPE)-mediated PCD played a crucial role in modulating susceptibility response during compatible interaction, which facilitated FocTR4 colonization in the host.

Development of PCR-Based Race-Specific Markers for Differentiation of Indian Fusarium oxysporum f. sp. cubense, the Causal Agent of Fusarium Wilt in Banana.

Fusarium wilt caused by Fusarium oxysporum f. sp. cubense (Foc), is the most lethal soil-borne fungal pathogen infecting bananas. Foc race 1 (R1) and 4 (R4) are the two most predominant races affecting the economically important Cavendish group of bananas in India. A total of seven vegetative compatibility groups (VCGs) from three pathogenic races were isolated during our field survey and were found to be highly virulent towards cv. Grande Naine. According to comparative genome analyses, these Indian Foc VCGs were diverse in genomic organization and effector gene profiles. As a result, false-positive results were obtained with currently available molecular markers. In this context, the study has been initiated to develop PCR-based molecular markers for the unambiguous identification of Indian Foc R1 and R4 isolates. Whole-genome sequences of Foc R1 (GCA_011316005.3), Foc TR4 (GCA_014282265.3), and Foc STR4 (GCA_016802205.1), as well as the reference genomes of Foc (ASM799451v1) and F. oxysporum f. sp. lycopersici (Fol; ASM14995v2), were aligned to identify unique variable regions among the Foc races. Using putative chromosome and predicted gene comparison, race-specific unique Foc virulence genes were identified. The putative lineage-specific identified genes encoding products secreted in xylem (SIX) that may be necessary for disease development in the banana. An in silico analysis was performed and primers were designed from a region where sequences were dissimilar with other races to develop a specific marker for Foc R1, R4, TR4, and STR4. These race-specific markers allowed target amplification in the characterized highly virulent Foc isolates, and did not show any cross-amplification to any other Foc races, VCGs or banana pathogens, Fusarium species, and non-pathogenic Fusarium oxysporum isolates. The study demonstrated that the molecular markers developed for all the three Foc races of India could detect the pathogen in planta and up to 0.025 pg µL−1 DNA levels. Thus, the markers developed in this study are novel and could potentially be useful for the accurate diagnosis and detection of the Indian Foc races which are important for the effective management of the disease.

FocECM33, a GPI-anchored protein, regulates vegetative growth and virulence in Fusarium oxysporum f. sp. cubense tropical race 4

ECM33, a glycosylphosphatidylinositol (GPI)-anchored protein, is important for fungal development and infection through regulating fungal cell wall integrity, however, the functions of its orthologs in pathogenesis have not been characterized in Fusarium oxysporum. Here, we discovered a GPI-anchored protein, FocECM33, which is required for vegetative growth and virulence of Fusasium oxysporum f. sp. cubense tropical race 4 (Foc TR4). FocECM33 was highly upregulated during the early infection process of Foc TR4 in banana roots. The targeted disruption of FocECM33 led to decreased hyphal growth, increased sensitivity to cell wall stresses and reduced virulence on banana plantlets. Furthermore, ΔFocECM33 mutant demonstrated a cell morphology defect, elevated ROS production and increased chitin content. Transcriptome analysis showed that FocECM33 has a significant influence on the production of various secondary metabolites and regulation of many biosynthetic processes in Foc TR4. Taken together, it seems FocECM33 contributes to the virulence of Foc TR4 through regulating the process of hyphal growth, ROS production and chitin synthesis.

Geographical Distribution and Genetic Diversity of the Banana Fusarium Wilt Fungus in Laos and Vietnam.

Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. cubense (Foc), poses a major threat to global banana production. The tropical race 4 (TR4) variant of Foc is a highly virulent form with a large host range, and severely affects Cavendish bananas. Foc TR4 was recently observed within the Greater Mekong Subregion, after Chinese private companies expanded Cavendish production to the region. In this study, extensive surveys conducted across Laos and Vietnam show that Foc TR4 is still mainly constricted to the northern regions of these countries and is limited to Cavendish cultivation settings. In Laos, Foc TR4 is associated with large-scale Cavendish plantations owned by or involved with Chinese companies through which infected planting material could have been imported. In Vietnam, mostly small-holder Cavendish farmers and backyard gardens were affected by Foc TR4. In Vietnam, no direct link is found with Chinese growers, and it is expected the pathogen mainly spreads through local and regional movement of infected planting materials. Foc TR4 was not recorded on banana cultivars other than Cavendish. The extensively cultivated ‘Pisang Awak’ cultivar was solely infected by VCGs belonging to Foc race 1 and 2, with a high occurrence of VCG 0123 across Laos, and of VCG 0124/5 in Vietnam. Substantial diversity of Foc VCGs was recorded (VCGs 0123, 0124/5, 01218 and 01221) from northern to southern regions in both countries, suggesting that Fusarium wilt is well established in the region. Interviews with farmers indicated that the local knowledge of Fusarium wilt epidemiology and options for disease management was limited. Clear communication efforts on disease epidemiology and management with emphasis on biosecurity practices need to be improved in order to prevent further spread of Foc TR4 to mixed variety smallholder settings.

Shared Core Microbiome and Functionality of Key Taxa Suppressive to Banana Fusarium Wilt.

Microbial contributions to natural soil suppressiveness have been reported for a range of plant pathogens and cropping systems. To disentangle the mechanisms underlying suppression of banana Panama disease caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc4), we used amplicon sequencing to analyze the composition of the soil microbiome from six separate locations, each comprised of paired orchards, one potentially suppressive and one conducive to the disease. Functional potentials of the microbiomes from one site were further examined by shotgun metagenomic sequencing after soil suppressiveness was confirmed by greenhouse experiments. Potential key antagonists involved in disease suppression were also isolated, and their activities were validated by a combination of microcosm and pot experiments. We found that potentially suppressive soils shared a common core community with relatively low levels of F. oxysporum and relatively high proportions of Myxococcales, Pseudomonadales, and Xanthomonadales, with five genera, Anaeromyxobacter, Kofleria, Plesiocystis, Pseudomonas, and Rhodanobacter being significantly enriched. Further, Pseudomonas was identified as a potential key taxon linked to pathogen suppression. Metagenomic analysis showed that, compared to the conducive soil, the microbiome in the disease suppressive soil displayed a significantly greater incidence of genes related to quorum sensing, biofilm formation, and synthesis of antimicrobial compounds potentially active against Foc4. We also recovered a higher frequency of antagonistic Pseudomonas isolates from disease suppressive experimental field sites, and their protective effects against banana Fusarium wilt disease were demonstrated under greenhouse conditions. Despite differences in location and soil conditions, separately located suppressive soils shared common characteristics, including enrichment of Myxococcales, Pseudomonadales, and Xanthomonadales, and enrichment of specific Pseudomonas populations with antagonistic activity against the pathogen. Moreover, changes in functional capacity toward an increase in quorum sensing, biofilm formation, and antimicrobial compound synthesizing involve in disease suppression.

Biological Control of Fusarium oxysporum f. sp. cubense Tropical Race 4 in Banana Plantlets Using Newly Isolated Streptomyces sp. WHL7 from Marine Soft Coral.

Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense is a disastrous fungal disease. Foc tropical race 4 (Foc TR4) infects almost all banana cultivars. Use of chemical fungicides caused serious environment pollution. Biological control with antagonistic microbes is a promising strategy for controlling Foc TR4. Here, strain WHL7 isolated from marine soft coral exhibited a high antifungal activity against Foc TR4. Based on the morphological and physicochemical profiles as well as the phylogenetic tree, the strain was assigned to Streptomyces sp. Fermentation broth of Streptomyces sp. WHL7 significantly increased the resistance of banana plantlets to Foc TR4 in the pot experiment. Analysis of antifungal mechanism showed that strain WHL7 extracts inhibited spore germination and mycelial growth of Foc TR4, and destroyed cell integrity and ultrastructure. Hence, Streptomyces sp. WHL7 is an important bioresource for exploring novel natural products and biofertilizer to manage Foc TR4.

Pest categorisation of Fusarium oxysporum f. sp. cubense Tropical Race 4.

The EFSA Plant Health Panel performed a pest categorisation of Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), an ascomycete fungus causing Fusarium wilt (Panama disease) on Musa spp. Foc TR4 is pathogenic to the commercial banana varieties including those of the ‘Cavendish’ group and is considered as the most destructive among Foc haplotypes. Uncertainty exists on the host range of Foc TR4, since it has not been demonstrated whether it can infect plant species other than Musa spp., which were previously reported as hosts of other Foc races. Foc TR4 is morphologically and physiologically identical to other representatives of the Fusarium oxysporum Species Complex (FOSC), but all Foc TR4 isolates belong to a single clonal lineage within the vegetative compatibility groups 01213‐01216. Several PCR protocols are described in the literature, but their specificity has been questioned as they may generate false positives. The pathogen is not included in EU Commission Implementing Regulation 2019/2072 and is not reported as present in the EU territory. Several potential entry pathways and means of spread were identified, including host plants for planting other than vitroplants, fresh fruits and leaves of host plants, soil and other substrates originating in infested third countries. Host availability and climate suitability occurring in some areas of the EU are favourable for the establishment of Foc TR4. Being a soil‐borne pathogen, eradication of Foc TR4 once it enters a new area is very difficult. Therefore, effective quarantine measures are essential in pathogen‐free areas. Although not specifically targeting against Foc TR4, phytosanitary measures are currently available to prevent the introduction of the pathogen into the EU. Considering that banana‐growing EU countries account for over 12% of the EU banana supply, it is expected that the economic impact of Foc TR4 on the European banana production areas would be devastating. Foc TR4 satisfies the criteria that are within the remit of EFSA to assess for this pathogen to be regarded as a potential Union quarantine pest.

Roving Survey of Panama Wilt Disease (Fusarium oxysporum f. sp. cubense) of Banana in Bihar

A survey of Panama wilt of banana was conducted in different districts of Bihar i.e. Muzaffarpur, Vaishali, Samastipur, Darbhanga, Katihar, Purnea, Bhagalpur, Kishanganj and Saharsa. This disease was present in all the districts with different levels of percent wilt index. In the Vaishali belt (Zone–I), the maximum percent wilt index was recorded in the Vaishali district (30-45%) in cv. Malbhog (AAB) having sucker planting material in the year 2019-20. In the Koshi belt (Zone-II), the percent wilt index (External symptom) was highest in Purnea district (34-42%) in cv. Robusta (AAA) followed by district Saharsa (28-40%) and Kishanganj (25-38%). The lowest percent wilt index was found in districts Bhagalpur (22-38%) and Katihar (24-38%) in the year 2019-20. Panama wilt of banana caused by Fusarium oxysporum f. sp. cubense TR4 was recorded in the Koshi belt (Zone-II) having cv. Grand naine (AAA) and cv. Robusta (AAA) in Bihar only. While in Vaishali belt (Zone-I) was free from incidence of Fusarium wilt of banana incited by Fusarium oxysporum f. sp. cubense TR4.

Exploration of Klebsiella pneumoniae M6 for paclobutrazol degradation, plant growth attributes, and biocontrol action under subtropical ecosystem.

In recent times, injudicious use of paclobutrazol (PBZ) in mango orchards deteriorates the soil quality and fertility by persistence nature and causes a serious ecosystem imbalance. In this study, a new Klebsiella pneumoniae strain M6 (MW228061) was isolated from mango rhizosphere and characterized as a potent plant growth promoter, biocontrol, and PBZ degrading agent. The strain M6 efficiently utilizes PBZ as carbon, energy and nitrogen source and degrades up to 98.28% (50 mgL-1 initial conc.) of PBZ at 15th day of incubation in MS medium. In the soil system first order degradation kinetics and linear model suggested 4.5 days was the theoretical half-life (t1/2 value) of PBZ with strain M6. Box Behnken design (BBD) model of Response surface methodology (RSM) showed pH 7.0, 31°C temperature, and 2.0 ml inoculum size (8 x 109 CFU mL-1) was optimized condition for maximum PBZ degradation with strain M6. Plant growth promoting attributes such as Zn, K, PO4 solubilization IAA, HCN and NH3 production of strain M6 showed positive results and were assessed quantitatively. The relation between plant growth promotion and PBZ degradation was analyzed by heat map, principal component analysis (PCA) and, clustal correlation analysis (CCA). Strain M6 was also showing a significant biocontrol activity against pathogenic fungi such as Fusarium oxysporum (MTCC–284), Colletotrichum gloeosporioides (MTCC– 2190), Pythium aphanidermatum (MTCC– 1024), Tropical race 1 (TR -1), and Tropical race 4 (TR -4). Hence, results of the study suggested that strain M6 can be utilized as an effective bio-agent to restore degraded land affected by persistent use of paclobutrazol.

Phenolics mediate suppression of Fusarium oxysporum f. sp. cubense TR4 by legume root exudates

Fusarium oxysporum Schlecht f. sp. cubense (E.F. Smith) Snyder & H.N. Hansen (Foc) is a notorious soil-borne fungal pathogen that causes Fusarium wilt disease of bananas (Musa spp. L.). Foc, especially Tropical Race 4 (TR4), is particularly devastating due its ability to infect diverse banana cultivars including Cavendish bananas. Fusarium wilt is difficult to manage, but intercropping has been reported to efficiently suppress Fusarium wilt through exudation of root metabolites with antagonistic effects on Foc. Yet, the metabolites that inhibit Foc and the underlying mode of action remain unclear. Hydroponic culture and targeted metabolite profiling were used to investigate the potential of root-exuded phenolic acids and flavonoids of two legume species, Desmodium uncinatum (Jacq.) DC. and Mucuna pruriens (L.) DC., to inhibit growth and the biosynthesis of the phytotoxins beauvericin and fusaric acid (FA) in Foc TR4. Of the twelve metabolites targeted, four phenolic acids (benzoic, t-cinnamic, p-coumaric, p-hydroxybenzoic) were common in root exudates of both legumes, whereas vanillin and the flavonoid quercetin were only detected in root exudates of M. pruriens. Bioassays using synthetic benzoic-, t-cinnamic-, or p-hydroxybenzoic acid, or a mixture thereof, showed a concentration-dependent suppressive effect on Foc TR4. Low concentrations of phenolic acids (0.01 and 0.1 mM) inhibited chlamydospore germination and the production of macro- and micro-conidia, as well as the biosynthesis of FA. Mycelial growth of Foc TR4 was only inhibited at high concentrations (1 mM) of benzoic acid, t-cinnamic acid, and a mixture of the three phenolics. This study demonstrates that phenolics in root exudates of D. uncinatum and M. pruriens suppress Foc TR4 by directly inhibiting the early stages of pathogen development. Intercropping banana with D. uncinatum and M. pruriens may thus be considered as a valuable option for suppression of Foc TR4.

Antifungal activity of volatile compounds generated by endophytic fungi Sarocladium brachiariae HND5 against Fusarium oxysporum f. sp. cubense.

The soil-born filamentous fungal pathogen Fusarium oxysporum f. sp. cubense (FOC), which causes vascular wilt disease in banana plants, is one of the most economically important Fusarium species. Biocontrol using endophytic microorganisms is among the most effective methods for controlling banana Fusarium wilt. In this study, volatile organic compounds (VOCs) showed strong antifungal activity against FOC. Seventeen compounds were identified from the VOCs produced by endophytic fungi Sarocladium brachiariae HND5, and three (2-methoxy-4-vinylphenol, 3,4-dimethoxystyrol and caryophyllene) showed antifungal activity against FOC with 50% effective concentrations of 36, 60 and 2900 μL/L headspace, respectively. Transmission electron microscopy (TEM) and double fluorescence staining revealed that 2-methoxy-4-vinylphenol and 3,4-dimethoxystyrol damaged the plasma membranes, resulting in cell death. 3,4-dimethoxystyrol also could induce expression of chitin synthases genes and altered the cell walls of FOC hyphae. Dichloro-dihydro-fluorescein diacetate staining indicated the caryophyllene induced accumulation of reactive oxygen species (ROS) in FOC hyphae. FOC secondary metabolism also responded to active VOC challenge by producing less fusaric acid and expressions of genes related to fusaric acid production were interrupted at sublethal concentrations. These findings indicate the potential of S. brachiariae HND5 as a biocontrol agent against FOC and the antifungal VOCs as fumigants.

The Vulnerability of Bananas to Globally Emerging Disease Threats.

Banana cultivation has increased significantly over the last century to meet the growing demand for this popular fruit. Originating in Southeast Asia, bananas are now produced in >135 different countries in tropical and subtropical regions of the world. Most of this expansion of production is based on a single banana variety, Cavendish, which makes up almost all the export trade grown in large-scale monocultures and a large part of the local trade and represents >40% of all bananas grown globally. Over the last century several major diseases of the banana have emerged and widely expanded their geographic ranges. Cultivars within the Cavendish variety are highly susceptible to these diseases, including yellow Sigatoka, black leaf streak, Eumusae leaf spot, freckle, Fusarium wilt tropical race 4, banana bunchy top, and the bacterial wilts Moko, Xanthomonas wilt, and banana Blood disease. This review graphically illustrates the emergence and rapid intercontinental spread of these diseases and discusses several major disease epidemics in bananas. Evidently, the large-scale monoculture based on the single variety Cavendish has resulted in an extreme level of genetic vulnerability. The resistance to diversification in the Cavendish production chain and the lack of investment in genetics and plant breeding in the recent past means that currently limited genetic solutions are available to replace the Cavendish banana with a set of market acceptable resistant varieties from a range of different genetic backgrounds.

English

The study aimed at understanding the spatial distribution of Fusarium wilt (FW) in different banana growing regions, ascertain the effect of management practices and plantation age on FW incidence, as well as investigate farmers’ knowledge regarding the symptoms and spread of FW in Uganda. Individual interviews were conducted in 119 farms using a pre-tested questionnaire and field observations during a survey in major banana growing regions. Results indicate that FW is widely distributed across the banana growing areas with more occurrences (70%) in Kapchorwa district and majority of respondents (63.4%) reported increasing disease prevalence. A chi-square test performed revealed significant association between FW incidence and plantation age with more incidences (51.6%) recorded in older plantations (>20 years of establishment) than newly (1-5 years) established ones (11.1%). FW incidence was significantly associated with plantation management with higher incidences (86.9%) recorded in well managed plantations. Half of interviewed farmers could explain and distinguish symptoms associated with FW from other diseases, but only 38.4% of these could tell how the disease spreads; thus, a need for more concerted efforts in building the capacity of farmers to identify the symptoms and spread of FW for effective management program. We identified preliminary evidence that field abandonment is sometimes used as a last option for coping up with FW. Understanding the mechanism behind this requires more detailed research as well as establishing how farmers are managing FW culturally.   Key words: Fusarium wilt, farmers’ knowledge, spatial distribution, Uganda.