Systematics of key phytopathogenic Fusarium species: current status and future challenges

This review is intended to provide plant pathologists and other scientists with a current overview of the most important Fusarium phytopathogens and mycotoxin producers. Knowledge of Fusarium species diversity and their evolutionary relationships has increased dramatically due to the application of multilocus molecular phylogenetics and genealogical concordance phylogenetic species recognition over the past 15 years. Currently Fusarium is estimated to comprise at least 300 genealogically exclusive phylogenetic species; however, fewer than half have been formally described. The most important plant pathogens reside in the following four groups: the F. fujikuroi species complex noted for Bakanae of rice, ear rot of maize, pitch canker of pine and several species that contaminate corn and other cereals with fumonisin mycotoxins; the F. graminearum species complex including the primary agents causing Fusarium head blight of wheat and barley that contaminate grain with trichothecene mycotoxins; the F. oxysporum species complex including vascular wilt agents of over 100 agronomically important crops; and the F. solani species complex, which includes many economically destructive foot and root rot pathogens of diverse hosts. Several other Fusarium phytopathogens reported from Japan and nested within other species complexes are reviewed briefly. With the abandonment of dual nomenclature, a broad consensus within the global community of Fusarium researchers has strongly supported the unitary use of the name Fusarium instead of several teleomorph names linked to it. Plant pathologists and other scientists needing accurate identifications of Fusarium isolates are encouraged to use Fusarium-ID and Fusarium MLST, Internet accessible websites dedicated to the molecular identification of Fusarium species.     

Molecular identification and mycotoxin production of <Emphasis Type="Italic">Lilium longiflorum</Emphasis>-associated fusaria isolated from two geographic locations in the United States

Fusarium diseases of Liliaceae crops cause significant losses worldwide. Yet some Fusarium species are found in planta without causing disease or even in a symbiotic relationship with its host. In this study we identified and characterized the Fusarium species isolated from soil, and from healthy and diseased bulbs of Lilium longiflorum grown in New Jersey and Oregon in the United States. The predominant Fusarium species from the Oregon location were F. solani (74%) and F. oxysporum (20%), whereas F. concentricum (43%) and F. proliferatum (26%), both belonging to the Gibberella fujikuroi species complex (GFSC), were the most commonly isolated species from New Jersey. To our knowledge, this is the first report of F. concentricum associated with Liliaceae. All of the isolates were characterized with sequences of the internal transcribed spacer and translation elongation factor 1-alpha genes. The 24 GFSC isolates were further characterized with mating type, mating population, and mycotoxin analysis. Results showed that all GFSC isolates were MAT-2, suggesting that the populations may be asexually reproducing in the region examined. The majority of the GFSC isolates produced beauvericin. Enniatin A, B, B₁ and fusaproliferin were produced by a few isolates. Enniatin A₁ and fumonisins were not detected in any of the isolates. Although F. oxysporum and F. solani are well-known bulb pathogens, many isolates of F. oxysporum and F. solani, and all of the F. concentricum and F. proliferatum were isolated from asymptomatic bulbs, suggesting their endophytic association with lilies.     

Five plant families support natural sporulation of <Emphasis Type="Italic">Cronartium ribicola</Emphasis> and <Emphasis Type="Italic">C. flaccidum</Emphasis> in Finland

Fusarium is one of the most destructive fungal genera whose members cause many diseases on plants, animals, and humans. Moreover, many Fusarium species secrete mycotoxins (e.g. trichothecenes and fumonisins) that are toxic to humans and animals. Fusarium isolates from date palm trees showing disease symptoms, e.g. chlorosis, necrosis and whitening, were collected from seven regions across Saudi Arabia. After single-sporing, the fungal strains were morphologically characterized. To confirm the identity of morphologically characterized Fusarium strains, three nuclear loci, two partial genes of translation elongation factor 1 α (tef1α) and β-tubulin (tub2), and the rDNA-ITS region, were amplified and sequenced. Of the 70 Fusarium strains, 70 % were identified as F. proliferatum that were recovered from six regions across Saudi Arabia. Fusarium solani (13 %), as well as one strain each of the following species: F. brachygibbosum, F. oxysporum, and F. verticillioides were also recovered. In addition, five Fusarium-like strains were recognized as Sarocladium kiliense by DNA-based data. The preliminary in vitro pathogenicity results showed that F. proliferatum had the highest colonization abilities on date palm leaflets, followed by F. solani. Although F. oxysporum f. sp. albedinis is the most serious date palm pathogen, F. proliferatum and F. solani are becoming serious pathogens and efforts should be made to restrict and control them. In addition, the potential toxin risks of strains belonging to F. proliferatum should be evaluated.     

A selective agar medium for isolation,enumeration and morphological identification of Fusarium proliferatum

A selective agar medium based on macerated date fruits was developed for the isolation, enumeration and morphological identification of Fusarium proliferatum from soil and from infected tissues of various plants (including: onion bulbs, corn ears and stems, and various weed tissues). The selective date medium enhances the formation of polyphialide and longer chains of conidia for better separation from other related Fusarium species which also grow and proliferate on this medium. Furthermore, the date medium enables microscopic distinction among other closely related Fusarium species, e.g. F. oxysporum and F. verticillioides. Fruits of the date cultivars Medjoul and Deglet Noor provided the most useful results as compared with other cultivars tested. The date medium can serve as a selective medium for direct isolation and enumeration of F. proliferatum, as it suppresses the development of other soil fungi and plant pathogens such as Macrophomina phaseolina, Sclerotium rolfsii and Rhizoctonia solani, as well as bacteria.     

Genetic variability among isolates of Fusarium oxysporum from sugar beet

Fusarium yellows, caused by the soil‐borne fungus Fusarium oxysporum f. sp. betae (Fob), can lead to significant yield losses in sugar beet. This fungus is variable in pathogenicity, morphology, host range and symptom production, and is not a well characterized pathogen on sugar beet. From 1998 to 2003, 86 isolates of F. oxysporum and 20 other Fusarium species from sugar beet, along with four F. oxysporum isolates from dry bean and five from spinach, were obtained from diseased plants and characterized for pathogenicity to sugar beet. A group of sugar beet Fusarium isolates from different geographic areas (including nonpathogenic and pathogenic F. oxysporum, F. solani, F. proliferatum and F. avenaceum), F. oxysporum from dry bean and spinach, and Fusarium DNA from Europe were chosen for phylogenetic analysis. Sequence data from β‐ tubulin, EF1α and ITS DNA were used to examine whether Fusarium diversity is related to geographic origin and pathogenicity. Parsimony and Bayesian MCMC analyses of individual and combined datasets revealed no clades based on geographic origin and a single clade consisting exclusively of pathogens. The presence of FOB and nonpathogenic isolates in clades predominately made up of Fusarium species from sugar beet and other hosts indicates that F. oxysporum f. sp. betae is not monophyletic.     

Resistance Reaction of Conifer Species (European Larch, Norway Spruce, Scots Pine) to Infection by Selected Necrotrophic Damping-off Pathogens

Three conifer species (European larch, Norway spruce, Scots pine) were investigated for their resistance to five damping-off pathogens (Rhizoctonia solani, Fusarium solani, F. oxysporum, F. culmorum, F. avenaceum). Inoculation of the primary roots of seedlings with these pathogens caused host cell death which did not prevent the invasive growth of these fungi; seedlings that had formed secondary and tertiary roots could overcome the infection to a significant degree. Infections with R. solani caused significant mortality to all the conifer species. In contrast, the tree species expressed different levels of resistance when challenged with the Fusarium isolates, with Norway spruce being the most resistant compared to uninoculated controls. Some of the Fusarium isolates were more pathogenic to certain hosts than others; F. oxysporum for European larch, F. avenaceum for Scots pine, F. solani for European larch; only F. culmorum was significantly pathogenic to Norway spruce. No significant differences in disease severity were observed at different soil pH (4.3–7.5). Disease progression was delayed at lower (10–15°C) rather than higher temperatures (20–25°C).     

兰州百合枯萎病病原菌鉴定及罹病组织超微结构观察

为明确引起甘肃省兰州百合主产区百合枯萎病的致病镰孢菌种类,对从百合主产区枯萎病罹病植株上分离纯化的4株镰孢菌株进行形态学鉴定、分子生物学鉴定以及致病性测定,同时利用电子显微镜对尖孢镰孢菌Fusarium oxysporum侵入百合鳞片后的细胞超微结构进行观察。结果表明:4株镰孢菌菌株经鉴定分别为尖孢镰孢菌、茄病镰孢菌F. solani、三线镰孢菌F. tricinctum和燕麦镰孢菌F. avenaceum。4株镰孢菌菌株的致病力由强到弱的顺序依次是尖孢镰孢菌、燕麦镰孢菌、茄病镰孢菌、三线镰孢菌;尖孢镰孢菌侵入后,鳞片细胞壁、细胞质膜和细胞核结构被破坏,细胞核附近出现大量线粒体,细胞中淀粉粒数量减少。表明尖孢镰孢菌是兰州百合枯萎病防治的重点防控对象。     

Pathogenic variation and molecular characterization of <Emphasis Type="Italic">Fusarium</Emphasis> species isolated from wilted Welsh onion in Japan

Thirty-two isolates of Fusarium species were obtained from wilted Welsh onion (Allium fistulosum) grown on nine farms from six regions in Japan and identified as F. oxysporum (18 isolates), F. verticillioides (7 isolates), and F. solani (7 isolates). The pathogenicity of 32 isolates was tested on five commercial cultivars of Welsh onion and two cultivars of bulb onion in a seedling assay in a greenhouse. The Fusarium isolates varied in the degree of disease severity on the cultivars. Five F. oxysporum isolates (08, 15, 17, 22, and 30) had a higher virulence on the cultivars than the other isolates. The host range of these five isolates was limited to Allium species. Molecular characterization of Fusarium isolates was performed using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of the internal transcribed spacer (ITS) regions of ribosomal DNA. The 32 isolates were grouped into eight types (four types for F. oxysporum, one for F. verticillioides, and three for F. solani). Restriction patterns of the ITS region were not related to pathogenicity. However, the haplotypes obtained with five enzymes (RsaI, HinfI, HaeIII, ScrFI, and MspI) and the phylogenetic analysis permitted the discernment of the three Fusarium species. The PCR-RFLP analysis should provide a rapid, simple method for differentiating Fusaruim species isolated from wilted Welsh onion in Japan.     

Morphology,phylogeny and pathogenicity of Fusarium species from Sansevieria trifasciata in Malaysia

Leaf blight is a common disease affecting Sansevieria trifasciata in many countries, including Malaysia. In the present study, Fusarium isolates were consistently recovered from the diseased leaves collected from various locations throughout the country. Based on morphology and multigene phylogenetic analysis using mitochondrial small subunit (mtSSU), intergenic spacer region (IGS) and translation elongation factor 1-α (TEF1-α) gene sequences, seven Fusarium species were identified, with F. oxysporum being the most prevalent (67.6%) among 34 isolates. Pathogenicity tests resulted in the discovery of pathogenic isolates that belonged to F. oxysporum, F. proliferatum, and F. pseudocircinatum, whereas all isolates of F. brachygibbosum, F. concentricum, F. mangiferae, and F. solani were nonpathogenic. The results suggest that several Fusarium species are accountable for causing disease on S. trifasciata in Malaysia.     

Footrot in asparagus caused byFusarium oxysporum f. sp.asparagi

In the asparagus crop at least four soil-borne diseases can be distinguished. Footrot is one which appears to be caused byFusarium oxysporum f. sp.asparagi and is characterized by brown oval lesions on the lower parts of stems. A method is described for testing for pathogenicity the species ofFusarium and other fungi isolated from diseased plants. A negative correlation was found between the number ofF. oxysporum f. sp.asparagi isolates and the ‘G-value’ which provides an indication of the development of an asparagus crop.     

Fusarium oxysporum schlecht. in Israel

Out of 208 isolates ofF. oxysporum, 158 produced toxic reactions when applied to rabbit skin. This fungus, ubiquitous in the soils of Israel, was the most prevalent component of theFusarium flora in the unirrigated (but not in the irrigated) soils sampled.F. oxysporum var.redolens was common only on unfertilized plots of heavy, unirrigated soils.F. oxysporum was a major constituent of theFusarium flora isolated from most of the 450 samples of 20 field and garden crops, but less common in 98 samples of citrus, avocado and mango fruits. It ranks among the most destructive pathogens of cucurbits (exceptCucurbita pepo), tomatoes, onions, and gladioli in Israel, but causes almost no damage to Brassicae, peas or cotton. In pathogenicity tests with 207 isolates from nine field and two fruit crops, numerous isolates caused seedling mortality, with watermelon, onion, cucumber, tomato and eggplant being the most susceptible. Spore measurements on 160 isolates from plants and 355 from soils, showed that spore size was not markedly affected by plant source or by irrigation, but did differ between soil samples taken at 5 and 20 cm depth, and from manured or unfertilized soils.     

Natural occurrence of <Emphasis Type="Italic">Fusarium</Emphasis> species and fumonisin on maize grains in Ethiopia

Fusarium species causing maize kernel rot are major threats to maize production, due to reduction in yield as well as contamination of kernels by mycotoxins that poses a health risk to humans and animals. Two-hundred maize kernel samples, collected from 20 major maize growing areas in Ethiopia were analyzed for the identity, species composition and prevalence of Fusarium species and fumonisin contamination. On average, 38 % (range: 16 to 68 %) of maize kernels were found to be contaminated by different fungal species. Total of eleven Fusarium spp. were identified based on morphological characteristics and by sequencing the partial region of translation elongation factor 1-alpha (EF-1α) gene. Fusarium verticillioides was the dominant species associated with maize kernels (42 %), followed by F. graminearum species complex (22.5 %) and F. pseudoanthophilium (13.4 %). The species composition and prevalence of Fusarium species differed among the areas investigated. Fusarium species composition was as many as eight and as few as four in some growing area. The majority of the maize samples (77 %) were found positive for fumonisin, with concentrations ranging from 25 μg kg^?1 to 4500 μg kg^?1 (mean: 348 μg kg^?1 and median: 258 μg kg^?1). Slight variation in fumonisin concentration was also observed among areas. Overall results indicate widespread occurrence of several Fusarium species and contamination by fumonisin mycotoxins. These findings are useful for intervention measures to reduce the impact of the main fungal species and their associated mycotoxins, by creating awareness and implementation of good agricultural practices.     

Fusarium oxysporum,F. proliferatum and F. redolens associated with basal rot of onion in Finland

In recent years in Finland, Fusarium infections in onions have increased, both in the field and in storage, and Fusarium species have taken the place of Botrytis as the worst pathogens causing post‐harvest rot of onion. To study Fusarium occurrence, samples were taken from onion sets, harvested onions and also from other plants grown in the onion fields. Isolates of five Fusarium species found in the survey were tested for pathogenicity on onion. Fusarium oxysporum was frequently found in onions and other plants, and, of the isolates tested, 31% caused disease symptoms and 15% caused growth stunting in onion seedlings. Fusarium proliferatum, a species previously not reported in Finland, was also identified. Over 50% of the diseased onion crop samples were infected with F. proliferatum, and all the F. proliferatum isolates tested were pathogenic to onion. Thus, compared to F. oxysporum, F. proliferatum seems to be more aggressive on onion. Also some of the F. redolens isolates were highly virulent, killing onion seedlings. Comparison of the translation elongation factor 1α gene sequences revealed that the majority of the aggressive isolates of F. oxysporum f. sp. cepae group together and are distinct from the other isolates. Incidence and relative proportions of the different Fusarium species differed between the sets and the mature bulbs. More research is required to determine to what extent Fusarium infections spoiling onions originate from infected onion sets rather than the field soil.     

Das Artenspektrum der Gattung Fusarium an Weizen und Roggen in Bayern in den Jahren 2003 und 2004

During the years 2003 and 2004 grains of wheat and rye were examined for the occurrence of different Fusarium species in Bavaria. The data obtained indicate that rye is infected with Fusarium spp. on a lower level than wheat. Overall F. graminearum was the most important Desoxynivalenol (DON) producing species with infected kernels per sample up to 18,5% whereas F. culmorum recedes more in the background. In addition F. poae was detected in high amounts: up to 28,5% kernels per sample were infected. Also the incidence of F. avenaceum was high. In contrast F. equiseti, F. tricinctum, F. sporotrichioides, F. oxysporum and Microdochium nivale were detected to a much lower extent.     

新疆南疆枣树根腐病病原的分离与鉴定

<正>枣为鼠李科(Rhamnaceae)枣属(Ziziphus M ill.)植物,在我国拥有悠久的栽培历史。枣树的喜光、喜温、耐寒、耐旱等栽培特点,非常适合在新疆这种气候条件特殊的地区生长。近年来,随着新疆红枣产业的迅速发展,枣树病害问题也逐渐突显。2010和2011年,新疆南疆多地枣园出现枣树根腐病,该病害主要造成枣树实生苗和多年生枣树根部腐烂,叶片黄化,植株树势衰弱,最终导致植株     

Phylogenetic relationships and virulence assays of Fusarium secorum from sugar beet suggest a new look at species designations

Fusarium spp. are responsible for significant yield losses in sugar beet (Beta vulgaris) with Fusarium oxysporum f. sp. betae most often reported as the primary causal agent. Recently, a new species, F. secorum, was reported to cause disease in sugar beet but little is known on the range of virulence within F. secorum or how this compares to the virulence and phylogenetic relationships previously reported for Fusarium pathogens of sugar beet. To initiate this study, partial translation elongation factor 1-α (TEF1) sequences from seven isolates of F. secorum were obtained and the data were added to a previously published phylogenetic tree that includes F. oxysporum f. sp. betae. Unexpectedly, the F. secorum strains nested into a distinct group that included isolates previously reported as F. oxysporum f. sp. betae. These results prompted an expanded phylogenetic analysis of TEF1 sequences from genomes of publicly available Fusarium spp., resulting in the additional discovery that some isolates previously reported as F. oxysporum f. sp. betae are F. commune, a species that is not known to be a sugar beet pathogen. Inoculation of sugar beet with differing genetic backgrounds demonstrated that all Fusarium strains have a significant range in virulence depending on cultivar. Taken together, the data suggest that F. secorum is more widespread than previously thought. Consequently, future screening for disease resistance should rely on isolates representing the full diversity of the Fusarium population that impacts sugar beet.     

Characterization of Fusarium diseases on commercially grown orchids in Hawaii

A decline of unknown aetiology has become a major problem for commercial orchid production in Hawaii, one of the primary orchid‐producing states in the USA. The major symptoms of decline include root degradation, foliar blight, pseudobulb rot and sheath rot. It was unclear whether all these symptoms are caused by the same or different pathogens, but preliminary research indicated that Fusarium species may be involved. In this study, the incidence of Fusarium species was examined across 186 plants, from 29 orchid genera and intergeneric hybrids across three islands in the state of Hawaii. The main five species associated with diseased orchids were F. proliferatum (38% of samples), F. solani (16%), F. oxysporum (16%) and two previously undescribed species (8% for both species combined). The two undescribed species were similar in appearance to F. subglutinans, and were designated FS‐A and FS‐B. Pathogenicity tests established that both F. proliferatum and FS‐B caused foliar spots, foliar blight and pseudostem rot on Dendrobium orchids, and that F. proliferatum isolates from diseased tissue of several genera could also induce symptoms on Dendrobium orchids. Although orchids have increasing importance in floriculture, relatively little is known about orchid pathogens, and previous studies focused primarily on Cymbidium and Phalaenopsis. This study provides new information concerning Dendrobium orchid pathogens and suggests a much wider host range than previously recognized for the five Fusarium species recovered from tissue with symptoms. These findings can contribute to better management of Fusarium diseases, which represent a significant challenge to orchid production in Hawaii.     

Diversity of <Emphasis Type="Italic">Fusarium</Emphasis> species associated with root rot of sugar beet in China

Sugar beet is widely grown throughout the world and represents the second largest crop used to produce sugar. Root rot in sugar beet, caused by Fusarium, significantly reduces yield, juice purity, and sugar concentration. Here, 307 Fusarium isolates were collected from sugar beet roots exhibiting typical root rot symptoms in eight provinces or autonomous regions of China from 2009 to 2012. Based on morphological characteristics and sequence data of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) and the translation elongation factor 1α (EF-1α), Fusarium oxysporum (38.4%) was identified as the most prevalent species, followed by F. solani (20.9%), and F. equiseti (18.9%). These three species were widely distributed in all eight of the provinces and autonomous regions. F. tricinctum (5.9%), F. brachygibbosum (4.6%), F. redolens (3.3%), F. proliferatum (3.3%), F. graminearum (2.3%), F. verticillioides (1.6%), F. nygamai (0.7%), and F. culmorum (0.3%) were less frequently obtained. Of the 307 Fusarium isolates, 117 representing different species and geographic locations were demonstrated to cause tip rot and vascular discoloration in sugar beet roots, with disease incidence ranging from 84.2 to 100.0% and disease index ranging from 41.94 to 75.83. This is the first detailed report of Fusarium species, in particular F. tricinctum, F. brachygibbosum, F. redolens, F. proliferatum, F. nygamai, and F. culmorum, causing sugar beet root rot in China.     

Morphological and molecular characterization of Fusarium spp. associated with yellowing disease of black pepper (Piper nigrum L.) in Malaysia

Yellowing disease is one of the most important diseases of black pepper (Piper nigrum L.). To characterize the pathogen(s) responsible for yellowing disease of black pepper in Malaysia, 53 isolates of Fusarium were collected from the roots of diseased black pepper plants and from rhizosphere soils from major growing areas in Sarawak and Johor. A total of 34 isolates of F. solani and 19 isolates of F. proliferatum were obtained and identified based on morphological characteristics and molecular techniques. DNA sequencing of the internal transcribed spacers (ITS1 and ITS2) and 5.8S ribosomal DNA regions was conducted to identify Fusarium species. Nucleotide sequence analysis of the ITS regions revealed that this molecular technique enabled identification of Fusarium at the species level as F. solani and F. proliferatum. In a pathogenicity test on 3-month-old black pepper plants, F. solani was pathogenic, but F. proliferatum was not. On the basis of morphology, DNA sequences and pathogenicity of the fungal isolates from the diseased plants, we showed that yellowing disease on black pepper is caused by F. solani     

Auftreten und Nachweis von Fusarium oxysporum und F. proliferatum an Steck- und Saatzwiebeln

The pathogen Fusarium oxysporum f. sp. cepae inducing the Fusarium basal rot mainly spreads in warmer cultivation regions due to its adaptibility to high temperature. Meanwhile the pathogen occurs in Germany as well, especially in years with relatively high average temperature during the growing season. Phytopathological investigations of 300 symptomless onion bulbs showed a contamination rate of approximately 10% with regard to Fusarium spp, with F.?oxysporum proving to be the predominant species. Onion sets planted in these fields were latently infected with F.?oxysporum at rates of 19?C98%. Unexpectedly, the contaminated sets did not indispensably lead to a high occurrence of plants exhibiting characteristic symptoms of Fusarium basal rot such as wet and dry rot. Presumably, the development of symptoms is particularly affected by given climatic conditions. The results of pathogenicity tests of isolated Fusarium spp. isolates under controlled conditions support this assumption. The inoculation of the substrate with selected Fusarium spp. isolates resulted in a reduction of emergence by up to 70% under controlled conditions, which are suboptimal with regard to the cultivation of onions. The emergence of plants was not affected by Fusarium spp. under optimal cultivation of onions. However, under optimal cultivation conditions a reduction of plant growth occurred in a subsequent growth stage. Beside F.?oxysporum, F.?proliferatum could be detected in onion bulbs as well as seeds. The proportion of contaminated seeds accounted to 62%. Both species F.?oxysporum and F.?proliferatum proved to be pathogenic in onion although their isolates varied much in their virulence.