Related mobile pathogenicity chromosomes in Fusarium oxysporum determine host range on cucurbits

Fusarium oxysporum f. sp. radicis-cucumerinum (Forc) causes severe root rot and wilt in several cucurbit species, including cucumber, melon, and watermelon. Previously, a pathogenicity chromosome, chr^RC, was identified in Forc. Strains that were previously nonpathogenic could infect multiple cucurbit species after obtaining this chromosome via horizontal chromosome transfer (HCT). In contrast, F. oxysporum f. sp. melonis (Fom) can only cause disease on melon plants, even though Fom contains contigs that are largely syntenic with chr^RC. The aim of this study was to identify the genetic basis underlying the difference in host range between Fom and Forc. First, colonization of different cucurbit species between Forc and Fom strains showed that although Fom did not reach the upper part of cucumber or watermelon plants, it did enter the root xylem. Second, to select candidate genomic regions associated with differences in host range, high-quality genome assemblies of Fom001, Fom005, and Forc016 were compared. One of the Fom contigs that is largely syntenic and highly similar in sequence to chr^RC contains the effector gene SIX6. After HCT of the SIX6-containing chromosome from Fom strains to a nonpathogenic strain, the recipient (HCT) strains caused disease on melon plants, but not on cucumber or watermelon plants. These results provide strong evidence that the differences in host range between Fom and Forc are caused by differences between transferred chromosomes of Fom and chr^RC, thus narrowing down the search for genes allowing or preventing infection of cucumber and watermelon to genes located on these chromosomes.     

Management Fusarium wilt on melon and watermelon by Penicillium oxalicum

The potential of the biological control fungus Penicillium oxalicum to suppress wilt caused by Fusarium oxysporum f. sp. melonis and F. oxysporum f. sp. niveum on melon and watermelon, respectively, was tested under different growth conditions. The area under disease progress curve of F. oxysporum f. sp. melonis infected melon plants was significantly reduced in growth chamber and field experiments. In glasshouse experiments, it was necessary to apply P. oxalicum and dazomet in order to reduce Fusarium wilt severity in melons caused by F. oxysporum f. sp. melonis. For watermelons, we found that P. oxalicum alone reduced the area under the disease progress curve by 58% in the growth chamber experiments and 54% in the glasshouse experiments. From these results, we suggested that P. oxalicum may be effective for the management of Fusarium wilt in melon and watermelon plants.     

Genetic Diversity of Fusarium oxysporum Populations Isolated from Tomato Plants in Tunisia

Fusarium crown and root rot of tomato (Lycopersicon esculentum) caused by Fusarium oxysporum f. sp. radicis‐lycopersici is a new devastative disease of tomato greenhouse crops in Tunisia. Nothing is known neither about the population of this pathogen in this region, nor about the population of F. oxysporum f. sp. lycopersici the causal agent of Fusarium wilt of tomato. In order to examine the genetic relatedness among the F. oxysporum isolates by intergenic spacer restriction fragment length polymorphism (IGS‐RFLP) analysis and to elucidate the origin of the formae specialesradicis‐lycopersici in Tunisia by looking for genetic similarity of Tunisians isolates with isolates from a foreign source, the genetic diversity among F. oxysporum f. sp. radicis‐lycopersici and F. oxysporum f. sp. lycopersici populations was investigated. A total of 62 isolates of F. oxysporum, obtained from symptomless tomato plants, were characterized using IGS typing and pathogenicity tests on tomato plants. All Fusarium isolates were highly pathogenic on tomato. Fusarium oxysporum f. sp. radicis‐lycopersici isolates were separated into five IGS types. From the 53 F. oxysporum f. sp. radicis‐lycopersici isolates, 34 isolates have the same IGS types (IGS type 25), and the remaining 19 isolates were distributed into four IGS types. However, the only nine isolates of F. oxysporum f. sp. lycopersici have six different IGS types. This difference of diversity between the two formae speciales suggests that F. oxysporum f. sp. radicis‐lycopersici isolates have a foreign origin and may have been accidentally introduced into Tunisia.     

Cross-pathogenicity between Formae Speciales of Fusarium oxysporum, the Pathogens of Cucumber and Melon

The population structure of Fusarium oxysporum f. sp. cucumerinum was studied using the vegetative compatibility grouping (VCG) approach. All 37 of the examined isolates from Israel were assigned to VCG 0180, the major VCG found in North America and the Mediterranean region. Approximately two‐thirds of the tested isolates were pathogenic to both cucumber and melon, but cumulatively they were more aggressive on cucumber, their major host, than on melon. Disease symptoms on melon plants were less destructive and often expressed as growth retardation. Melon cultivars differing in Fom genes for resistance to F. oxysporum f. sp. melonis were inoculated with three isolates of F. oxysporum f. sp. cucumerinum. Results showed that Fom genes do not confer resistance to F. oxysporum f. sp. cucumerinum, although different horticultural types may respond differently to this pathogen. The reciprocal inoculation of F. oxysporum f. sp. melonis on cucumber, using four physiological races, did not result in disease symptoms or growth retardation. It is concluded that cucumerinum and melonis should remain two distinct formae speciales.     

Gerbera jamesonii, Osteospermum sp. and Argyranthemum frutescens: New Hosts of Fusarium oxysporum f. sp. chrysanthemi

The pathogenicity of five isolates of Fusarium oxysporum obtained from infected gerbera (Gerbera jamesonii), chrysanthemum (Chrysanthemum morifolium), Paris daisy (Argyranthemum frutescens) and African daisy (Osteospermum sp.) plants was tested on some varieties of the following Compositae hosts: C. morifolium, G. jamesonii, Argyranthemum frutescens (Paris daisy) and Osteospermum sp. and compared with the host range and pathogenicity of an isolate of F. oxysporum f. sp. chrysanthemi obtained from the ATCC collection. The results indicated that isolates of F. oxysporum from G. jamesonii as well as those from A. frutescens and Osteospermum sp. belong to the forma specialischrysanthemi. The isolate from gerbera was virulent on all tested varieties of gerbera, C. morifolium, A. frutescens and Osteospermumsp. Similar results were obtained testing the isolates obtained from A. frutescens and Osteospermumsp. The strain from C. morifolium infected cultivar of gerbera, A. frutescens and Osteospermum sp. The pathogenicity of isolate of F. oxysporum f. sp. chrysanthemi obtained from the ATCC showed a different cultivar range particularly in the case of chrysanthemum and gerbera.     

First Record of Fusarium Wilt of Tobacco in Greece Imported as Seedborne Inoculum

Leaf yellowing and brown discoloration was observed in tobacco plants cv. Burley TN97 in tobacco fields of central Greece in 2002. Fusarium oxysporum f. sp. nicotianae was isolated from symptomatic plants and Koch's postulates were fulfilled. The pathogenicity of the isolated fungus was examined on five tobacco cultivars (Burley TN97, BurleyB21, VirginiaBE9, Virginia Niki and Anatolika KE26/2). The pathogen was present in tobacco seed batches imported in 2000 and 2001, which indicates that the infected seed is most probably the primary source of the disease in Greece. As Fusarium oxysporum f. sp. vasinfectum can also cause vascular wilt in tobacco, the hypothesis that the isolated F. oxysporum strain belongs to f. sp. vasinfectum was excluded by a pathogenicity test to cotton cv. Acala SJ‐2. This is the first report of F. oxysporum f. sp. nicotianae in Greece and the second in the European Union, although the seedborne nature of the pathogen has not been previously reported in Europe.     

Characterization of emerging populations of Fusarium oxysporum f. sp. conglutinans causing cabbage wilt in China

Fusarium oxysporum f. sp. conglutinans (FOC) causes Fusarium wilt, a disease of cabbage that has brought about significant economic loss throughout northern China since it was first detected in 2001. To characterize the Chinese FOC isolates, we compared the cultural characteristics, pathogenicity and races between the Chinese isolates and the type strains (race 1: 52,557 and race 2: 58,385). The Chinese FGL‐03‐6 isolate had cultural characteristics similar to those of strain 52,557, including colony growth rate, colony and spore characteristics and responses to temperature changes, while the strain 58,385 grew faster, produced more pigment and spores and was more adaptable to temperature fluctuations. The lethal temperature for all strains was 60°C, and the optimal temperatures for pathogen growth on potato dextrose agar and pathogenicity on plants were 25°C and 25 to 30°C, respectively. Tests for race and pathogenicity indicated that different cabbage cultivars had similar resistance reactions to FGL‐03‐6 and 52,557. However, the pathogenicity of FGL‐03‐6 was similar to that of 58,385, which infected quickly and caused more severe disease symptoms. This study further provides information regarding characterizing different strains of F. oxysporum f. sp. conglutinans.     

Presence of Fusarium oxysporum f. sp. melonis Race 1 in Soils Cultivated with Melon in the State of Colima (Mexico)

During years 2001, 2002 and 2003 the gravity of the Fusarium wilt in 1000 hectares of melon culture was evaluated in Colima (Mexico). In spite of the soil disinfections with methyl bromide, the losses could reach 25% of the final production. The analysis of 4 soil samples from the fields with ill plants, in a selective medium for Fusarium, allowed to detect the presence of F. oxysporum. By means of the presented technique “soil phytopathometry”, 31 isolates of F. oxysporum f. sp. melonis were obtained from the soil samples. The isolates were inoculated on melon plants to evaluate their pathogenicity. The 31 isolates inoculated, produced the symptoms of chlorosis and wilting, in melon cultivars that allowed us to affirm that all isolates were race 1 of F. oxysporum f. sp. melonis. Being this the first news of the presence of F. oxysporum f. sp. melonis in the state of Colima (Mexico).     

Fungus gnats vector Fusarium oxysporum f.sp. radicislycopersici1

Fungus gnat adults transported Fusarium oxysporum f.sp. radicis-lycopersici from Petri dish culture and infected host plants to the roots and hypocotyls of healthy tomato and bean plants. The source of the fungus did not affect the ability of fungus gnats to transport the fungus to healthy hosts. The presence of fungus gnat larvae in media in which young tomato plants were grown did not increase the incidence of plant infection by the pathogen. Fungus gnat adults appear to aid in the dissemination of F. oxysporum f.sp. radicis-lycopersici.     

嫁接西瓜对西瓜枯萎病菌侵染的防御机制研究

为了揭示嫁接提高西瓜抗枯萎病的机制,该研究以嫁接西瓜为材料,采用扫描电镜观察了枯萎病菌侵染下寄主的组织结构变化,荧光定量分析了相关防卫基因的表达,比较了嫁接西瓜对枯萎病菌侵染的抗感反应。结果显示:(1)枯萎病菌侵染后,与自根西瓜相比,嫁接西瓜的根部木质部导管通过快速形成膜状物、侵填体及细胞壁增厚阻塞菌丝入侵;自根西瓜防御反应较嫁接西瓜晚,严重侵染时薄壁细胞降解,导管组织脱落导致维管系统空洞,从而使植株呈现萎蔫症状,该现象在嫁接西瓜中没有发现。(2)枯萎病菌侵染后,嫁接西瓜比自根西瓜具有较高的防卫基因表达水平,其中:嫁接西瓜中,CHI、APX和PPO基因的表达随枯萎病菌侵染时间的延长而升高,而PAL呈现先升高后降低的表达趋势,但仍高于本底表达;自根西瓜中,仅PPO基因在枯萎病菌侵染后表达上调,而其他基因的表达则是先升高后降低,与嫁接西瓜中的PAL基因表达一致。研究表明,嫁接植株一方面通过快速的组织结构响应,另一方面从转录水平提高了相关防卫基因的表达,最终使植株具有抗病性;推测防御基因在嫁接植株与枯萎病菌互作中的强烈诱导响应可能是嫁接植株抗病的分子机制之一。     

Identification of pathogenicity‐related genes in Fusarium oxysporum f. sp. cepae

Pathogenic isolates of Fusarium oxysporum, distinguished as formae speciales (f. spp.) on the basis of their host specificity, cause crown rots, root rots and vascular wilts on many important crops worldwide. Fusarium oxysporum f. sp. cepae (FOC) is particularly problematic to onion growers worldwide and is increasing in prevalence in the UK. We characterized 31 F. oxysporum isolates collected from UK onions using pathogenicity tests, sequencing of housekeeping genes and identification of effectors. In onion seedling and bulb tests, 21 isolates were pathogenic and 10 were non‐pathogenic. The molecular characterization of these isolates, and 21 additional isolates comprising other f. spp. and different Fusarium species, was carried out by sequencing three housekeeping genes. A concatenated tree separated the F. oxysporum isolates into six clades, but did not distinguish between pathogenic and non‐pathogenic isolates. Ten putative effectors were identified within FOC, including seven Secreted In Xylem (SIX) genes first reported in F. oxysporum f. sp. lycopersici. Two highly homologous proteins with signal peptides and RxLR motifs (CRX1/CRX2) and a gene with no previously characterized domains (C5) were also identified. The presence/absence of nine of these genes was strongly related to pathogenicity against onion and all were shown to be expressed in planta. Different SIX gene complements were identified in other f. spp., but none were identified in three other Fusarium species from onion. Although the FOC SIX genes had a high level of homology with other f. spp., there were clear differences in sequences which were unique to FOC, whereas CRX1 and C5 genes appear to be largely FOC specific.     

Inheritance and genetic linkage of fusarium wilt {Fusarium oxysporum f.sp. cucumerinum race 1) and scab {Cladosporium cucumerinum) resistance genes in cucumber (Cucumis sativus)

The inheritance of resistance of the cucumber cv. SMR 18 to the race 1 of Fusarium oxysporum f.sp. cucumerinum, the linkage relationship between resistance to race 1 of F. oxysporum f.sp. cucumerinum, resistance to Cladosporium cucumerinum and fruit spine colour, and the reactions of several cucumber cultivars to inoculations with race 1 of F. oxysporum f.sp. cucumerinum and C. cucumerinum were examined. The inbred line Straight 8 (P,), which has white fruit spines and is susceptible to both fusarium wilt and scab was crossed with the inbred line SMR 18 (P₂), which has black fruit spines and resistance to both diseases. When F, F₂, F₃, BC₁P₁ BC₁P₂ and BC₁P₁ selfed progenies were inoculated at the cotyledon stage with a suspension of spores of race 1 of F. oxysporum f.sp. cucumerinum, the ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene, designated Fcu-1. When 171 BC^! plants were selfed and from each resulting F₂ family different groups of 15–25 seedlings each were tested for resistance to either disease, segregation data indicated that the Fcu-1 locus and the Ccu locus for C. cucumerinum resistance were completely linked. No evidence for linkage was found between the Fcu-1 (Ccu) locus and the B locus for fruit spine colour. Among the 59 cultivars tested at the seedling stage, 15 were susceptible, while the remainder were highly resistant to inoculations with both pathogens.     

Pattern-triggered immunity restricts host colonization by endophytic fusaria,but does not affect endophyte-mediated resistance

Fusarium oxysporum (Fo) is best known as a host-specific vascular pathogen causing major crop losses. Most Fo strains, however, are root endophytes potentially conferring endophyte-mediated resistance (EMR). EMR is a mechanistically poorly understood root-specific induced resistance response induced by endophytic or nonhost pathogenic Fo strains. Like other types of induced immunity, such as systemic acquired resistance or induced systemic resistance, EMR has been proposed to rely on the activation of the pattern-triggered immunity (PTI) system of the plant. PTI is activated upon recognition of conserved microbe-associated molecular patterns (MAMPs) of invading microbes. Here, we investigated the role of PTI in controlling host colonization by Fo endophytes and their ability to induce EMR to the tomato pathogen Fo f. sp. lycopersici (Fol). Transgenic tomato and Arabidopsis plants expressing the Fo effector gene Avr2 are hypersusceptible to bacterial and fungal infection. Here we show that these plants are PTI-compromised and are nonresponsive to bacterial- (flg22) and fungal- (chitosan) MAMPs. We challenged the PTI-compromised tomato mutants with the EMR-conferring Fo endophyte Fo47, the nonhost pathogen Fom (a melon pathogen), and with Fol. Compared to wild-type plants, Avr2-tomato plants became hypercolonized by Fo47 and Fom. Surprisingly, however, EMR towards Fol, induced by either Fo47 or Fom, was unaffected in these plants. These data show that EMR-based disease resistance is independent from the conventional defence pathways triggered by PTI, but that PTI is involved in restricting host colonization by nonpathogenic Fo isolates.     

Sciarid and shore flies as aerial vectors of Fusarium oxysporum f. sp. cucumerinum in greenhouse cucumbers

The options for managing Fusarium wilt in greenhouse cucumbers are limited by our poor understanding of the modes of survival and dissemination of the pathogen. This study uses a specific quantitative real‐time PCR assay for Fusarium oxysporum f. sp. cucumerinum to investigate the significance of flying insects as aerial vectors of the pathogen in a commercial cucumber greenhouse. Shore flies were more frequently detected (35.5%) carrying F. oxysporum f. sp. cucumerinum than sciarids (25%), with both species carrying between 1 × 10² and 1 × 10⁶ pathogen genome copies/individual. Sciarid and shore flies acquired F. oxysporum f. sp. cucumerinum following exposures to agar cultures of the pathogen of up to 94 h. Light microscopy revealed that spores were carried externally on the bodies of the adult flies. The ability of adult sciarid flies to vector the pathogen from peat‐grown diseased cucumber plants and infect healthy cucumber plants was demonstrated in a caged glasshouse trial. An inoculum density trial showed that vascular wilt disease was initiated after inoculation of peat‐grown seedlings with as few as 1000 conidia. We conclude that sciarid and shore flies play significant roles as vectors of F. oxysporum f. sp. cucumerinum in greenhouse cucumbers and need to be recognized in developing integrated crop management strategies.     

Effect of Simulated Soil Solarization and Organic Amendments on Fusarium Wilt of Rocket and Basil Under Controlled Conditions

Pot trials were carried out under controlled conditions to evaluate the effectiveness against Fusarium wilt of rocket (Fusarium oxysporum f.sp. conglutinans) and basil (F. oxysporum f.sp. basilici) of soil amendments based on a patented formulation of Brassica carinata defatted seed meal and compost, combined or not with a simulation of soil solarization. The soil solarization treatment was carried out in a growth chamber by heating the soil for 7 and 14 days at optimal (55–52°C for 6 h, 50–48°C for 8 h and 47–45°C for 10 h/day) and sub‐optimal (50–48°C for 6 h, 45–43°C for 8 h and 40–38°C for 10 h/day) temperatures similar to those observed in summer in solarized soil in greenhouses in Northern Italy. Two subsequent cycles of plant cultivation were carried out in the same soil. Even at sub‐optimal temperature regimes, 7 days of thermal treatment provided very valuable results in terms of disease control on both rocket and basil. In general, the thermal treatment was more effective against F. oxysporum f.sp. basilici than against F. oxysporum f.sp. conglutinans. Control of Fusarium wilt of rocket is improved with 14 days of thermal treatment. The combination of organic amendments with a short period of soil solarization (7 or 14 days), although not providing any improvement to the level of disease management, did significantly increase biomass and positively affected yield.     

Microsatellite marker-based detection of Fusarium wilt pathogens of Psidium guajava L.

Wilt of Psidium guajava L., incited by Fusarium oxysporum f. sp. psidii and Fusarium solani is a serious soil-borne disease of guava in India. Forty-two isolates each of F. oxysporum f. sp. psidii (Fop) and F. solani (Fs) collected from different agro climatic zones of India showing pathogenicity were subjected to estimate the genetic and molecular characterisation in terms of analysis of microsatellite marker studies. Out of eight microsatellite markers, only four microsatellite markers, viz. MB 13, MB 17, RE 102 and AY212027 were amplified with single band pattern showing the character of identical marker for molecular characterisation and genetic identification. Microsatellite marker MB 13 was amplified in F. oxysporum f. sp. psidii and F. solani isolates. Product size of 296 bps and 1018 bps were exactly amplified with a single banding pattern in all the isolates of F. oxysporum f. sp. psidii and F. solani, respectively. Microsatellite markers, viz. MB 17, RE 102 and AY212027 were also exactly amplified with a single banding pattern. MB 17 was amplified in F. oxysporum f. sp. psidii isolates with a product size of 300 bp. RE 102 and AY212027 were amplified in F. solani isolates with the product size of 153 bp and 300 bp, respectively. Therefore, amplified microsatellite marker may be used as identifying DNA marker.     

Microconidia germination of the tomato pathogen Fusarium oxysporum in the presence of root exudates

Abstract

In this study we assessed microconidia germination of the tomato pathogens F. oxysporum f. sp. lycopersici (Fol) and F. oxysporum f. sp. radicis-lycopersici (Forl) in the presence of root exudates. Tomato root exudates stimulated microconidia germination and the level of stimulation was affected by plant age. Treatment of root exudates with insoluble polyvinylpolypyrrolidone, which binds phenolic compounds, indicated that tomato root exudates contain phenolic compounds inhibitory to F. oxysporum microconidia germination. Our study indicates that tomato root exudates similarly stimulate microconidia germination of both Fol and Forl. However, individual F. oxysporum strains differ in the degree of germination response to the root exudates. Furthermore, root exudates from non-host plants also contain compounds that stimulate microconidia germination of Fol. In general, the effects of root exudates from non-host plants did not differ considerably from those of tomato. The ability of phenolic compounds to inhibit germination of Fol seems not to be plant-specific.     

First record of Fusarium vascular wilt on grapevine in Egypt

During the summer season of 2003 and 2004, wilt syndromes of grapevine leaves (Cv. crimson) and vascular discolouration of roots have been observed in 2-year-old grapevine plants in the field at two sides in Gharbeia Governorate, Egypt. First, symptoms of wilt began on bottom leaves borderline as chlorosis and then these turned to necrotic spots and the leaves died. Wilt symptoms were spread to apical associated with vascular discolouration of roots and stem basal. Routine isolations of discoloured root tissue from diseased plant yielded eight isolates of Fusarium oxysporum Schlechtend only where no other fungi were developed. Microscopic examination revealed the presence of three shapes of microconidia, first is avoid shape non-septate measuring 2.5–3.0 μm × 6–10 μm, second is cylindrical with one septa measuring 2.6 μm × 17.0 μm and third shape also cylindrical with two septate measuring 3.0 μm × 20.0 μm. Macroconidia was rarely with three septate measuring 3.5– 4.0 μm × 35.0–38.0 μm, and chlamydospores were found singly or in pairs or chains. F. oxysporum isolates attacked grapevine plants (Cv. crimson) causing vascular wilt (66.7%) and root-rot syndrome (33.3%). In vitro isolates of F. oxysporum causing wilt of grapevine (Cv. crimson) varied for producing lytic enzymes, i.e. polygalacturonase (PG) and cellulase. The reactions of several grapevines (Cvs.) with a virulent isolate of F. oxysporum indicated the presence of two different symptoms, i.e. vascular wilt only on grapevine plants (Cv. crimson) and root-rot on the other grapevine (Cvs.), i.e. superior, Thompson, King robi and flame seedless. All F. oxysporum isolates caused vascular wilt of grapevine Cv. crimson, successfully reisolated from symptomatic vascular infected tissue and complete identification on the basis of colony, conidia morphology and host range at formae speciales level as F. oxysporum f. sp. herbemontis (Tochetto) Gordan. This is the first report of Fusarium wilt on grapevine in Egypt.     

Antifungal activity of streptavidin C1 and C2 against pathogens causing Fusarium wilt

Fusarium wilt is caused by the soil-inhabiting fungus Fusarium oxysporum ff. spp. and is one of the most devastating plant diseases, resulting in losses and decreasing the quality and safety of agricultural crops. We recently reported the structures and biochemical properties of two biotin-binding proteins, streptavidin C1 and C2 (isolated from Streptomyces cinnamonensis strain KPP02129). In the present study, the potential of the biotin-binding proteins as antifungal agent for Fusarium wilt pathogens was investigated using recombinant streptavidin C1 and C2. The minimum inhibitory concentration of streptavidin C2 was found to be 16 µg ml^–1 for inhibiting the mycelial growth of F. oxysporum f.sp. cucumerinum and F. oxysporum f.sp. lycopersici, while that of streptavidin C1 was found to be 64 µg ml^–1. Compared with the nontreated control soil, the population density of F. oxysporum f.sp. lycopersici in the soil was reduced to 49·5% and 39·6% on treatment with streptavidin C1 (500 µg ml^–1) and C2 (500 µg ml^–1), respectively. A greenhouse experiment revealed that Fusarium wilt of tomato plants was completely inhibited on soil drenching using a 50-ml culture filtrate of the streptavidin-producing strain KPP02129.