Host-specific Fusarium oxysporum Causes Wilt of Jojoba

Jojoba [Simmondsia chinensis (Link) Schneider] plantations in Israel originated from vegetative propagation, planted during 1991–92, have shown symptoms of wilting and subsequent death. Verticillium dahliae was only rarely isolated from these plants and artificial inoculation showed only mild disease symptoms. Fusarium oxysporum caused severe chlorosis, desiccation, defoliation and wilt in leaves of jojoba plants, resulting in plant death. Recovery of the fungus from artificially inoculated stem cuttings and seedlings showed for the first time that F. oxysporum was the primary pathogen. Inoculated cuttings exhibited wilt within 3 weeks, while in seedlings wilt occurred 10–24 weeks after inoculation. Seedlings and cuttings of jojoba which were inoculated with other Fusarium isolates originating from different crops (F. oxysporum f. sp. vasinfectum from cotton, F. oxysporum f. sp. dianthi from carnation, F. oxysporum f. sp. lycopersici from tomato and F. oxysporum f. sp. basilicum from basil) did not develop symptoms. Moreover, cotton, tomato, melon and cucumber seedlings inoculated with several virulent F. oxysporum isolates from jojoba did not show any symptoms of wilt or defoliation. These results indicate a high degree of specificity of the Fusarium isolates from jojoba; therefore, it is suggested that this isolate be defined as F. oxysporum f. sp. simmondsia.     

The effects of Fusarium oxysporum f.sp. lycopersici (Sacc.) Snyder & Hansen on tomato in diphenamid-treated soil

Pre-emergence soil application of the herbicide diphenamid in concentrations exceeding the normal field rate increased the resistance of tomato plants towards infection by the wilt fungus Fusarium oxysporum f.sp. lycopersici. This was detected as significant increases in the percentage emergence of seedlings although growth parameters of the raised seedlings were reduced. Treated plants exhibited no wilt symptoms, although the pathogen maintained its population at detectable levels in the rhizosphere of tomato plants. However, the growth inhibition caused by diphenamid alone was much less than that reported for the combined application of pathogen and herbicide. Growth activities of F. oxysporum f.sp. lycopersici were inhibited by high concentrations of diphenamid in vitro. It is possible that the biodegradation of this herbicide by species such as Aspergillus candidus (present in substantial counts in treated rhizospheres) was one of the causes of increased tolerence of the pathogen to the herbicide in situ.     

<Emphasis Type="Italic">Fusarium sambucinum</Emphasis> isolate FS-94 induces resistance against fusarium wilt of tomato via activation and priming of a salicylic acid-dependent signaling system

The wheat rhizosphere-inhabiting nonpathogenic Fusarium sambucinum isolate FS-94 protected tomato from Fusarium wilt (F. oxysporum f. sp. lycopersici) in laboratory experiments. Seed soaking or immersion of seedling roots in a FS-94 spore suspension prior to inoculation with the pathogen delayed the appearance of wilt symptoms and significantly reduced disease severity in plants of a susceptible tomato cultivar. Quantification of fungal ergosterol in infected tomato showed that protection against wilt agent was related to limitation of the pathogen growth in plants exposed to FS-94. Incubation of tomato seedlings in a FS-94 spore suspension for 48 or 72 h led to plant protection and increased the salicylic acid (SA) concentration in their roots, suggesting that this isolate was involved in a plant-mediated mode of action and induced resistance. Soaking tomato seeds in the spore suspension did not induce SA accumulation in seedling roots, but nevertheless resulted in a significant reduction in wilt severity when the seedlings were challenged with the pathogen. In response to pathogen attack, the SA content in susceptible seedlings grown from FS-94-treated seeds started to increase within 1 day and remained elevated for 72 h. This suggests that F. sambucinum isolate FS-94 primed a SA-dependent signaling system in tomato.     

Influence of salicylic acid and Glomus fasciculatum on fusarial wilt of tomato and brinjal

Wilt of tomato caused by Fusarium oxysporum f.sp. lycopersici and wilt of brinjal caused by Fusarium solani are very common in different agricultural fields of West Bengal, India. The study on the effect of salicylic acid (SA) on in vitro growth of the pathogens revealed that SA completely checks the growth at 0.7 mM concentration and above. On the other hand, the percentage of mycorrhisation in the host plants with an arbuscular mycorrhizal (AM) fungus, Glomus fasciculatum was found to be decreased in the presence of SA in the plants with or without pathogenic infection. Treatment of tomato and brinjal plants either singly with AM fungus or with SA (0.5 and 1.0 mM) and also with their combined treatment showed amelioration of plant height, length of root, fresh weight of root and fresh weight of plants. However, the AM fungus-treated plants showed highest growth responses. The result also reveals that integrated treatment with AM plus SA has significant effect on reduction of infection where application of AM + SA (1.0 mM) resulted in 67.16 and 69.70% reduction of fusarial wilt infection in tomato and brinjal plants, respectively.     

Interaction between Fusarium oxysporum F. vasinfectum and Cephalosporium maydis on cotton and maize

The interaction between Fusarium oxysporum (cause of cotton wilt) and Cephalosporium maydis (cause of maize late-wilt) on cotton roots is associated with an appreciable decrease in the severity of the cotton wilt disease. Reduction in infection is more pronounced when the latter fungus precedes the former in the soil than when they are inoculated simultaneously. C. maydis exerts little or no such effect when it follows F. oxysporum in the soil. C. maydis grows on the surface of cotton roots near growing points as a root-surface inhabitant. Dark red lesions are produced but these disappear, as does the fungus, when the root becomes hardened either naturally or in response to the growth of the fungus on the surface. The presence of the fungus is associated with increased production of root laterals. Cotton plants, including those which may appear healthy, show only mild internal symptoms of Fusarium infection when grown in soil inoculated with the two fungi, suggesting that the decreased severity of wilt is largely due to increased tolerance of the plants to infection with the disease as a result of increased number of root laterals. It is also possible that cotton roots with C. maydis on their surface become less suitable for the progress of F. oxysporum. F. oxysporum produces in culture a metabolite inhibitory to C. maydis. This may partly account for the little effect that the latter fungus exerts on the severity of wilt when it follows F. oxysporum in the soil. It appears that the interaction between F. oxysporum and C. maydis does not affect the pathogenicity of the latter fungus to maize.     

Effect of Meloidogyne incognita,M. hapla,and M. javanica on the Severity of Fusarium Wilt of Chrysanthemum

Rooted cuttings of Chrysanthemum morifolium ''Yellow Delaware'' (Fusarium-susceptible) and ''White Iceberg'' (Fusarium-resistant) were greenhouse-grown in: (i) non-infested soil; (ii) soil infested with Fusarium oxysporum alone; (iii) soil infested with Meloidogyne incognita, M. javanica or M. hapla; and (iv) each nematode separately plus the fungus. All nematode species infected roots of both cultivars and caused characteristic root-knot symptoms but did not appreciably affect growth meassured by plant weight. Nematodes did not break Fusarium wilt resistance of ''White Iceberg''; however, wilt symptoms appeared earlier and were more severe among ''Yellow Delaware'' plants inoculated with Meloidogyne javanica and F. oxysporum than with similar combinations of the fungus and M. incognita or M. hapla or with the fungus alone.     

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.     

Host perception of jasmonates promotes infection by Fusarium oxysporum formae speciales that produce isoleucine‐ and leucine‐conjugated jasmonates

Three pathogenic forms, or formae speciales (f. spp.), of Fusarium oxysporum infect the roots of Arabidopsis thaliana below ground, instigating symptoms of wilt disease in leaves above ground. In previous reports, Arabidopsis mutants that are deficient in the biosynthesis of abscisic acid or salicylic acid or insensitive to ethylene or jasmonates exhibited either more or less wilt disease, than the wild‐type, implicating the involvement of hormones in the normal host response to F. oxysporum. Our analysis of hormone‐related mutants finds no evidence that endogenous hormones contribute to infection in roots. Mutants that are deficient in abscisic acid and insensitive to ethylene show no less infection than the wild‐type, although they exhibit less disease. Whether a mutant that is insensitive to jasmonates affects infection depends on which forma specialis (f. sp.) is infecting the roots. Insensitivity to jasmonates suppresses infection by F. oxysporum f. sp. conglutinans and F. oxysporum f. sp. matthioli, which produce isoleucine‐ and leucine‐conjugated jasmonate (JA‐Ile/Leu), respectively, in culture filtrates, whereas insensitivity to jasmonates has no effect on infection by F. oxysporum f. sp. raphani, which produces no detectable JA‐Ile/Leu. Furthermore, insensitivity to jasmonates has no effect on wilt disease of tomato, and the tomato pathogen F. oxysporum f. sp. lycopersici produces no detectable jasmonates. Thus, some, but not all, F. oxysporum pathogens appear to utilize jasmonates as effectors, promoting infection in roots and/or the development of symptoms in shoots. Only when the infection of roots is promoted by jasmonates is wilt disease enhanced in a mutant deficient in salicylic acid biosynthesis.     

Induction of defense-related proteins in tomato roots treated with Pseudomonas fluorescens Pf1 and Fusarium oxysporum f. sp. lycopersici

Pseudomonas fluorescens isolate Pf1 was found to protect tomato plants from wilt disease caused by Fusarium oxysporum f. sp. lycopersici. Induction of defense proteins and chemicals by P. fluorescens isolate Pf1 against challenge inoculation with F. oxysporum f. sp. lycopersici in tomato was studied. Phenolics were found to accumulate in bacterized tomato root tissues challenged with F. oxysporum f. sp. lycopersici at one day after pathogen challenge. The accumulation of phenolics reached maximum at the 5^th day after pathogen challenge. In pathogen-inoculated plants, the accumulation started at the 2^nd day and drastically decreased 4 days after the pathogen inoculation. Activities of phenylalanine ammonia-lyase (PAL), peroxidase (PO) and polyphenol oxidase (PPO) increased in bacterized tomato root tissues challenged with the pathogen at one day after pathogen challenge and activities of PAL and PO reached maximum at the 4^th day while activity of PPO reached maximum at the 5^th day after challenge inoculation. Isoform analysis revealed that a unique PPO1 isoform was induced and PO1 and PPO2 isoforms were expressed at higher levels in bacterized tomato root tissues challenge inoculated with the pathogen. Similarly, -1,3 glucanase, chitinase and thaumatin-like proteins (TLP) were induced to accumulate at higher levels at 3-5 days of challenge inoculation in bacterized plants. Western blot analysis showed that chitinase isoform Chi2 with a molecular weight of 46 kDa was newly induced due to P. fluorescens isolate Pf1 treatment challenged with the pathogen. TLP isoform with molecular weight of 33 kDa was induced not only in P. fluorescens isolate Pf1-treated root tissues challenged with the pathogen but also in roots treated with P. fluorescens isolate Pf1 alone and roots inoculated with the pathogen. These results suggest that induction of defense enzymes involved in phenylpropanoid pathway and accumulation of phenolics and PR-proteins might have contributed to restriction of invasion of F. oxysporum f. sp. lycopersici in tomato roots.     

Antagonistic effects of soil bacteria onFusarium oxysporum Schlecht f.sp.dianthii (Prill and Del.) Snyd. and Hans.

Summary Thirty two bacteria antagonistic to a number of phytopathogenic fungi were isolated from soil samples. One bacterial strain, designated as M 51, appeared to be particularly active towardsF. oxysporum f. sp.dianthii, in vitro andin vivo and it was inhibitoryin vitro to three otherFusarium spp. used. Tests to find if there was protection against fusarium wilt were carried out by three different methods of inoculation of the cuttings: a) dipping of cuttings for ten minutes in bacterial suspension; b) spraying of suspension on perlite where the rooted cuttings were planted; c) spraying the greenhouse bench rooting boxes, where the non-rooted cuttings were planted, with bacterial suspension. Following this all the cuttings were transplanted into soil naturally highly infested withFusarium oxysporum f. sp.dianthii (3000 units/g). Good protection against fusarium wilt was obtained for cuttings inoculated by method (b). However protection decreased gradually about 60 days after they were transplanted; both control and inoculated cuttings showed a comparable mortality rate. Method of inoculation and the development of the protective effect are discussed.     

Mycorrhizal fungi induced activation of tomato defense system mitigates Fusarium wilt stress

The fungus Fusarium oxysporum f. sp. lycopersici (FOL) is known to cause vascular wilt on tomato almost over the world. Inoculation of FOL reduced plant growth and increased wilt of tomato. The following study examined the possible role of arbuscular mycorrhizal fungi (AMF) consortium comprising of Rhizophagus intraradices, Funneliformis mosseae and Claroideoglomus etunicatum against FOL in tomato and explored in an inducing plant systemic defense. AMF inoculation reduced the wilt disease within vascular tissue and in vivo production of fusaric acid was observed which may be responsible in reduced wilting. FOL had an antagonistic effect on AMF colonization, reduced the number of spores, arbuscules and vesicles. AMF also inhibited the damage induced by Fusarium wilt through increasing chlorophyll contents along with the activity of phosphate metabolising enzymes (acid and alkaline phosphatases). Moreover, tomato plants with mycorrhizal inoculation showed an increase in the level of antioxidant enzymes including glutathione reductase, catalase, and etc. with an ultimate influence on the elimination of reactive oxygen species. Moreover, rise in phosphatase along with antioxidant enzymatic systems and enhanced photosynthetic performance contributed to induced resistance against FOL in tomato.     

The effect of root-knot nematode on vascular resistance to Fusarium oxysporum f. sp. vasinfectum in the stems of cotton plants

The effect of root-knot nematode (Meloidogyne incognita) on external wilt symptoms and on the cotton plant's vascular response to stem-inoculation with Fusarium oxysporum f. sp. vasinfectum was investigated. Wilt symptoms were more severe in all plants inoculated with both organisms than with the fungus alone but relative wilt resistance of the cultivars was maintained. Greater symptom severity was associated with greater fungal proliferation in the stele and this was related to the ability of the nematode to reduce the efficiency of vascular occlusion. The nematode had no effect on the accumulation of infection-induced terpenoid aldehyde compounds.     

Effect of nitrogen nutrition on Fusarium wilt of tomato plants

The role of nitrate-nitrogen (NO₃-N) in relation to the development of tomato wilt caused by Fusarium oxysporum f. sp. lycopersici R1 was studied. Plants receiving 284 μg/ml nitrogen in the nutrient solution exhibited the same severe wilt symptoms as the control plants in soil. Disease decreased with increasing nitrogen levels (420, 630 and 1050 μg/ml). Apparently, the plants were also less susceptible to the disease when the concentration of nitrogen was 70 μg/ml, i. e. below the optimal level (284 μg/ml). Protein content in tissues of plants grown with different amounts of nitrogen was also determined. High nitrogen levels, which decreased disease severity, increased the protein content in leaf tissues. Of 17 amino acids only proline content increased with increasing nitrogen supply. High doses of NO₃–N decreased the phenol content and the activity of peroxidase in stem and leaf tissues of tomato plants. It is suggested that phenolic compounds and the activity of peroxidase are not significant in the resistance of tomato to Fusarlum wilt associated with high nitrogen supply.     

Insight into the molecular requirements for pathogenicity of Fusarium oxysporum f. sp. lycopersici through large-scale insertional mutagenesis

Background  

Fusarium oxysporum f. sp. lycopersici is the causal agent of vascular wilt disease in tomato. In order to gain more insight into the molecular processes in F. oxysporum necessary for pathogenesis and to uncover the genes involved, we used Agrobacterium-mediated insertional mutagenesis to generate 10,290 transformants and screened the transformants for loss or reduction of pathogenicity.     

Biocontrol efficacy of Trichoderma asperellum MSST against tomato wilting by Fusarium oxysporum f. sp. lycopersici

Tomato is a popular vegetable widely grown in the tropics, which is mainly attacked by fusarium wilt incited by Fusarium oxysporum f. sp. lycopersici (FOL). In the present scenario, an ecofriendly alternative strategy such as use of fungi from rhizosphere is being explored to combat the phytopathogen invasion. This study was carried out to evaluate the efficacy of Trichoderma asperellum MSST to promote the growth and yield parameters of tomato S-22, a susceptible variety. This study was also undertaken to manage fusarium wilt disease under in vitro and in vivo conditions. Significant increase in vegetative parameters like root length, shoot length, plant weight and chlorophyll content 60 days after sowing (DAS) was observed. There was reduction in the incidence of fusarium wilt in tomato up to 85%. Increase in the level of total phenol, peroxidase, polyphenoloxidase and phenylalanine ammonium lyase activity at 10th day of pathogen inoculation showed enhancement of plant defence mechanism by T. asperellum MSST against FOL. Overall study revealed that isolate MSST was proven to be potential biocontrol agent showing induced resistance against FOL.     

Fusaric acid contributes to virulence of Fusarium oxysporum on plant and mammalian hosts

Fusaric acid (FA) is amongst the oldest identified secondary metabolites produced by Fusarium species, known for a long time to display strong phytotoxicity and moderate toxicity to animal cells; however, the cellular targets of FA and its function in fungal pathogenicity remain unknown. Here, we investigated the role of FA in Fusarium oxysporum, a soil‐borne cross‐kingdom pathogen that causes vascular wilt on more than 100 plant species and opportunistic infections in humans. Targeted deletion of fub1, encoding a predicted orthologue of the polyketide synthase involved in FA biosynthesis in F. verticillioides and F. fujikuroi, abolished the production of FA and its derivatives in F. oxysporum. We further showed that the expression of fub1 was positively controlled by the master regulator of secondary metabolism LaeA and the alkaline pH regulator PacC through the modulation of chromatin accessibility at the fub1 locus. FA exhibited strong phytotoxicity on tomato plants, which was rescued by the exogenous supply of copper, iron or zinc, suggesting a possible function of FA as a chelating agent of these metal ions. Importantly, the severity of vascular wilt symptoms on tomato plants and the mortality of immunosuppressed mice were significantly reduced in fub1Δ mutants and fully restored in the complemented strains. Collectively, these results provide new insights into the regulation and mode of action of FA, as well as on the function of this phytotoxin during the infection process of F. oxysporum.     

Biocontrol of Fusarium and Verticillium Wilt of Tomato by Penicillium oxalicum under Greenhouse and Field Conditions

Treatments with conidia of Penicillium oxalicum produced in a solid‐state fermentation system were applied at similar densities (6 × 10⁶ spores/g seedbed substrate) to tomato seedbeds in water suspensions (T1: 5 days before sowing, or T2: 7 days before transplanting; 15 days after sowing), or in mixture with the production substrate (T3: 7 days before transplanting; 15 days after sowing). Treatments T2 and T3 significantly (P = 0.05) reduced fusarium wilt of tomato in both greenhouse (artificial inoculation) (33 and 28%, respectively) and field conditions (naturally infested soils) (51 and 72%, respectively), while treatment T1 was efficient only in greenhouse (52%). Verticillium wilt disease reduction was obtained with T3 in two field experiments (56 and 46%, respectively), while T1 and T2 reduced disease only in one field experiment (52% for both T1 and T2). Treatment with conidia of P. oxalicum plus fermentation substrate (T3) resulted in better establishment of a stable and effective population of P. oxalicum in seedbed soil and rhizosphere providing populations of approx. 10⁷ CFU/g soil before transplanting. Results indicate that it will be necessary to apply P. oxalicum at a rate of approx. 10⁶–10⁷ CFU/g in seedbed substrate and rhizosphere before transplanting for effective control of fusarium and verticillium wilt of tomato, and that formulation of P. oxalicum has a substantial influence on its efficacy.