Nonpathogenic Fusarium Oxysporum Research Articles

Induced Resistance in Tomato Plants Against Fusarium Wilt Invoked by Nonpathogenic Fusarium, Chitosan and Bion

The potential of. nonpathogenic Fusarium oxysporum strain Avr5, either alone or in combination with chitosan and Bion, for inducing defense reaction in tomato plants inoculated with F. oxysporum f. sp lycopersici, was studied in vitro and glasshouse conditions. Application Bion at concentration of 5, 50, 100 and <TEX>$500{\mu}g$</TEX>/ml, and the highest concentration of chitosan reduced in vitro growth of the pathogen. Nonpathogenic F. oxysporum Avr5 reduced the disease severity of Fusarium wilt of tomato in split plants, significantly. Bion and chitosan applied on tomato seedlings at concentration <TEX>$100{\mu}g$</TEX> a.i./plant; 15, 10 and 5 days before inoculation of pathogen. All treatments significantly reduced disease severity of Fusarium wilt of tomato relative to the infected control. The biggest disease reduction and increasing tomato growth belong to combination of nonpathogenic Fusarium and Bion. Growth rate of shoot and root markedly inhibited in tomato plants in response to tomato Fusarium wilt as compared with healthy control. These results suggest that reduction in disease incidence and promotion in growth parameters in tomato plants inoculated with nonpathogenic Fusarium and sprayed with elicitors could be related to the synergistic and cooperative effect between them, which lead to the induction and regulation of disease resistance. Combination of elicitors and non-pathogenic Fusarium synergistically inhibit the growth of pathogen and provide the first experimental support to the hypothesis that such synergy can contribute to enhanced fungal resistance in tomato. This chemical could provide a new approach for suppression of tomato Fusarium wilt, but its practical use needs further investigation.

Efficacy of chemical and fluorescent protein markers in studying plant colonization by endophytic non-pathogenic Fusarium oxysporum isolates

In studying plant colonization by inoculated Fusarium oxysporum endophytes, it is important to be able to distinguish inoculated isolates from saprophytic strains. In the current study, F. oxysporum isolates were transformed with the green (GFP) and red fluorescent protein (DsRed) genes, and benomyl- and chlorate-resistant mutant isolates were also developed. The benomyl- and chlorate-resistant mutants, and the fluorescently labelled transformants, were able to grow on potato dextrose agar amended with 20 mg Benlate® l−1, 30 g chlorate l−1 and 150 μg hygromycin ml−1, respectively. Single spores of all mutants remained stable after several transfers on non-selective media. Most mutants and transformants produced colony diameters that did not differ significantly from that of their wild-type progenitors after 7 days of growth on non-selective media. Few mutants, however, had growth rates that were either slower or faster than for their wild-types. Plant colonization studies showed that root and rhizome tissue colonization by most benomyl- and chlorate-resistant mutants was similar to that of their wild-type isolates. Unlike GFP transformants, DsRed transformants were difficult to visualize in planta. Both the mutants and transformants can be used for future studies to investigate colonization, distribution and survival of biocontrol F. oxysporum endophytes in banana plants.

Mode of action of a non-pathogenic Fusarium oxysporum strain against Verticillium dahliae using Real Time QPCR analysis and biomarker transformation

Verticillium wilt is a devastating disease of a wide range of herbaceous and woody plant hosts. It is incited by the soilborne fungus Verticillium dahliae. Management strategies are mainly focused on preventive measures. In a previous study, the efficacy of a non-pathogenic Fusarium oxysporum strain, designated as F2, isolated from a suppressive compost amendment, has been reported to reduce Verticillium wilt symptom development in eggplants under greenhouse and field conditions; in addition, antibiosis or parasitism were ruled out by using a dual culture test. In the present study, we investigated the mode of action of F2 against V. dahliae. For this purpose, the F2 and V. dahliae strains were transformed with the EGFP and DsRed2 reporter genes respectively, so as to visualize their presence on the root surface of eggplants. In addition, the ramification of both fungi into the plant vascular system was monitored by Real Time QPCR analysis. It was shown that F2 colonizes the root surface along the intercellular junctions excluding V. dahliae from the same ecological niche. In parallel, QPCR analysis showed that application of F2 reduces the levels of V. dahliae vascular colonization along with the disease severity. In a split root experiment it was demonstrated that F2 does not trigger the defense mechanisms of eggplants against V. dahliae. Therefore, it seems that competition for space or nutrients on the root surface are the main mechanism of action of F2 against V. dahliae.

RFLP analysis of rDNA-ITS regions of native non-pathogenicFusarium oxysporumisolates and their field evaluation for the suppression of Fusarium wilt disease of banana

Fusarium wilt caused by Fusarium oxysporum f. sp. cubense is the most devastating disease of banana affecting commercial cultivars grown worldwide. An attempt has been made to identify antagonistic, non-pathogenic Fusarium oxysporum (npFo) isolates from banana soil. A total of 200 rhizosphere soil samples were collected from different commercial cultivars, as well as wild bananas. Forty Fusarium isolates were recovered, of which 33 were identified as Fo based on mycelial and spore characters. The identity of Fo isolates was confirmed by specific primers FOF1 and FOR1 inPCR reactions. The 33 isolates were confirmed as non-pathogenic following inoculation studies. Molecular characterisation of the npFo isolates was determined by ribosomal internally transcribed spacer (ITS) restriction fragment length polymorphism. Pathogenic isolates were also included for comparison. Among the npFo isolates, 14 ITS haplotypes were observed of which groups 6 and 10 were major groups consisting of 6 and 5 npFo isolates, respectively. The in vitro evaluation of these npFo isolates by dual culture plate and spore germination technique against the Fusarium wilt pathogen revealed the most inhibiting isolates were Ro-3 and Ra-1. Under pot culture studies, these npFo isolates were evaluated by application: (i) at planting; (ii) at planting + 2 months after planting; and (iii) at planting + 2 months after planting + 4 months after planting, in both tissue-cultured as well as sucker-derived plants of the cultivar Rasthali. The Fusarium wilt severity score observed after 6 months demonstrated that the application of Ro-3 three times resulted in the reduction of Fusarium wilt severity by up to 89% as well as increasing the plant growth parameters significantly when compared with the pathogen-inoculated control plants. Field application of these npFo isolates also reduced Fusarium wilt disease by 80%. The results of this study suggest that npFo strain Ro-3 could be used for the management of Fusarium wilt disease of banana to sustain banana production.

Germination of Fusarium oxysporum in root exudates from tomato plants challenged with different Fusarium oxysporum strains

The response of microconidia from pathogenic and non-pathogenic Fusarium oxysporum to root exudates from tomato plants inoculated with different pathogenic and non-pathogenic F. oxysporum strains was studied. Root exudates from non-inoculated tomatoes highly stimulated the microconidial germination of the two tomato pathogens, F. oxysporum f.sp. lycopersici strain Fol 007 and F. oxysporum f.sp. radicis-lycopersici strain Forl 101587. In root exudates from tomato plants challenged with the pathogen Fol 007 the microconidial germination of Fol 007 was increased, whereas in root exudates from plants challenged with Forl 101587 the microconidial germination of Fol 007 was reduced. Root exudates of tomato plants challenged with the non-pathogenic unspecific F. oxysporum strain Fo 135 and the biocontrol strain Fo 47 clearly reduced microconidial germination of the pathogenic strain Forl 101587. Moreover, the microconidial germination rate of the biocontrol strain Fo 47 was increased in the presence of root exudates of tomato plants challenged with the tomato wilt pathogen Fol 007. These results indicate that pathogenic and non-pathogenic F. oxysporum strains alter the root exudation of tomato plants differently and consequently the fungal propagation of pathogenic and non-pathogenic F. oxysporum strains in the rhizosphere is affected differently.

Defense-related gene expression in susceptible and tolerant bananas ( Musa spp.) following inoculation with non-pathogenic Fusarium oxysporum endophytes and challenge with Radopholus similis

Radopholus similis is a major pest of East African highland cooking bananas ( Musa spp.) in Uganda. Non-pathogenic Fusarium oxysporum endophytes, isolated from bananas in farmers’ fields, have shown potential to reduce R. similis numbers in tissue culture banana. The mechanism through which endophytes confer resistance to nematodes has previously been demonstrated to involve induced resistance. In this study, the expression of eight defense-related genes in banana was investigated using quantitative real-time reverse transcription PCR. Plants of susceptible (cv. Nabusa, genomic group AAA-EA) and tolerant (cv. Kayinja, genomic group ABB) banana cultivars were inoculated with endophytic F. oxysporum strain V5w2. Gene expression levels were analysed following endophyte inoculation and nematode challenge. Endophyte colonization of roots of the tolerant cultivar induced transient expression of POX and suppressed expression of PR-3, lectin, PAE, PAL and PIR7A. Catalase and PR-1 activities were up-regulated in the tolerant cultivar 33 days after endophyte colonization of roots, but their expressions were further up-regulated following nematode challenge. Apart from POX and lectin, the other genes analysed were not responsive to endophyte colonization or R. similis challenge in the susceptible cv. Nabusa. This is the first report of endophyte-induced defense-related gene expression in banana.

Fusarium equiseti GF191 as an effective biocontrol agent against Fusarium crown and root rot of tomato in rock wool systems

Six isolates of plant growth-promoting fungi (PGPF), non-pathogenic Fusarium oxysporum, and five isolates of bacteria were tested in hydroponic rock wool systems as potential biocontrol agents of Fusarium crown and root rot (FCRR) of tomato caused by Fusarium oxysporum f. sp. radicis-lycopersici (FORL). PGPF Fusarium equiseti proved the most effective organism in controlling FCRR, and the reduction rate by F. equiseti against FCRR was consistently high and significant in four experiments. The numbers of colony-forming units of FORL per gram fresh weight of stems were significantly reduced ( P=0.05) in plants treated with F. equiseti. Stem extracts from F. equiseti-treated and pathogen-challenged plants significantly inhibited the germination and germ tube length of FORL microconidia. Moreover, stem extracts from F. equiseti-treated plants (not treated with pathogen) significantly inhibited the germination of FORL microconidia and production of FORL budding-cells.

Solid-state cultures of Fusarium oxysporum transform aromatic components of olive-mill dry residue and reduce its phytotoxicity

The present study mainly investigated the ability of solid-state cultures of the non-pathogenic Fusarium oxysporum strain BAFC 738 to transform aromatic components to reduce the phytotoxicity in olive-mill dry residue (DOR), the waste from the two-phase manufacturing process. Lignin, hemicellulose, fats and water-soluble extractives contents of DOR colonized by the fungus for 20 weeks were reduced by 16%, 25%, 71% and 13%, respectively, while the cellulose content increased by 25%. In addition, the ethyl acetate-extractable phenolic fraction of the waste was reduced by 65%. However, mass-balance ultra-filtration and size-exclusion chromatography experiments suggested that the apparent removal of that fraction, mainly including 2-(3,4-dihydroxyphenyl)ethyl alcohol and 2-(4-hydroxyphenyl)ethyl alcohol, was due to polymerization. Mn-peroxidase and Mn-independent peroxidase activities were found in F. oxysporum solid-state cultures, while laccase and aryl alcohol oxidase activities were not detected. Tests performed with seedlings of tomato ( Lycopersicum esculentum L.), soybean ( Glycine maximum Merr.), and alfalfa ( Medicago sativa L.) grown on soils containing 6% (w/w) of bioconverted DOR (kg soil) −1 showed that the waste’s phytotoxicity was removed by 20 weeks-old fungal cultures. By contrast, the same material exhibited a high residual toxicity towards lettuce ( Lactuca sativa L.).

Influence of the mutualistic endophyte Fusarium oxysporum 162 on Meloidogyne incognita attraction and invasion

Abstract The non-pathogenic endophytic Fusarium oxysporum strain 162 (FO162) has been selected for its capacity to reduce root-knot nematode galling on tomato. The objectives of this study were to investigate the influence of this fungus on invasion of tomato roots by Meloidogyne incognita. The number of M. incognita that invaded roots of plants treated with FO162 was significantly lower (reductions of 36.0-55.9%) than in the non-treated controls in linked twin-pot chambers. The results demonstrated that colonisation of the roots by FO162 affects the number of M. incognita that penetrate the roots. The results of a choice test in the absence of a plant indicated that the tomato root exudates collected from plants colonised by FO162 are either less attractive or they exude substances that have repellent activity toward M. incognita juveniles.

Identification and characterization of non-pathogenic Fusarium oxysporum capable of increasing and decreasing Fusarium wilt severity

Fusarium wilt of banana is a potentially devastating disease throughout the world. Options for control of the causal organism, Fusarium oxysporum f.sp. cubense ( Foc) are limited. Suppressive soil sites have previously been identified where, despite the presence of Foc, Fusarium wilt does not develop. In order to understand some aspects of this disease suppression, endophytic Fusarium oxysporum isolates were obtained from banana roots. These isolates were genetically characterized and compared with an isolate of Fusarium oxysporum previously identified as being capable of suppressing Fusarium wilt of banana in glasshouse trials. Three additional isolates were selected for glasshouse trials to assess suppression of Fusarium wilt in two different cultivars of banana, Cavendish and Lady Finger. One isolate (BRIP 29089) was identified as a potential biocontrol organism, reducing the disease severity of Fusarium wilt in Lady Finger and Cavendish cultivars. Interestingly, one isolate (BRIP 45952) increased Fusarium wilt disease severity on Cavendish. The implications of an isolate of Fusarium oxysporum, non-pathogenic on banana, increasing disease severity and the potential role of non-pathogenic isolates of Fusarium oxysporum in disease complexes are discussed.

Improved Colonization of East African Highland Musa Tissue Culture Plants by Endophytic Fusarium oxysporum

Non-pathogenic endophytic Fusarium oxysporum inoculated into banana (Musa spp.) tissue culture plants can provide protection against banana weevils (Cosmopolites sordidus) and nematodes (Radopholus similis). The degree of control probably depends, in part, upon the level of endophyte establishment following inoculation. In this study, we compared three methods of inoculating endophytic fungi into eight week-old tissue culture plants: (1) Dipping the roots and rhizomes in a spore suspension, (2) dipping the roots and rhizomes in a spore suspension after the plants had been grown in an additional nutrient solution to enhance root growth, and (3) using a solid substrate inoculum after plants were grown in an additional nutrient solution. Irrespective of the inoculation method, rhizomes were colonized to a higher extent (91%) than roots (64%). The use of a solid substrate inoculum resulted in a higher percentage root colonization (96%) than when plants were dipped in a spore suspension. When roots and rhizomes were dipped in a spore suspension, root colonization was higher (71%) for plants grown in an additional nutrient solution than for plants obtained directly from rooting medium (49%). Use of a solid substrate inoculum and increased root development by growing the plants in a nutrient solution are necessary to maximize root colonization by fungal endophytes.

Selection of endemic nonpathogenic endophytic Fusarium oxysporum from bean roots and rhizosphere competent fluorescent Pseudomonas species to suppress Fusarium-yellow of beans

A less resource intensive procedure was used to select an endophytic non-pathogenic isolate of Fusarium oxysporum (NPFo) and an antibiotic producing rhizosphere/rhizoplane (RS-RP) competent fluorescent pseudomonad to suppress Fusarium yellow (F. oxysporum (Schlecht) f. sp. phaseoli Kendrick and Snyder) of bean (Phaseolus vulgaris L.). Using the combined criteria of in vitro and in planta competitiveness resulted in selection of three isolates for determination of their disease suppressing attributes. Several isolates of fluorescent pseudomonads obtained from RS-RP and internal root tissues of bean were tested for antibiosis in vitro. The isolates producing large inhibition zones were selected to determine RS-RP competence through seed treatment. The three most competent isolates were tested for their capacity to reduce root colonization by the pathogen, which resulted in the selection of only one isolate, identified as Pseudomonas putida (Trevisan) Migula biotype A. The NPFo isolates Fo10 and Fo6 endophytically occupied about 80% of the root length compared to only 60% by the isolate Fo16. Mixing chlamydospores of Fo10 in non-sterile field soil, reduced RS-RP and internal root colonization by the pathogen by 95 and 89%, respectively. In the greenhouse trial, mixing chlamydospores of either of the three NPFo isolates, the area under the chlorophyll retention curve (AUCRC) increased significantly with Fo10 compared to Fo6 or F16, over the positive control. The grain yield/plants from Fo10 treated soil was 177 or 291% higher than from Fo6-soil or positive control, respectively, but remained 50% lower than in pathogen-free negative control soil. In the field plots with pathogen ID of 850 ± 150/g soil, mixing chlamydospores of Fo10 (4250 ± 113/g), increased AUCRC and grain yield by about 100% over the control. However, if the bean seeds were treated with P. putida, the grain yield increased by 211% without further increase in AUCRC. The suppressive effect of Fo10 on disease development appeared to be related to its high saprophytic competitiveness with the pathogen in the soil matrix, which resulted in lower ID in the root zone, and parasitic competitiveness by reducing the availability of infection sites and internal root colonization by the pathogen. Seed treatment with P. putida provided additional protection in the root zone.

Influence of plant root exudates, germ tube orientation and passive conidia transport on biological control of fusarium wilt by strains of nonpathogenic Fusarium oxysporum.

In earlier studies, biological control of Fusarium wilt of cucumber induced by Fusarium oxysporum f. sp. cucumerinum was demonstrated using nonpathogenic strains C5 and C14 of Fusarium oxysporum. Strain C14 induced resistance and competed for infection sites whether roots were wounded or intact, whereas strain C5 required wounds to achieve biocontrol. In the current work, additional attributes involved in enhanced resistance by nonpathogenic biocontrol agents strains to Fusarium wilt of cucumber and pea were further investigated. In pre-penetration assays, pathogenic formae specials exhibited a significantly higher percentage of spore germination in 4-day-old root exudates of cucumber and pea than nonpathogens. Also, strain C5 exhibited the lowest significant reduction in spore germination in contrast to strain C14 or control. One-day-old cucumber roots injected with strain C14 resulted in significant reduction in germ tube orientation towards the root surface, 48-96 h after inoculation with F. o. cucumerinum spores, whereas strain C5 induced significantly lower spore orientation of the pathogen and only at 72 and 96 h after inoculation. In post-penetration tests, passive transport of microconidia of pathogenic and nonpathogens in stems from base to apex were examined when severed plant roots were immersed in spore suspension. In repeated trials, strain C5, F. o. cucumerinum and F. o. pisi were consistently isolated from stem tissues of both cucumber and pea at increasing heights over a 17 days incubation period. Strain C14 however, was recovered at a maximum translocation distance of 4.6 cm at day 6 and later height of isolation significantly declined thereafter to 1.2 cm at day 17. In pea stem, the decline was even less. Significant induction of resistance to challenge inoculation by the pathogen in cucumber occurred 72 and 96 h after pre-inoculation with biocontrol agents. Nonetheless, strain C14 induced protection as early as 48 h and the maximum resistance was reached at 96 h. The presented data confirm the previous findings that attributes important for nonpathogenic fusaria to induce resistant are: rapid spore germination and orientation in response to root exudate; active root penetration and passive conidia transport in stem to initiate defence reaction without pathogenicity and enough lag period between induction and challenge inoculation. Strain C14 possesses all these qualifications and hence its ability to enhance host resistance is superior than strain C5.

Biological Control Efficiency of Fusarium Wilt of Tomato by Nonpathogenic Fusarium oxysporum Fo-B2 in Different Environments

ABSTRACT Efficiency of nonpathogenic Fusarium oxysporum Fo-B2 for the biological control of Fusarium wilt of tomato, caused by F. oxysporum f. sp. lycopersici CU1, was examined in different environments: a growth chamber with sterile soil-less medium, a greenhouse with fumigated or nonfumigated soil, and nonfumigated field plots. Inoculation of Fo-B2 onto tomato roots significantly reduced the severity of disease, but the efficiency of disease suppression decreased as the experimental environment became less controlled. Relationships between the recovery of Fo-B2 from hypocotyls and the disease severity indicated that the biocontrol agent was most effective when it colonized vascular tissues intensively. Moreover, the degree of Fo-B2 colonization was greatly reduced when the seedlings were grown in nonfumigated soil. Dose-response models (negative exponential, hyperbolic saturation, and logistic) were fit to observed data collected over a range of inoculum densities of the pathogen and the antagonist; the logistic model provided the best fit in all environments. The ratios of an 50% effective dose parameter for Fo-B2 to that of CU1 increased as the environment became less controlled, suggesting that environmentally related efficiency reduction impacted the antagonist more than the pathogen. The results suggest that indigenous soil microbes were a primary factor negatively influencing the efficiency of Fo-B2. Therefore, early establishment of the antagonist in a noncompetitive environment prior to outplanting could improve the efficacy of biological control.

Potential of non-pathogenic Fusarium oxysporum isolates for control of Fusarium wilt of tomato

This study was done to evaluate the efficiency of non-pathogenic Fusarium oxysporum isolates (141/3, 233, 233/1, 245, 245/1, 251, 251/2, 251/5, and 257) in controlling vascular wilt caused by F. oxysporum f. sp. lycopersici, race 2 (isolates C-21A, TO11, and TO245) in tomato (Lycopersicon esculentum) cv. Viradoro seedlings. In order to determine the effect of non-pathogenic F. oxysporum isolates in tomato plants, the root system of 30-day-old seedlings was immersed in conidial suspensions (10(6) ml-1) of each isolate and the seedlings were transplanted to a cultivation substrate. Thirty-five days after transplanting it was observed that the non-pathogenic F. oxysporum isolates were not pathogenic to the cv. Viradoro nor did they affect seedling development. The efficiency of the non-pathogenic F. oxysporum isolates in controlling Fusarium wilt was determined by immersing the tomato seedling roots in the conidial suspension (10(6) ml-1) of each isolate and then transplanting them into substrates previously infested with isolates of F. oxysporum f.sp. lycopersici, race 2 (10(5) conidia ml-1 of substrate). Evaluations were performed 35 days after transplanting, for severity in scale with 1=healthy plant to 6=dead plant or plant showing vessel browning and wilted leaves up to the leader shoot and seedling height. The non-pathogenic F. oxysporum isolates were efficient in reducing the severity of the disease and maintaining normal plant development. These results provide evidence of the antagonistic activity of non-pathogenic F. oxysporum isolates in controlling vascular wilt caused by F. oxysporum f. sp. lycopersici race 2 in tomato.

Biological Control of Fusarium Wilt of Tomato – A Review

Fusarium wilt of tomato (Lycopersicum esculentum) caused by Fusarium oxysporum f.sp. lycopersici leads to high losses of tomatoes in many countries. Increasing restraints on the use of pesticides encourages adoption of use of alternative strategies of controlling the disease. Alternative strategies include use of biocontrol agents. Bacterial biocontrol agents with promising biocontrol activities against F. 0xysporum f. sp. lycopersici include Pseudomonas flourescens, P. putida, P. chlororaphis, P. alcaligenes, Bacillus subtilis, Streptomyces pulcher, S. canascens, S. citreoflourescens, S. corchorusii and S. mutabilis. Fungal biocontrol agents that are efficacious against F. oxysporum f.sp. lycopersici include Penicillium oxalicum, P. purpurogenum, Trichoderma harzianum, T. piluliferum, Pythium oligandrum, non-pathogenic Fusarium oxysporum Fo47 and Trametes versicolor. Biological control agents including bacteria and fungi have shown some promise for the control of Fusarium wilt of tomato. This paper discusses the mode of action of non-pathogenic strains of F. oxysporum, other fungi and bacteria including fluorescent pseudomonads in the management of Fusarium wilt of tomato. The relative importance of systemic induced resistance in the suppression of Fusarium wilt is discussed. Key Words: biocontrol, induced resistance, rhizosphere, tomato wilt. J. Trop. Microbiol Vol.1 2002: 74-78

Cell interactions between a nonpathogenic Fusarium oxysporum strain and root tissues of Eucalyptus viminalis

Nonpathogenic isolates of Fusarium oxysporum can be successful antagonists of pathogenic forms of the same fungal species that commonly attacks crop plants. The characteristics that distinguish nonpathogenic from pathogenic forms are not well understood. In this study, the mode of root colonization of Eucalyptus viminalis seedlings by a nonpathogenic F. oxysporum strain is described at the ultrastructural level. Root systems of E. viminalis plants were inoculated with nonpathogenic F. oxysporum strain Fo47 in an in vitro model system. Changes in the occurrence of nonesterified and methyl-esterified pectins in colonized E. viminalis roots were evaluated by in situ immunolabeling using two monoclonal antibodies, JIM 5 and JIM 7. Modes of penetration and root colonization patterns in E. viminalis seedlings by the nonpathogenic fungus were similar to those described for pathogenic forms of F. oxysporum. However, root interactions differed in that the nonpathogenic fungus did not induce host tissue damage. No papilla-like appositions were observed in host cells in response to invading hyphae, which did not disrupt the host plasma membrane in many cases, suggesting that a biotrophic relationship was established. Root colonization by the nonpathogenic strain did not induce alteration in JIM 7 labeling of methyl-esterified pectin in E. viminalis cell walls, whereas nonesterified pectin was detected to a significantly greater extent in cell walls of roots colonized by the fungus. Pectin components decreased slightly only at points of hyphal contact with host cells. Because nonpathogenic strains utilize pectin in pure culture, host control over enzyme activity or production by the fungi may at least partly explain their compatible interactions with host tissues.

Effects of arbuscular mycorrhizal fungi and a non-pathogenic Fusarium oxysporum on Meloidogyne incognita infestation of tomato.

Arbuscular mycorrhizal (AM) fungi and non-pathogenic strains of soil-borne pathogens have been shown to control plant parasitic nematodes. As AM fungi and non-pathogenic fungi improve plant health by different mechanisms, combination of two such partners with complementary mechanisms might increase overall control efficacy and, therefore, provide an environmentally safe alternative to nematicide application. Experiments were conducted to study possible interactions between the AM fungus Glomus coronatum and the non-pathogenic Fusarium oxysporum strain Fo162 in the control of Meloidogyne incognita on tomato. Pre-inoculation of tomato plants with G. coronatum or Fo162 stimulated plant growth and reduced M. incognita infestation. Combined application of the AM fungus and Fo162 enhanced mycorrhization of tomato roots but did not increase overall nematode control or plant growth. A higher number of nematodes per gall was found for mycorrhizal than non-mycorrhizal plants. In synergisms between biocontrol agents, differences in their antagonistic mechanisms seem to be less important than their effects on different growth stages of the pathogen.

Effect of fusaric acid and phytoanticipins on growth of rhizobacteria and Fusarium oxysporum.

Suppression of soilborne diseases by biocontrol agents involves complex interactions among biocontrol agents and the pathogen and between these microorganisms and the plant. In general, these interactions are not well characterized. In this work, we studied (i) the diversity among strains of fluorescent Pseudomonas spp., Bacillus spp., and Paenibacillus sp. for their sensitivity to fusaric acid (FAc) and phytoanticipins from different host plants, (ii) the diversity of pathogenic and nonpathogenic Fusarium oxysporum isolates for their sensitivity to phytoanticipins, and (iii) the influence of FAc on the production of pyoverdine by fluorescent Pseudomonas spp. tolerant to this compound. There was a great diversity in the response of the bacterial strains to FAc; however, as a group, Bacillus spp. and Paenibacillus macerans were much more sensitive to FAc than Pseudomonas spp. FAc also affected production of pyoverdine by FAc-tolerant Pseudomonas spp. strains. Phytoanticipins differed in their effects on microbial growth, and sensitivity to a phytoanticipin varied among bacterial and fungal strains. Biochanin A did not affect growth of bacteria, but coumarin inhibited growth of Pseudomonas spp. strains and had no effect on Bacillus circulans and P. macerans. Conversely, tomatine inhibited growth of B. circulans and P. macerans. Biochanin A and tomatine inhibited growth of three pathogenic isolates of F. oxysporum but increased growth of three nonpathogenic F. oxysporum isolates. Coumarin inhibited growth of all pathogenic and nonpathogenic F. oxysporum isolates. These results are indicative of the complex interactions that can occur among plants, pathogens, and biological control agents in the rhizosphere and on the root surface. Also, these results may help to explain the low efficacy of some combinations of biocontrol agents, as well as the inconsistency in achieving disease suppression under field conditions.

Effects of Varying Environmental Conditions on Biological Control of Fusarium Wilt of Tomato by Nonpathogenic Fusarium spp.

ABSTRACT The influence of varying environmental and cropping conditions including temperature, light, soil type, pathogen isolate and race, and cultivar of tomato on biological control of Fusarium wilt of tomato by isolates of nonpathogenic Fusarium oxysporum (CS-20 and CS-24) and F. solani (CS-1) was evaluated in greenhouse and growth chamber experiments. Liquid spore suspensions (10(6)/ml) of the biocontrol isolates were applied to soilless potting mix at the time of tomato seeding, and the seedlings were transplanted into pathogen-infested field soil 2 weeks later. Temperature regimes ranging from 22 to 32 degrees C significantly affected disease development and plant physiological parameters. Biocontrol isolate CS-20 significantly reduced disease at all temperature regimes tested, yielding reductions of disease incidence of 59 to 100% relative to pathogen control treatments. Isolates CS-24 and CS-1 reduced disease incidence in the greenhouse and at high temperatures, but were less effective at the optimum temperature for disease development (27 degrees C). Growing plants under shade (50% of full light) versus full light affected some plant growth parameters, but did not affect the efficacy of biocontrol of any of the three bio-control isolates. Isolate CS-20 effectively reduced disease incidence (56 to 79% reduction) in four different field soils varying in texture (sandy to clayey) and organic matter content (0 to 3.2%). Isolate CS-1 reduced disease in the sandy and loamy soils (49 to 66% reduction), but was not effective in a heavy clay soil. Both CS-1 and CS-20 were equally effective against all three races of the pathogen, as well as multiple isolates of each race (48 to 66% reduction in disease incidence). Both isolates, CS-1 and CS-20, were equally effective in reducing disease incidence (66 to 80% reduction) by pathogenic races 1, 2, and 3 on eight different tomato cultivars containing varying levels of inherent resistance to Fusarium wilt (susceptible, resistant to race 1, or resistant to races 1 and 2). These results demonstrate that both these Fusarium isolates, and particularly CS-20, can effectively reduce Fusarium wilt disease of tomato under a variety of environmental conditions and have potential for further development.