The subtilisins of the invertebrate mycopathogens Verticillium chlamydosporium and Metarhizium anisopliae. are serologically and functionally related

Abstract The major extracellular proteases from the nematophagous fungus Verticillium chlamydosporium and the entomophagous fungus Metarhizium anisopliae , VCP1 and Pr1, respectively, are closely related both functionally and serologically. Antibodies raised against either enzyme cross-reacted with both antigens, suggesting that they have common epitopes. The VCP1 and Prl antisera labelled bovine pancreatic elastase and proteinase K, respectively. Neither antiserum reacted with commercial chymotrypsin. An antiserum to a serine protease from the closely related V. suchlasporium also cross-reacted with VCP1 and Prl. In contrast, a polyclonal antibody to an isoform of Pr1 exclusive to M. anisopliae isolate ME1 failed to recognize Prl from M. anisopliae V245 or VCP1. The N-terminal amino acid sequence of VCP1 revealed similarities with subtilisin-like enzymes from other fungi, but the closest match was with Pr1. The pure enzymes, VCP1 and Prl, failed to hydrolyse mono-aminoacyl-naphthylamide substrates but demonstrated dipeptidyl peptidase activity against Gly-Pro-βNA and Leu-Ala-βNA, respectively. These results are discussed in the context of specificity of invertebrate mycopathogens.     

Effect of temperature on in vitro interactions between Verticillium chlamydosporium and other Meloidogyne-associated micro-organisms

In vitro interaction between Verticillium chlamydosporium (Goddard) andbacteria or fungi was studied at 20 °Cand 15 °C. The frequency of antagonismwas lower at 15 °C for the bacteria, butnot for the fungi. Interaction was affected bytemperature for 33% of the bacteria and 47%of the fungi.     

Meloidogyne incognita Survival in Soil Infested with Paecilomyces lilacinus and Verticillium chlamydosporium

Meloidogyne incognita-infected tomato seedlings were transplanted into sterilized soil or unsterilized soil collected from 20 California tomato fields to measure suppression caused by Paecilomyces lilacinus, Verticillium chlamydosporium, and other naturally occurring antagonists. Unsterilized soils Q, A, and H contained 35, 39, and 55% fewer M. incognita second-stage juveniles (J2) than did sterilized soil 1 month after infected tomato seedlings were transplanted to these soils and placed in a greenhouse. Three months after infected seedlings were transplanted to unsterilized or sterilized soil, unsterilized soils K, L, and Q had 97, 62, and 86% fewer J2 than the corresponding sterilized soils. Unsterilized soils of M. incognita-infected seedlings that were maintained 1 month in a greenhouse followed by 1 or 2 months of post-harvest incubation contained J2 numbers equal to, or greater than, numbers in the corresponding sterilized soil. The most suppressive of the unsterilized soils, K and Q, were not infested with V. chlamydosporium. Paecilomyces lilacinus and V. chlamydosporium increased in colony forming units in unsterilized soil of all bioassays, but they were not associated with lower numbers of J2.     

Activity of Nemathorin,natural product and bioproducts against root-knot nematodes on tomatoes

An experimental study was carried out in pots to investigate the activity of Nemathorin, natural product and biopesticides against root-knot nematodes (RKNs) on tomatoes. Fosthiazate and abamectin proved to be the most effective treatments which suppressed the RKN population by 82.1%. Furthermore, arbuscular mycorrhizal fungus 2 (AMF2) was the superior treatment that reduced galls/root system followed by abamectin with the values of 72.5% and 67.2%, respectively. In addition, fosthiazate, cadusafos and crustacean2 gave the highest increase in the root length with the values of 55.8%, 54.6% and 54.6%, respectively. AMF2 was the most effective treatment which increases the root weight by 43.9%, while azadirachtin decreased the rootweight by 12.2% compared to untreated check. AMF2, cadusafos and crustacean2 not only increased the shoot length but also increased the shoot weight. Azadirachtin recorded the minimum increase in shoot system length and weight.     

New citriculture system suppresses native and augmented entomopathogenic nematodes

Since its first detection in 2005, the bacterial disease huanglongbing (HLB or citrus greening) has emerged as a critical threat to the citrus industry in Florida. An “Advanced Production System” (APS) could mitigate the impact of HLB by bringing citrus trees into production more quickly and economically than conventional citriculture methods. However, unlike conventional practices, APS fertigates plants daily, thereby changing the soil properties in ways that might impact soil biota. We tested the hypothesis that changes to soil properties caused by APS would affect the abundance of native entomopathogenic nematodes (EPNs) and/or the survival of augmented EPNs. The densities of organisms at different trophic levels were measured by real-time qPCR in three experiments conducted in an ongoing field experiment. Target organisms included 6 entompathogenic nematodes, 5 nematophagous fungi (NF) and a phoretic bacterium, Paenibacillus sp. Soil properties, free-living nematodes and citrus fibrous roots were also measured. Compared to soil under conventional citriculture (CC), APS increased soil pH and Mg content, while reducing the electrical conductivity, and content of K, Mn and Fe. The naturally occurring EPN Steinernema diaprepesi was 5 times less abundant in APS plots where these nematodes were more heavily encumbered by the phoretic bacterium Paenibacillus sp., which limits the foraging success of EPNs. In general, when EPNs were augmented in either treatment, fewer Steinernema riobrave than Heterorhabditis indica were recovered and recovery of both species declined rapidly over time. As seen with native S. diaprepesi, fewer augmented S. riobrave were recovered from APS plots in two of the three experiments, whereas the management system did not affect the recovery of H. indica. More of some endoparasitic and trapping NF were recovered from soil augmented with S. riobrave than with H. indica. However, variation in the responses of NF to the management systems suggests that these NF species were not primarily responsible for the steinernematid responses to APS. Although APS has the potential to reduce EPN populations and exacerbate herbivory by subterranean pests such as the root weevil Diaprepes abbreviatus, additional study of the physical causes of this effect may reveal ways to avoid the problem.     

Isolation and screening of fungi associated with eggs and females of root-knot nematodes and their biocontrol potential against Meloidogyne incognita in Bangladesh

A total of 297 fungal isolates belonging to 20 genera and 33 species were isolated and identified from eggs and females of Meloidogyne spp. in Bangladesh. The predominant genera were Fusarium, Aspergillus and Penicillium; and the significant ones were Purpureocillium, Trichoderma and Pochonia. The 24 well tissue culture plate screening technique was applied for pathogenicity tests against Meloidogyne incognita in vitro. The average percentages of egg parasitism, egg hatch inhibition and juvenile mortality varied significantly and were ranged from 8.2 to 64.9% (p = 0.05), 24.8 to 72.4% (p = 0.05), and 2.3 to 33.1% (p = 0.05), respectively. Two isolates of Purpureocillium lilacinum (PLSAU 1 and PLSAU 2) and one isolate of Pochonia chlamydosporia (PCSAU 1) reduced more than 60% average root galls of tomato, eggplant and cucumber in greenhouse experiments. This is the first investigation of fungi associated with nematodes in the country and their biological control potential against M. incognita.     

Association of Verticillium chlamydosporium and Paecilomyces lilacinus with Root-knot Nematode Infested Soil

Population densities of Meloidogyne incognita and the nematophagous fungi, Paecilomyces lilacinus and Verticillium chlamydosporium, were determined in 20 northern California tomato fields over two growing seasons. Paecilomyces lilacinus was isolated from three fields, V. chlamydosporium was isolated from one field, and both fungi were isolated from 12 fields. Verticillium chlamydosporium numbers were positively correlated with numbers of M. incognita and P. lilacinus. Paecilomyces lilacinus numbers were positively correlated with V. chlamydosporium numbers, but they did not correlate with M. incognita numbers. The correlation coefficients were low (R < 0.5) but significant (P < 0.05). All P. lilacinus and V. chlamydosporium field isolates parasitized M. incognita eggs in vitro. In a greenhouse study, numbers of V. chlamydosporium and P. lilacinus increased more in soils with M. incognita-infected tomato plants than in soil with uninfected tomato plants. After 10 weeks, the Pf/ Pi of second-stage juveniles in soils infested with P. lilacinus, V. chlamydosporium, and M. incognita was 47.1 to 295.6. The results suggest V. chlamydosporium and P. lilacinus are not effectively suppressing populations of M. incognita in California tomato fields.     

茄子黄萎病菌抗性根际芽孢杆菌的筛选与鉴定

利用离体抑菌圈法从茄子根际土壤筛选出8株对黄萎病菌大丽轮枝菌(Verticillium dahliae)抑菌效果明显的芽孢杆菌,其中F53菌株分泌几丁质酶和-β1,3-葡聚糖酶,对所检测的12种植物病原菌具明显的抑制作用,其发酵液热、酸碱稳定性强,用F53菌株制成的菌剂对茄子黄萎病盆栽试验防效为52.4%~75.8%,初步鉴定F53菌株为环状芽孢杆菌(Bacillus circulans)。     

The compatibility of the fungicide azoxystrobin with Pochonia chlamydosporia, a biological control agent for potato cyst nematodes (Globodera spp.)

The biological control potential of Pochonia chlamydosporia against root-knot ( Meloidogyne spp.) and cyst-forming ( Heterodera and Globodera spp.) nematodes is widely appreciated. In spite of this, little has been undertaken to determine the compatibility of this fungus with modern fungicides. A series of experiments were undertaken to investigate the effect of azoxystrobin on P. chlamydosporia . Initial Petri dish experiments found a significant reduction in the growth of P. chlamydosporia . EC₅₀ values were calculated at 7 and 14 mg L⁻¹, respectively, for hyphal growth and chlamydospore germination. A microcosm experiment based on counts of the colony-forming units of the re-isolated fungus showed a reduction in EC₅₀ values for azoxystrobin in soil after 12, 20 and 40 days incubation (1.25, 1.00 and 3.00 mg kg⁻¹ soil, respectively). There was evidence the fungus recovered over time in response to azoxystrobin application. This was also demonstrated in a glasshouse experiment where EC₅₀ values of 1.32 and 4.4 mg kg⁻¹ soil were obtained for 35 and 49 days after planting (DAP), respectively.     

Effect of potato used as a trap crop on potato cyst nematodes and other soil pathogens and on the growth of a subsequent main potato crop

A field experiment in which main‐crop potatoes were grown every other year was conducted on a sandy soil from 1994 to 1999. The aim of the experiment was to control soil‐borne pathogens of potato with ecologically sound methods. Potato grown as a trap crop from the end of April to the end of June (8 wk) was used to control potato cyst nematodes (PCN) (Globodera pallida), and its effects on other important soil pathogens and on the growth of a subsequent potato crop were also assessed. Additional experimental treatments were a potato crop from which the haulm was removed and a green manure crop. Three potato cultivars with different degrees of resistance to PCN were grown as the main crop. Duplicate sets of the experiment were run concurrently. The PCN were effectively controlled by the potato trap crop. When a highly resistant potato cultivar was grown as a main crop after the trap crop, the post‐harvest soil infestation was very low. When a moderately resistant cultivar was grown after the trap crop the soil infestation also remained low. When the trap crop was alternated with a susceptible potato cultivar as a main crop, soil infestation increased slightly, but the degree of control when compared with no trap crop averaged 96%. Soil infestation with root‐knot nematodes (mainly Meloidogyne hapla) increased when potato was grown as a trap crop, but soil infestation with the root‐lesion nematode Pratylenchus crenatus was not affected. Stem canker caused by Rhizoctonia solani was not affected by the trap crop but black scurf (sclerotia of R. solani) on tubers was reduced. Soil infestation with Verticillium dahliae declined in one of the duplicate sets of the experiment but not in the other. However, stem infections by V. dahliae were significantly decreased in both sets, although the effect depended on the PCN‐resistance level of the potato cultivar. When a highly resistant potato cultivar was grown Verticillium stem infections were not significantly affected, they were decreased with a moderately resistant cultivar but the decrease was most pronounced with a PCN‐susceptible cultivar. Senescence of a following potato crop was not influenced by the trap crop when a highly PCN‐resistant cultivar was grown, but it was delayed in the case of a moderately resistant or a susceptible cultivar, resulting in higher tuber yields for those cultivars. The experiment proved that a trap crop can be an alternative to chemical soil disinfection but, for several reasons, the potato itself is not an ideal crop for this purpose; a trap crop other than potato must be developed.     

Studies on the ability of two isolates of Bacillus thuringiensis,an isolate of Clonostachys rosea f. rosea and a diatomaceous earth product to control gastrointestinal nematodes of sheep

Abstract

Nematode parasites have developed resistance to anthelmintics. Biological control of gastrointestinal nematodes (GIN) in sheep is a promising non-chemical control method. Two experiments were done using Merino sheep. In each experiment, gender, initial egg count per gram (EPG) and initial body weight (BW) aided in placing animals into four groups, each of which was randomly fed with one of four treatments. Experiment 1 evaluated anthelmintic effects of Bacillus thuringiensis Berliner (Bt), Clonostachys rosea f. rosea Schroers (C. rosea) and diatomaceous earth (DE) in sheep. Bacillus thuringiensis and C. rosea were fed to sheep at a rate of 1 g kg^?1 BW, and DE was fed at 2% of sheep diet. Relative to the control, treatments had no effect (P>0.05) on EPG, but reduced (P<0.001) larvae per gram (LPG) in faecal culture. Efficacy varied with time (P<0.001). On Day 7, Bt, C. rosea and DE had efficacies of 76, 87 and 61%, respectively. In experiment 2, efficacy of feeding 1 g kg^?1 BW of C. rosea chlamydospores to sheep every day, every second day and every third day was tested. Daily feeding of fungal chlamydospores had no effect on EPG (P>0.05), but reduced (P<0.001) LPG (12±1.67) more than every second day (39±0.77) or third day (58±1.77). On Day 12, feeding daily, every second day and every third day had efficacies of 90, 63 and 49%, respectively. These studies suggest that each of Bt, C. rosea isolates and DE products has a potential to affect nematode larvae, and daily use of C. rosea had the highest effect as a biological control of nematodes in this study.     

A new control strategy for nematodes of sheep using chlamydospores of a fungus,Clonostachys rosea f. rosea,and an ethanolic extract of a plant,Ananas comosus

An experiment was conducted at Ukulinga Research Farm using 24 Merino sheep to test a combination of plant Ananas comosus (Ac) and fungus Clonostachys rosea (Cr) in controlling nematode parasites. Four treatments were assigned: 100 mg kg^?1 body weight (BW) ethanol extract of Ac, 1 g of Cr product kg^?1 BW, the combination of both treatments (AcCr), and a Control. Sheep within a treatment were paired and penned in individual paddocks, and rectal faecal samples were taken for counting of eggs per gram (EPG) of faeces and third-stage (L3) larvae. Grass samples were taken every 21 days for counting of L3 larvae. A. comosus and AcCr reduced (P < 0.001) EPG. A. comosus, C.rosea and AcCr reduced (P < 0.001) L3 counts. The efficacy of treatments increased with time (P < 0.001). C. rosea and AcCr reduced (P < 0.001) L3 on grass. In conclusion, AcCr is better at controlling nematodes than either Ac or Cr individually.     

云南省玉溪地区昆虫线虫资源分布调查及应用初探

昆虫线虫尤其是昆虫病原线虫可以部分替代化学农药防治有害昆虫。本文通过大蜡螟诱捕法对云南省玉溪市8县1区的昆虫线虫资源进行了调査。从270份土壤样品中分别得到57个昆虫线虫样本,分属于4个属,包括小杆科Rhabditidae的3个属,双胃科Diplogasteridae的1个属。结果表明,线虫的分布与土壤类型和植被类型密切相关,砂壤土、壤土中线虫检出率分别为100.0%、19.9%,较粘土12.1%的检出率高。在未受干扰的地块(林地和未耕地)线虫检出率为20.4%,受人类干扰的地块(烟田和农田)线虫检出率为16.1%,说明砂壤土和壤土适合昆虫线虫种群的建立,且在人活动较少的地块中线虫分布较多。用本土线虫在实验室条件下利用大蜡螟和蛴螬(棕色鳃金龟幼虫)对分离到的线虫进行致病力测定,结果表明:YN43(Oscheius sp.2)是大蜡螟最敏感的昆虫线虫品系,校正死亡率为76%;YN93(Oscheius sp.1)对蛴螬致病力最强,校正死亡率为54.5%,在防治云南地下害虫蛴螬方面具有很大的潜力。     

Evaluation of the effectiveness of a consortium of three plant-growth promoting rhizobacteria for biocontrol of cotton Verticillium wilt

We investigated the biocontrol efficacy of a consortium of three rhizobacteria (hereafter BBS) against cotton Verticillium wilt, along with the effect on plant growth, in a series of greenhouse and field experiments. BBS treatment inhibited germination of Verticillium dahliae spores by 88.4%. Under greenhouse conditions, BBS reduced Verticillium wilt by 86.1% compared to the untreated control, and promoted plant growth by 49.9%. In field experiments, nine L/acre of BBS suspension reduced Verticillium wilt by 76.0% and increased cotton yields by 13.7%. Soil inoculation with BBS also improved the breaking tenacity and uniformity index of harvested cotton. Soil properties improved with BBS treatment in field experiments, including an increase in organic matter and the availability of nitrogen (N), phosphorus (P) and potassium (K). There was no significant difference in the biocontrol efficacy of BBS among eight tested cotton cultivars.     

Biosurfactants are involved in the biological control of Verticillium microsclerotia by Pseudomonas spp

AIMS: To examine the effect of previously described bacterial antagonists on the viability of Verticillium microsclerotia in vitro and to elucidate the possible modes of action of bacterial strains in the suppression of Verticillium microsclerotia viability. METHODS AND RESULTS: A microplate assay was developed to test the suppressive effect of well-defined Pseudomonas spp. on the viability of Verticillium microsclerotia in vitro. Experiments using phenazine- and biosurfactant-deficient mutants indicated that biosurfactants and phenazine-1-carboxylic acid play a role in the suppression of microsclerotia viability by Pseudomonas spp. In addition, microsclerotia colonization tests revealed that Pseudomonas spp. are able to colonize the surface of the microsclerotia, but not the inner matrix. Growth response curves showed that the population levels of Pseudomonas spp. increased when they were in the vicinity of Verticillium microsclerotia, indicating that Pseudomonas spp. may utilize nutrients from the microsclerotia for their growth. CONCLUSIONS: Pseudomonas spp. seem to be good candidates for Verticilllium microsclerotia biocontrol. Biosurfactant production is one of the main mechanisms involved in their mode of action. SIGNIFICANCE AND IMPACT OF THE STUDY: This line of work may contribute to a better understanding of biological control agents and their working mechanisms.     

Hirsutella rhossiliensisand Verticillium chlamydosporium as Biocontrol Agents of the Root-knot Nematode Meloidogyne hapla on Lettuce

Hirsutella rhossiliensis and Verticillium chlamydosporium infected second-stage juveniles (J2) and eggs of Meloidogyne hapla, respectively, in petri dishes and in organic soil in pots planted to lettuce in the greenhouse. In vitro, H. rhossiliensis produced 78 to 124 spores/infected J2 of M. hapla. The number of J2 in roots of lettuce seedlings decreased exponentially with increasing numbers of vegetative colonies of H. rhossiliensis in the soil. At an infestation of 8 M. hapla eggs/cm³ soil, 1.9 colonies of H. rhossiliensis/cm³ soil were needed for a 50% decrease in J2 penetration of lettuce roots. Egg-mass colonization with V. chlamydosporium varied from 16% to 43% when soil was infested with 8 M. hapla eggs and treated with 5,000 or 10,000 chlamydospores of V. chlamydosporium/cm³ soil. This treatment resulted in fewer J2 entering roots of bioassay lettuce seedlings planted in the infested soils after harvesting the first lettuce plants 7 weeks after infestation with M. hapla. Hirsutella rhossiliensis (0 to 4.3 colonies/cm3 soil), V. chlamydosporium (500 to 10,000 chlamydospores/cm3 soil), or their combination, added to organic soils with 8 M. hapla eggs/cm³ soil, generally did not affect lettuce weight, root galling, or egg production of M. hapla. However, when lettuce was replanted in a mix of infested and uninfested soil (1:3 and 1:7, v:v), egg production was lower in soils with V. chlamydosporium than in soils without the fungus. Both fungi have potential to reduce the M. hapla population, but at densities below 8 eggs/cm³ soil.     

Experimental parasite community perturbation reveals associations between Sin Nombre virus and gastrointestinal nematodes in a rodent reservoir host

Individuals are often co-infected with several parasite species, yet measuring within-host interactions remains difficult in the wild. Consequently, the impacts of such interactions on host fitness and epidemiology are often unknown. We used anthelmintic drugs to experimentally reduce nematode infection and measured the effects on both nematodes and the important zoonosis Sin Nombre virus (SNV) in its primary reservoir (Peromyscus spp.). Treatment significantly reduced nematode infection, but increased SNV seroprevalence. Furthermore, mice that were co-infected with both nematodes and SNV were in better condition and survived up to four times longer than uninfected or singly infected mice. These results highlight the importance of investigating multiple parasites for understanding interindividual variation and epidemiological dynamics in reservoir populations with zoonotic transmission potential.     

Ultrastructure and germinability of Verticillium dahliae microsclerotia parasitized by Talaromyces flavus on agar medium and in treated soil

The interactions between microsclerotia (ms) of the fungal plant pathogen Verticillium dahliae and the mycoparasite Talaromyces flavus were followed in soil and on agar medium. Germinability of ms, which had been incubated for 14 days in soil treated with 0.5% of a T. flavus ‐ wheat bran preparation, decreased from 84% to 17%, as compared with 81% and 74% in untreated soil and in soil treated with a sterilized biocontrol preparation respectively. Germinability of ms which had been buried in treated soil for 4 days decreased to 70%, all ms being parasitized by T. flavus. Upon transfer of the ms to untreated soil for 10 more days, germinability decreased further to 20%, indicating that T. flavus continued to parasitize sclerotia in the untreated soil. Scanning electron micrographs showed heavy fungal colonization and typical T. flavus conidia on the surface of the ms buried in the treated soil, but not in control soils. Transmission electron micrographs of ms incubated with T. flavus on agar revealed parsitism involving invasion of some host cells by means of small penetration pegs; the host cell walls were mainly lysed at their site of contact with the parasite hyphal tips. Further colonization of the ms cells occurred simultaneously with the degradation of the invaded host cell contents, rather than the cell walls. Mycoparasitism of V. dahliae ms by T. flavus hyphae may be involved in the biological control of verticillium wilt disease.     

Phenylacetic acid-producing Rhizoctonia solani represses the biosynthesis of nematicidal compounds in vitro and influences biocontrol of Meloidogyne incognita in tomato by Pseudomonas fluorescens strain CHA0 and its GM derivatives

AIMS: The aim of the present investigation was to determine the influence of Rhizoctonia solani and its pathogenicity factor on the production of nematicidal agent(s) by Pseudomonas fluorescens strain CHA0 and its GM derivatives in vitro and nematode biocontrol potential by bacterial inoculants in tomato. METHODS AND RESULTS: One (Rs7) of the nine R. solani isolates from infected tomato roots inhibited seedling emergence and caused root rot in tomato. Thin layer chromatography revealed that culture filtrates of two isolates (Rs3 and Rs7) produced brown spots at Rf-values closely similar to synthetic phenylacetic acid (PAA), a phytotoxic factor. Filtrates from isolate Rs7, amended with the growth medium of P. fluorescens, markedly repressed nematicidal activity and PhlA'-'LacZ reporter gene expression of the bacteria in vitro. On the contrary, isolate Rs4 enhanced nematicidal potential of a 2,4-diacetylphloroglucinol overproducing mutant, CHA0/pME3424, of P. fluorescens strain CHA0 in vitro. Therefore, R. solani isolates Rs4 and Rs7 were tested more rigorously for their potential to influence biocontrol effectiveness of the bacterial agents. Methanol extract of the culture filtrates of PAA-producing isolate Rs7 resulting from medium amended with phenylalanine enhanced fungal repression of the production of nematicidal agents by bacteria, while amendments with zinc or molybdenum eliminated such fungal repression, thereby restoring bacterial potential to cause nematode mortality in vitro. A pot experiment was carried out, 3-week-old tomato seedlings were infested with R. solani isolates Rs4 or Rs7 and/or inoculated with Meloidogyne incognita, the root-knot nematode. The infested soil was treated with aqueous cell suspensions (10(8) CFU) of P. fluorescens strain CHA0 or its GM derivatives or left untreated (as a control). Observations taken 45 days after nematode inoculation revealed that, irrespective of the bacterial treatments, galling intensity per gram of fresh tomato roots was markedly higher in soil amended with isolate Rs4 than in Rs7-amended soils. Soil amendments with R. solani and the bacterial antagonists resulted in substantial reductions of the number of galls per gram of root. These results are contradictory to those obtained under in vitro conditions where culture filtrates of PAA-positive Rs7 repressed the production of nematicidal compounds. Plants grown in Rs7-amended soils, with or without bacterial inoculants, had lesser shoot and root weights than plants grown in nonamended or Rs4-amended soils. Moreover, amendments with Rs7 substantially retarded root growth and produced necrotic lesions that reduced the number of entry sites for invasion and subsequent infection by nematodes. Populations of P. fluorescens in the tomato rhizosphere were markedly higher in Rs7-amended soils. CONCLUSIONS: PAA-producing virulent R. solani drastically affects the potential of P. fluorescens to cause death of M. incognita juveniles in vitro and influences bacterial effectiveness to suppress nematodes in tomato roots. SIGNIFICANCE AND IMPACT OF THE STUDY: As most agricultural soils are infested with root-infecting fungi, including R. solani, it is likely that some PAA-producing isolates of the fungus may also be isolated from such soils. The inhibitory effect of PAA-producing R. solani on the biosynthesis of nematicidal agent(s) critical in biocontrol may reduce or even eliminate the effectiveness of fluorescent pseudomonads against root-knot nematodes, both in nursery beds and in field conditions. Introduction of bacterial inoculants, for the control of any plant pathogen, should be avoided in soils infested with PAA-producing R. solani. Alternatively, the agents could be applied together with an appropriate quantity of fungicide or chemicals such as zinc to create an environment more favourable for bacterial biocontrol action.