Suppression of rice blast,cabbage black leaf spot,and tomato bacterial wilt diseases by Meyerozyma guilliermondii TA-2 and the nature of protection

This study was conducted to evaluate the efficacy of yeast strain TA-2 for controlling rice blast, cabbage black leaf spot, and tomato bacterial wilt diseases. Microscopic and phylogenetic analyses based on rDNA-internal transcribed region (ITS) and rDNA-D1/D2 sequences indicated that yeast strain TA-2 is Meyerozyma guilliermondii. Pretreatment with TA-2 by soil drenching significantly reduced the severity of black leaf spot disease caused by Alternaria brassicicola and leaf blast disease caused by Magnaporthe oryzae. Symptom development of tomato bacterial wilt caused by Ralstonia solanacearum in both soil drench and needle inoculation tests was significantly reduced in TA-2-pretreated plants under soil drenching. Disease severity and R. solanacearum growth were significantly reduced in tomato plants pretreated with yeast culture, cell suspension, or culture filtrate of TA-2 under soil drenching. TA-2 does not produce antibiotics. The present study indicates that disease suppression is systemic, as the roots were treated with TA-2 and the pathogens were inoculated onto leaves or stems, thereby separating the two spatially. M. guilliermondii TA-2 could become a promising natural antimicrobial agent against rice blast, cabbage black leaf spot, and tomato bacterial wilt diseases and might be useful as an eco-friendly control measure, contributing to sustainable agriculture.     

生物质炭载体联合有益菌防控番茄土传青枯病的效果研究

土传青枯病是由青枯菌（Ralstonia solanacearum）引起的一种细菌性病害。根际有益细菌在青枯病的防控中发挥着重要作用,其在根际有效定殖是发挥生防作用的前提。以玉米秸秆、木块（松木）和稻壳为原料制成的3种生物质炭为有益菌Bacillus amyloliquefaciens T-5的载体,探究生物质炭对有益菌防控番茄土传青枯病效果的影响,并利用室内模拟试验探究生物质炭对青枯菌的吸附、固持以及对根系分泌物的吸附作用,旨在阐述施用生物质炭提升有益菌T-5抑制病原青枯菌能力的可能机制。温室试验结果表明：单独施用3种生物质炭均显著降低青枯病的发病率和根际青枯菌的数量,其中具有高比表面积的木块生物质炭的防控效率达到60.56%。3种生物质炭作为有益菌T-5的载体均能够显著提升有益菌T-5的根际定殖数量及其防病效率,其中木块生物质炭的提升效果最好。与仅接种青枯菌的对照相比,木块生物质炭与有益菌T-5组合处理的根际青枯菌数量降幅达97.42%;与单独有益菌T-5处理相比,有益菌T-5以木块生物质炭为载体使其根际定殖数量提高了5.71倍。进一步研究发现,木块生物质炭能够有效吸附青枯菌,吸附...     

The rhizosphere soil of diseased tomato plants as a source for novel microorganisms to control bacterial wilt

Plant growth promoting rhizobacteria (PGPRs) are used for biocontrol of bacterial wilt caused by Ralstonia solanacearum. They are commonly isolated from the rhizosphere of healthy plants and are scarce in the rhizosphere of diseased plants. We hypothesized that a pathogen-prevalent environment, such as the rhizosphere of infected plants, would be a good or better source for isolating PGPRs than the rhizosphere of healthy plants. In order for these PGPRs to survive successfully in a pathogen-prevalent environment, they must have particularly well-developed survival strategies under the stresses exerted by pathogen activities, which would be of value for their use as biocontrol agents. To test this hypothesis, R. solanacearum-antagonistic bacteria were screened from the rhizospheres of diseased and healthy tomato plants. In total, 110 rhizobacteria were isolated, 18 of which showed antagonism to R. solanacearum in vitro. Among the 18 antagonistic strains, 11 (out of 60) were from the rhizosphere of diseased plants, with inhibition diameter zones ranging from 11.2 to 15.2 mm, whereas 7 (out of 50) were from the rhizosphere of healthy plants, with inhibition diameter zones ranging from 11.5 to 30.5 mm. Strains WR4, WR21, and WR42 from diseased plants rhizosphere, and HR61, HR62, and HR92 from healthy plants rhizosphere, were chosen to investigate their biocontrol efficacies (BCEs) in greenhouse condition. Results showed that WR-isolates performed better in reducing disease incidence (DI) than those HR-isolates. Population densities of R. solanacearum in the rhizosphere soil and crown section of tomato plants were lower in WR-isolate treatments than those in HR-isolate treatments. The best biocontrol effect was achieved by inoculating the strain WR21, followed by WR4, WR42, HR92, HR62, and HR61. Root colonization test showed WR21 had the highest root-colonizing capacity compared with 5 other antagonists. BCEs were positively (r = 0.747) correlated with root-colonizing capacities, but were negatively (r = −0.797) correlated with inhibition zones. In conclusion, the rhizosphere of diseased tomato plants is a good reservoir of biocontrol bacteria.     

Effect of Ralstonia solanacearum on Mineral Nutrients and Infrared Temperatures in Two Tomato Cultivars

ABSTRACT

The objective of this study was to determine how the responses of two tomato cultivars to Ralstonia solanacearum relate to their leaf infrared temperature and acquiring of nutrients from soil. Tomato (Solanum lycopersicum L.) cultivars of disease susceptible-‘FL 47’ and resistant-‘H 7998’ were grown in soil inoculated with R. solanacearum. Bacterial wilt incidence, leaf infrared temperatures, and uptake of nutrients were measured for 28 d. In bacterial wilt-resistant cultivar ‘H 7998’, concentration of sulfur (S; +77%), calcium (Ca; +66%), boron (B; +60%) were found higher and nitrogen (N; ?26%) were found lower, compared with susceptible ‘FL 47’. Infrared temperatures were correlated with wilt percentage at 14 d, but not at 7 d. These results provide evidence that there is a correlation between bacterial wilt resistance and translocation of some nutrients in the shoots. Additionally, data indicates that the infrared thermometer could only detect wilting after obvious symptoms were visibly incited by R. solanacearum in tomato.     

Control of tomato bacterial wilt and root-knot diseases by Bacillus thuringiensis CR-371 and Streptomyces avermectinius NBRC14893

Ralstonia solanacearum and Meloidogyne incognita are two soilborne pathogens that cause serious damage and great losses in the production of tomato. For this purpose, a bacterial isolate, Bacillus thuringiensis CR-371, and an actinomyces isolate, Streptomyces avermectinius NBRC14893, were examined for their ability to protect tomato from root-knot nematode and bacterial wilt diseases under glasshouse conditions. Treatment of tomato roots with B. thuringiensis CR-371 and S. avermectinius NBRC14893 followed by challenge inoculation with R. solanacearum and M. incognita significantly decreased disease severity of bacterial wilt alone, root-knot nematode alone, or mixed infection by both pathogens compared to the control. Furthermore, pretreatment of tomato roots with B. thuringiensis CR-371 and S. avermectinius NBRC14893 significantly reduced bacterial proliferation of R. solanacearum both in pathogen alone inoculated plants and in plants co-inoculated with R. solanacearum and M. incognita. In conclusion, our results suggest that the treatment of tomato roots with B. thuringiensis CR-371 and S. avermectinius NBRC14893 simultaneously suppresses bacterial wilt and root-knot nematode diseases. Therefore, B. thuringiensis CR-371 and S. avermectinius NBRC14893 could provide new options for integrated pest management strategies against plant diseases, especially against bacterial-nematode disease complexes that cause synergistic yield losses.     

生物有机肥对田间蔬菜根际土壤中病原菌和功能菌组成的影响

利用实时荧光定量PCR方法对田间条件下连作番茄和辣椒施用生物有机肥(BOF)和常规施肥(CK)的根际土壤微生物中青枯病原菌和功能菌群(固氮菌和荧光假单胞菌)的数量进行定量研究.结果表明:与CK相比,BOF处理的番茄和辣椒产量分别提高了26.0％和19.9％,青枯病发病率分别降低了41.5％和44.7％,番茄和辣椒植株根际土壤固氮菌数量分别增加了23.5％和25.8％、荧光假单胞菌数量分别增加了29.5％和20.2％、病原菌数量分别减少了73.2％和90.1％.生物有机肥能够调控根际微生物区系的组成,降低土传病害的发病率,促进作物健康生长;实时荧光定量PCR方法能够快速准确地检测根际土壤中功能微生物种群数量变化.     

Physiological Responses of Calcium-Efficient and Calcium-Inefficient Tomato Genotypes to Differential Bacterial Wilt Conditions

Bacterial wilt (BW) disease, caused by Ralstonia solanacearum, can severely limit tomato (Solanum lycopersicum) production in southern United States. Tomato genotypes display variable responses to BW disease. The physiological mechanism underlying BW resistance is not well understood. In this study, experiments were conducted to compare two tomato genotypes, PI117566 [calcium (CA)–efficient] and PI109315 (Ca-inefficient), for wilting, shoot growth, final fresh weight, and shoot Ca concentrations when inoculated with R. solanacearum. The inoculation concentration of R. solanacearum varied from 0 to 10⁸ colony forming units/ml. Genotype PI109315 appear to be more BW-resistant compared with genotype PI117566 under sufficient Ca conditions. Furthermore, we found that genotype PI109315 had greater shoot growth and final fresh weight than genotype PI117566. These findings implied that Ca-efficient tomato genotypes may not play a key role in BW resistance of tomato under sufficient Ca conditions because Ca efficiency showed no effect on the suppression of BW.     

病原青枯菌土壤存活的影响因素研究进展

土传青枯病是一种毁灭性的细菌性病害,广泛分布于热带、亚热带和温带地区,严重威胁世界粮食安全。病原青枯菌主要从土壤中侵染作物根系,其在土壤中存活能力强,因此防治极为困难。明确病原青枯菌土壤存活的关键影响因素有助于开发高效阻控土传青枯病的措施。国内外学者在青枯菌的土壤存活方面开展了大量研究,但由于影响青枯菌土壤存活的因素复杂,而相关研究多围绕单一因素展开,缺乏针对青枯菌土壤存活规律和影响因素的系统性认识。本文系统梳理了青枯菌的自身特性（基因、行为和代谢产物）及土壤生物、非生物因素对其在土壤中存活的影响,阐明了青枯菌在寄主存在时土体存活、向寄主根表方向运动迁移时根际存活以及入侵寄主根系时根表存活的主要影响因子,以期为土传青枯病的系统阻控提供参考。     

Effects of Arbuscular Mycorrhizal Colonization on Microbial Community in Rhizosphere Soil and Fusarium Wilt Disease in Tomato

Fusarium wilt is caused by soil-borne pathogen Fusarium oxysporum. Tomato (Lycopersicon esculentum Mill.) is susceptible to Fusarium oxysporum f. sp. lycopersici race 1 and was infected with wilt disease. A pot experiment was conducted to investigate effects of inoculating arbuscular mycorrhizal (AM) fungus (Glomus etunicatium) on the microbial community in the rhizosphere soil and Fusarium wilt in tomato (cv. Oogatafukuju). The results indicated that AM fungal inoculation suppressed the Fusarium number in the rhizosphere soil of tomato and decreased the Fusarium wilt disease index. Compared to the control, AM fungal inoculation increased the actinomycete number but increased bacterial number. Bacterial and fungal numbers were high but actinomycetes number was low when tomato basal stems became discolored brown. Fusarium inoculation significantly suppressed development of AM colonization and decreased polyphenol oxidase (PPO) activity in leaves and roots of tomato. Inoculation with AM fungi and Fusarium maintained high PPO activity in leaves and roots. The AM colonization increased root growth of tomato, whereas Fusarium inoculation had no significant effect on tomato growth. These findings suggest that because AM fungal inoculation changes microbial communities and enhances PPO activity, it should suppress occurrence of Fusarium wilt in tomato.     

Evaluation of Bacillus-fortified organic fertilizer for controlling tobacco bacterial wilt in greenhouse and field experiments

Bacterial wilt caused by Ralstonia solanacearum is one of the most serious tobacco diseases worldwide, and no effective control measures are available to date. Three Bacillus isolates (Bacillus amyloliquefaciens SQR-7 and SQR-101 and Bacillus methylotrophicus SQR-29) were obtained from the rhizosphere soil of tobacco. These bacilli exhibited strong inhibition against R. solanacearum and produced indole acetic acid and siderophores. The three antagonistic strains were used to fortify organic fertilizers to produce bioorganic fertilizers (BOFs named for each isolate) for the control of tobacco bacterial wilt. The application of BOFs delayed wilt development and effectively decreased the disease incidence under both greenhouse and field conditions. The tobacco bacterial wilt control efficacy was 44.3%, 70.5%, and 85.1% using BOF101, BOF29, and BOF7 in the greenhouse. Although the control efficacies in the field were lower, the application of BOF7 still achieved 58.0% and 56.2% control efficacies in two years field experiments. The application of bioorganic fertilizer significantly (p < 0.001) repressed the pathogen R. solanacearum in soil in both pot and field experiments, though the abundance of R. solanacearum increased as during the growth period of the tobacco plants. In general, the populations of the antagonistic bacterial strains declined after soil application and as the tobacco plants grew; however, the density of SQR-7 and SQR-29 in the rhizosphere soil remained at a high level (≥10⁶ cfu/g) in the later growth stages. Additionally, the application of bioorganic fertilizers promoted tobacco growth and increased the leaf yield.     

健康与罹患青枯病的番茄土壤细菌群落特征比较

应用实时荧光定量PCR及MiSeq高通量测序技术,全面地研究了连作番茄田块中健康与感染青枯病植株周围土体及根际土壤细菌群落结构和组成.结果表明:健康番茄土体土壤的pH及全碳含量显著高于感病番茄土体土壤;土体及根际土壤的细菌群落结构和组成明显不同于感病番茄土体及根际土壤细菌群落.与感病番茄根际相比,健康番茄根际细菌的数量...     

Tobacco bacterial wilt can be biologically controlled by the application of antagonistic strains in combination with organic fertilizer

Bacterial wilt caused by Ralstonia solanacearum is one of the most serious tobacco diseases worldwide. Brevibacillus brevis (L-25) and Streptomyces rochei (L-9) with strong inhibitory effects on R. solanacearum in vitro were isolated from the rhizosphere of a healthy tobacco plant in a severely wilt-diseased field. Pot and field experiments were conducted to evaluate the biocontrol effect of the isolated antagonists alone and in combination with organic fertilizer. In pot experiment, the control efficacy was 92.3–100 % in the treatments applied with L-25 and L-9 alone or together with organic fertilizers. When bioorganic fertilizer containing L-9 and L-25 was applied to the soil in field condition, the control efficacies were 95.4 and 30.0 in the Anhui and Guizhou field plots, respectively. The counts of bacteria and actinomycetes in rhizosphere soil were significantly increased (p?≤?0.05) under all antagonist applications compared with CK (P_R). In contrast, fungal and R. solanacearum densities in the rhizosphere soil applied with antagonists were much lower than the CK (P_R) rhizosphere. Combined application of the two antagonists had better effect than single antagonist treatments. The antagonists were more effective when they were combined with organic fertilizer as compared with the antagonistic strains only. These results allow us to conclude that a combination of the biocontrol agents, L-25 and L-9, together with organic fertilizers can effectively control bacterial wilt by affecting soil microbial structure.     

Fusarial wilt suppression and crop improvement through two rhizobacterial strains in chick pea growing in soils infested with Fusarium oxysporum f.sp. ciceris

 Bacterization of chick pea seeds with a siderophore-producing fluorescent Pseudomonas strain RBT13 and an antibiotic-producing Bacillus subtilis strain AF1, isolated from tomato rhizoplane and pigeon pea rhizosphere repectively, increased the shoot height, root length, fresh weight, dry weight and yield in soils infected with Fusarium oxysporum f.sp. ciceris. Seed bacterization also resulted in a significant reduction in chick pea wilt caused by the same pathogen. Addition of iron to the soil completely eliminated disease suppression by RBT13 but not by AF1. Dual drug-resistant mutant strains derived from the rhizobacteria were used to monitor and confirm root colonization. The results indicate the potential for development of both strains for the biological control of chick pea wilt. Received: 29 April 1998     

Effect of calcium concentration in nutrient solution before and after inoculation with Ralstonia solanacearum on resistance of tomato seedlings to bacterial wilt

We previously reported that calcium (Ca) nutrition in tomato (Lycopersicon esculentum Mill.) significantly affected the resistance to bacterial wilt caused by Ralstonia solanacearum Smith. To elucidate the mechanisms underlying the Ca-dependent resistance, the effect of the Ca concentration in the nutrient solution applied before and after inoculation with the pathogen on the resistance of tomato seedlings to bacterial wilt was studied. One week before inoculation, seedlings were transferred to nutrient solutions containing Ca at concentrations of 0.4, 4.4, or 20.4 mM. Soon after inoculation, the seedlings that were treated with each concentration of Ca before inoculation were transferred to solutions containing the same three concentrations of Ca. Although the disease development was not affected by the concentration of Ca in the solution before inoculation, a higher concentration of Ca after inoculation reduced the disease severity. This result suggests that the concentration of Ca in the host, especially in the cell walls, before infection may not be directly involved in the Ca-dependent resistance of tomato seedlings to bacterial wilt.     

移栽定殖根际有益菌番茄苗的田间效应研究

为评估移栽定殖根际有益菌(PGPR)番茄苗对果实产量和青枯病防控效果的影响,通过连续3季田间试验,研究了基于生物有机肥的施用,移栽生物育苗基质(在普通育苗基质中添加分离自根际的解淀粉芽孢杆菌)所育种苗(BIONS),相比于移栽普通育苗基质所育种苗(BIO),对设施番茄产量、发病率、收获期植株土体与根际微生物数量和土壤基本理化性质的影响。连续3季田间试验结果表明:相比于BIO处理,BIONS处理第一、二、三季的增产幅度分别达38.86%、47.87%、34.60%,产量差异均达到显著性水平;BIONS处理的发病率每季均极显著低于BIO处理;BIONS处理增加了根际细菌数量,降低了根际真菌数量;土壤基本理化性质方面,BIONS处理的硝态氮含量和铵态氮含量每季均高于BIO处理,且硝态氮含量与产量呈显著正相关,硝态氮含量和铵态氮含量与发病率呈显著负相关。因此,以生物有机肥为底肥,移栽生物育苗基质所育种苗,能够有效防控番茄青枯病的发生,进而提高产量。     

Suppression of Ralstonia solanacearum in soil following colonization by other strains of R. solanacearum

The effect of prior colonization of a sterile loam soil and a sterile clay loam soil by individual soil bacteria on the subsequent growth of a bacterial wilt pathogen Ralstonia solanacearum YU1Rif43 (tRNA type III: Seal et al. 1992: Appl. Environ. Microbiol., 58, 3759–3761) was investigated. Various strains, belonging to the same type, the same species, the same genus, Gram-negative, Grampositive, or fungi, were used. The degree of suppression of the growth of R. solanacearum YU1Rif43 was markedly different depending on the species that had previously colonized the soil, hereafter referred to as priorcolonists. All the strains belonging to R. solanacearum type III suppressed the growth of R. solanacearum YU1Rif43 markedly, while strains of R. solanacearum type I and type II showed a moderate suppressive effect on R. solanacearum YU1Rif43. The suppressive effect of the strains belonging to species other than R. solanacearum, including fungal strains, was relatively limited, or some strains did not show any suppressive effect. The production of bacteriocin did not appear to be related to the strong suppressive effect of the R. solanacearum type III strains. Possible mechanisms for the suppressive effect of priorcolonists on R. solanacearum YU1Rif43 are discussed in relation to nutrients and physical sites in soil available for growth.     

不同基质生物有机肥防控番茄土传青枯病及促生效果研究

本文旨在研究功能菌株解淀粉芽孢杆菌配合不同原料的有机肥，制备不同生物有机肥对番茄防控土传青枯病以及促生的效果研究，并考察不同生物有机肥对土壤养分及根际细菌群落的影响，获得能够有效防控番茄青枯病的生物有机肥。本研究以解淀粉芽孢杆菌T-5为功能菌株，分别配合秸秆有机肥及鸡粪有机肥，经二次发酵分别制得秸秆生物有机肥（BIO1）和鸡粪生物有机肥（BIO2）。于江苏省南京市一处青枯病多发的番茄大棚进行试验。考察不同施肥处理下番茄的发病情况、生长情况以及土壤理化性质，并利用高通量测序探究根际细菌群落的变化。两种生物有机肥均能显著降低番茄土传青枯病的发病率，且BIO1处理的防控效果最佳，使发病率降低了94.45%。两种生物有机肥均能显著促进番茄生长，增加了番茄地上部的生物量并提高了产量，但对于番茄果实品质并无显著提升。此外，施用BIO1可显著增加土壤有机质含量，BIO2则显著提高了土壤pH。土壤有机质与番茄的发病率呈负相关，与番茄地上部生物量和产量呈显著正相关关系。基于高通量测序结果，发现生物有机肥可显著提高根际细菌多样性，同时改变了细菌群落结构。防控效果最好的BIO1处理中，变形菌门相对丰度最高，放线菌门相对丰度最低，土壤硝态氮和pH与这两个门水平的细菌类群具有显著相关性。以解淀粉芽孢杆菌为功能菌株、秸秆为原料制备的秸秆生物有机肥可有效防控番茄土传青枯病，效果优于鸡粪生物有机肥。该生物有机肥还可显著促进番茄生长及产量提升，并通过调控根际细菌群落中变形菌门和放线菌门的相对丰度，提高土壤抑病能力，减少土传青枯病的发生。     

Effects of bio-organic fertilizer plus soil amendment on the control of tobacco bacterial wilt and composition of soil bacterial communities

Tobacco bacterial wilt (TBW) is caused by Ralstonia solanacearum (R. solanacearum), a severe pathogenic agent with a wide host range. In this study, lime?+?ammonium bicarbonate (L?+?AB), organic fertilizer (OF), bio-organic fertilizer (BOF), and integrated treatment (L?+?AB?+?BOF) were assessed for the ability to control TBW and to influence the composition of native soil bacterial communities. The results showed that disease incidence of L?+?AB?+?BOF for two growth seasons in pot experiment was the lowest, with only 15.56 and 11.11 % at seasons 1 and 2, respectively. The integrated treatment could also significantly suppress TBW in the field, with a disease incidence of only 14.27 % compared with 35.41, 50.03, and 31.32 % in L?+?AB, OF, and BOF treatments, respectively. With application of the integrated treatment in pot and field experiments, the abundances of R. solanacearum were both significantly lower than those with other treatments. Denaturing gradient gel electrophoresis (DGGE) patterns showed that application of BOF significantly affected composition of bacterial communities of rhizosphere. The analysis of 454 sequencing data showed that application of integrated treatment recruited more beneficial bacteria than other treatments, such as Bacillus, Paenibacillus, Arthrobacter, and Streptomyces, while the abundance of Ralstonia with the integrated treatment was decreased. Overall, these results suggested that application of integrated agricultural management could effectively suppress bacterial wilt by affecting the composition of bacterial community and reducing the population of R. solanacearum.     

烟草青枯病菌拮抗菌的筛选、鉴定及生防特性研究

烟草青枯病危害严重,以拮抗菌进行防病的生物防治手段成为研究热点。从不同烟田分离纯化出238株细菌菌株,首先经牙签接种初筛,选取对青枯病菌抑制效果较好的菌株制备其抑菌物质的粗提物,以牛津杯法复筛,最终获得3株对烟草青枯病菌有明显抑制作用的拮抗细菌。全细胞脂肪酸、16S rDNA及gyrB基因测序等分析结果表明,菌株H19、Y6为解淀粉芽孢杆菌(Bacillus amyloliquefaciens),菌株H34为甲基营养型芽孢杆菌(B.methylotrophicus)。3株拮抗菌经CAS检测平板法和Salkowski比色法,发现均具有产铁载体和吲哚-3-乙酸(IAA)的能力,以菌株H19能力最强。温室促生试验结果表明,3株拮抗菌能显著促进烟草株高、鲜重及干重等指标,与对照相比,平均增长率分别达到70%~115%、40%~49%和32%~42%。温室控病试验结果表明,菌株H19、H34和Y6明显降低烟草青枯病的发病率,防效达76.57%、60.98%和69.83%,稍逊于农用链霉素处理的78.66%。     

The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae

Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.