Interactions between Xanthomonas translucens pv. translucens,the Causal Agent of Bacterial Leaf Streak of Wheat,and Bacterial Epiphytes in the Wheat Phyllosphere

The abilities of 44 bacterial epiphytes to reduce leaf-associated population sizes of Xanthomonas translucens pv. translucens strain Xtt4Rif-2 and bacterial leaf streak were quantified in growth-chamber experiments. Bacterial epiphytes were inoculated individually onto 10-day-old wheat seedlings at a density of 10⁸ CFU/ml. Pathogen strain Xtt4Rif-2 was inoculated onto wheat seedlings 48 h later at a density of 10⁷ CFU/ml. Population sizes of the bacterial epiphytes and the pathogen were quantified 48 h after inoculation of the pathogen and disease severity was assessed 5–6 days later. Thirteen bacterial epiphytes were identified by their ability to reduce Xtt4Rif-2 populations, disease severity, or both compared to a pathogen-only control (subsequently referred to as successful antagonists). The potential roles of antibiosis and competition for nutrient resources in mediating the observed interactions between the epiphytes and the pathogen were also investigated. Only one epiphyte inhibited Xtt in vitro. Thus, antibiosis probably was not a major mechanism by which pathogen population sizes and disease severity were reduced. Similarity in nutrient utilization between bacterial epiphytes and pathogen strain Xtt4Rif-2 was estimated using nutrient-overlap indices. Nutrient-overlap indices were not predictive of the ability of epiphytes to reduce pathogen populations or disease severity. However, successful antagonists utilized both sucrose and inositol more frequently than poor antagonists.     

Phylogenetic relatedness of Xanthomonas axonopodis pv. punicae, the causal agent of bacterial blight of pomegranate based on two loci, 16S rRNA and gyrB

Bacterial blight caused by Xanthomonas axonopodis pv. punicae (Xap) is a major disease in pomegranate (Punica granatum) cultivation in India. The Xap strains from three distinct geographical origins, Delhi, Maharashtra and Andhra Pradesh were studied for their genetic variability and phylogenetic relationship with other Xanthomonads targeting two important loci 16S rRNA and gyrB. All Xap strains showed 100 % sequence conservation in both the loci, suggesting that geographical origin does not necessarily reflect variation to genetic make-up of the Xap. Phylogeny derived from 16S rRNA gene revealed that two Xanthomonas species, Xanthomonas citri subsp. malvacearum DSM 3849 T and X. axonopodis pv. manihotis NCPPB1834 formed a single cluster along with Xap. Further, analysis in the gyrB locus indicated that X. citri subsp. malvacearum shared 99.4 % identity while pathovars X. axonopodis pv. manihotis shared only 95 % identity with the Xap strains. Thus, we established that gyrB was the preferred locus over 16S rRNA gene to discriminate the Xap strains from closely related Xanthomonas species type strains. Nevertheless, our study demonstrated for the first time that pomegranate bacterial blight pathogen is phylogenetically very close to Xanthomonas citri subsp. malvacearum infecting cotton.     

Bacillus spp. and Pseudomonas putida as inhibitors of the Colletotrichum acutatum group and potential to control Glomerella leaf spot

The control of Glomerella leaf spot (GLS) in Brazil is solely based on fungicide sprays and new alternatives are needed. In apple, few biological control methods have been evaluated, and most have focused on post-harvest pathogens. Therefore, the objectives of this work were to study the mode of action of three bacterial strains and the commercial product Serenade® (Bacillus subtilis) against the Colletotrichum acutatum group, the causal agents of GLS, and to evaluate the influence of bacterial isolates and Serenade® on the development of the first cycle of infection disease under controlled conditions. To assess the mode of action of the bacterial isolates against strains of the C. acutatum group, in vitro tests were performed. It was tested the effect of the bacteria on conidial germination and mycelial growth, using three methodologies, (i) fungal-bacterial co-cultivation, (ii) bacterial thermostable metabolites and (iii) bacterial volatile compounds. The influence of the bacterial isolates on the GLS development was assessed using apple seedlings. The seedlings were first sprayed weekly with bacterial suspension for 5 weeks, and were then inoculated with conidia suspensions (10⁴ conidia mL⁻¹) of C. acutatum group isolates. Seedlings were maintained in chambers (CONVIRON) at 25 °C and a 12-h light regime. Disease severity of GLS was evaluated daily by counting typical lesions caused by C. acutatum group on all leaves during 12 consecutive days. The disease progress curve was fitted to nonlinear models for incidence and severity data. The treatments were compared by contrasting epidemiological parameters. Bacillus sp. isolated from the apple phylloplane inhibited more than 60% of the C. acutatum group conidial germination. The mode of action of Bacillus sp. and Bacillus alcalophilus on the C. acutatum group was through the production of fixed and volatile compounds, which inhibited mycelial growth. The primary mode of action of Serenade® on the C. acutatum group was the production of thermostable metabolites capable of completely inhibiting mycelial growth. In the GLS disease cycle, it was possible to adjust the monomolecular model for incidence and the number of lesions. There were significant differences between the epidemiological parameters of GLS in seedlings treated with apple phylloplane bacteria or with Serenade® as compared to the controls, indicating a potential for the use of biological control to manage GLS in apple orchards.     

Microbial Community Composition Affects Soil Fungistasis

Most soils inhibit fungal germination and growth to a certain extent, a phenomenon known as soil fungistasis. Previous observations have implicated microorganisms as the causal agents of fungistasis, with their action mediated either by available carbon limitation (nutrient deprivation hypothesis) or production of antifungal compounds (antibiosis hypothesis). To obtain evidence for either of these hypotheses, we measured soil respiration and microbial numbers (as indicators of nutrient stress) and bacterial community composition (as an indicator of potential differences in the composition of antifungal components) during the development of fungistasis. This was done for two fungistatic dune soils in which fungistasis was initially fully or partly relieved by partial sterilization treatment or nutrient addition. Fungistasis development was measured as restriction of the ability of the fungi Chaetomium globosum, Fusarium culmorum, Fusarium oxysporum, and Trichoderma harzianum to colonize soils. Fungistasis did not always reappear after soil treatments despite intense competition for carbon, suggesting that microbial community composition is important in the development of fungistasis. Both microbial community analysis and in vitro antagonism tests indicated that the presence of pseudomonads might be essential for the development of fungistasis. Overall, the results lend support to the antibiosis hypothesis.     

Assessment of the Importance of Similarity in Carbon Source Utilization Profiles between the Biological Control Agent and the Pathogen in Biological Control of Bacterial Speck of Tomato

Bacterial speck of tomato, caused by Pseudomonas syringae pv. tomato, was used to determine whether similarity in carbon source utilization between a preemptive biological control agent and the pathogen was significant in determining the ability of the bacterium to suppress disease. Similarity in carbon source utilization was quantified as the ratio of the number of tomato carbon sources utilized in vitro by the biological control agent to the number of tomato carbon sources utilized in vitro by the target pathogen (the niche overlap index [NOI]). Suppression of the disease was quantified as the percent reduction in disease severity compared to the pathogen-only control when nonpathogenic bacteria were applied to foliage 48 h prior to the pathogen. In the collection of 36 nonpathogenic bacterial strains, there was a significant (P < 0.01), but weak (r² = 0.25), correlation between reduction in disease severity and similarity in carbon source utilization, suggesting that similarity in carbon source use was significant in determining ability to suppress disease. The relationship was investigated further using catabolic mutants of P. syringae strain TLP2, an effective biological control agent of speck. Catabolic mutants exhibited lower levels of similarity (NOI = 0.07 to 0.90) than did wild-type TLP2 (NOI = 0.93). With these catabolic mutants there was a significant (P < 0.01), and stronger (r² = 0.42), correlation between reduction in disease severity and similarity in carbon source utilization. This suggests that similarity in carbon source utilization was a more important component of biological control ability for the catabolic mutants than for the nonpathogenic bacteria. Together, these studies indicate that suppression of bacterial speck of tomato was correlated with nutritional similarity between the pathogenic and nonpathogenic bacteria and suggest that preemptive utilization of carbon sources was probably involved in the biological control of the disease by both the naturally occurring nonpathogenic bacteria and the catabolic mutants.     

Mixtures of plant growth-promoting rhizobacteria for induction of systemic resistance against multiple plant diseases

Studies of induced systemic resistance using strains of plant growth-promoting rhizobacteria (PGPR) have concentrated on the use of individual PGPR as inducers against multiple diseases of a single crop. To date, few reports have examined the potential of PGPR strain mixtures to induce systemic resistance against diseases of several different plant hosts. The objective of this study was to select mixtures of compatible PGPR strains with the capacity to elicit induced systemic resistance in four hosts. The specific diseases and hosts tested in this study included: bacterial wilt of tomato (Lycopersicon esculentum) caused by Ralstonia solanacearum, anthracnose of long cayenne pepper (Capsicum annuum var. acuminatum) caused by Colletotrichum gloeosporioides, damping off of green kuang futsoi (Brassica chinensis var. parachinensis) caused by Rhizoctonia solani, and cucumber mosaic virus (CMV) on cucumber (Cucumis sativus). To examine compatibility, seven selected PGPR strains were individually tested for in vitro antibiosis against all other PGPR strains and against three of the tested pathogens (R. solanacearum, C. gloeosporioides, and R. solani). No in vitro antibiosis was observed among PGPR strains or against pathogens. Twenty-one combinations of PGPR and seven individual PGPR were tested in the greenhouse for induced resistance activity. Results indicated that four mixtures of PGPR and one individual strain treatment significantly reduced the severity of all four diseases compared to the nonbacterized control: 11 mixtures reduced CMV of cucumber, 16 mixtures reduced bacterial wilt of tomato, 18 mixtures reduced anthracnose of long cayenne pepper, and 7 mixtures reduced damping off of green kuang futsoi. Most mixtures of PGPR provided a greater disease suppression than individual PGPR strains. These results suggest that mixtures of PGPR can elicit induced systemic resistance to fungal, bacterial, and viral diseases in the four hosts tested.     

Suppression of bacterial wilt in Eucalyptus urophylla by fluorescent Pseudomonas spp. in China

Bacterial wilt caused by Ralstonia solanacearum race 1, biovar III has become a severe problem in Eucalyptus plantations in south China. The disease mainly attacks young eucalypt trees, and no effective control measures are available yet. To explore possibilities to develop biological control of the disease, strains of fluorescent Pseudomonas spp. that are effective in suppressing plant diseases by known mechanisms, were tested for their potential to control bacterial wilt in Eucalyptus. Pseudomonas putida WCS358r, Pseudomonas fluorescens WCS374r, P. fluorescens WCS417r, and Pseudomonas aeruginosa 7NSK2 antagonize R. solanacearum in vitro by siderophore-mediated competition for iron, whereas inhibition of pathogen growth by P. fluorescens CHA0r is antibiosis-based. No correlations were found between antagonistic activities of these Pseudomonas spp. in vitro and biocontrol of bacterial wilt in Eucalyptus in vivo. None of the strains suppressed disease when mixed together with the pathogen through the soil or when seeds or seedlings were treated with the strains one to four weeks before transfer into soil infested with R. solanacearum. However, when the seedlings were dipped with their roots in a bacterial suspension before transplanting into infested soil, P. fluorescens WCS417r significantly suppressed bacterial wilt. P. putida WCS358r was marginally effective, whereas its siderophore-minus mutant had no effect at all, indicating that siderophore-mediated competition for iron can contribute but is not effective enough to suppress bacterial wilt in Eucalyptus. A derivative of P. putida WCS358r, constitutively producing 2,4-diacetylphloroglucinol (WCS358::phl) reduced disease. Combined treatment with P. fluorescens WCS417r and P. putida WCS358::phl did not improve suppression of bacterial wilt.     

Intensive aquaculture selects for increased virulence and interference competition in bacteria

Although increased disease severity driven by intensive farming practices is problematic in food production, the role of evolutionary change in disease is not well understood in these environments. Experiments on parasite evolution are traditionally conducted using laboratory models, often unrelated to economically important systems. We compared how the virulence, growth and competitive ability of a globally important fish pathogen, Flavobacterium columnare, change under intensive aquaculture. We characterized bacterial isolates from disease outbreaks at fish farms during 2003–2010, and compared F. columnare populations in inlet water and outlet water of a fish farm during the 2010 outbreak. Our data suggest that the farming environment may select for bacterial strains that have high virulence at both long and short time scales, and it seems that these strains have also evolved increased ability for interference competition. Our results are consistent with the suggestion that selection pressures at fish farms can cause rapid changes in pathogen populations, which are likely to have long-lasting evolutionary effects on pathogen virulence. A better understanding of these evolutionary effects will be vital in prevention and control of disease outbreaks to secure food production.     

Colony Age of Trichoderma azevedoi Alters the Profile of Volatile Organic Compounds and Ability to Suppress Sclerotinia sclerotiorum in Bean Plants

Common bean (Phaseolus vulgaris L.) is one of the most important crops in human food production. The occurrence of diseases, such as white mold, caused by Sclerotinia sclerotiorum can limit the production of this legume. The use of Trichoderma has become an important strategy in the suppression of this disease. The aim of the present study was to evaluate the effect of volatile organic compounds (VOCs) emitted by Trichoderma azevedoi CEN1241 in five different growth periods on the severity of white mold in common bean. The in vitro assays were carried out in double-plate and split-plate, and the in vivo assays, through the exposure of the mycelia of S. sclerotiorum to the VOCs of T. azevedoi CEN1241 and subsequent inoculation in bean plants. Chemical analysis by gas chromatography coupled to mass spectrometry detected 37 VOCs produced by T. azevedoi CEN1241, covering six major chemical classes. The profile of VOCs produced by T. azevedoi CEN1241 varied according to colony age and was shown to be related to the ability of the biocontrol agent to suppress S. sclerotiorum. T. azevedoi CEN1241 VOCs reduced the size of S. sclerotiorum lesions on bean fragments in vitro and reduced disease severity in a greenhouse. This study demonstrated in a more applied way that the mechanism of antibiosis through the production of volatile compounds exerted by Trichoderma can complement other mechanisms, such as parasitism and competition, thus contributing to a better efficiency in the control of white mold in bean plants.     

Biological control of common blight of bean (Phaseolus vulgaris) causedby Xanthomonas axonopodis pv. phaseoli by using the bacteriumRahnella aquatilis

The aim of this study was to evaluate the bacterium Rahnella aquatilis (Ra) for protection of bean plants against common blight disease caused by Xanthomonas axonopodis pv. phaseoli (Xap). Xap isolates were isolated from a naturally blighted leaves of bean plants grown in Assiut governorate. The blight symptoms were produced by all three isolates, but the isolates differed in their degree of the pathogenicity. Xap1 was the most virulence one against bean plants. The effect of Ra against common blight of bean plant was tested. In vitro studies, Ra exhibited inhibitor effect against the pathogen. Under greenhouse and field conditions, beanvariety “Giza 6” treated by Ra resulted in marked disease suppression. Ahigh decrease of the disease was correlated with a reduction of the bacterial multiplication. In physiological studies, bean plants treated by Ra exhibited higher phenolic compounds contents and higher activity of peroxidase (PO) enzyme than untreated plants. In conclusion, application of Ra was effective and could be recommended for controlling the bean common blight disease.     

Effects of freshwater bacterial siderophore on Microcystis and Anabaena

Six siderophore-producing bacterial strains were isolated from the freshwater, in which five strains belonged to Pseudomona genus, and the other belonged to Stenotrophomonas genus. The strain, Stenotrophomonas maltophilia 15, which produced hydroxamate-type siderophore, was selected for siderophore preparation. Its siderophore production was inhibited by FeCl₃, especially when FeCl₃ concentration was higher than 20 μM. Effects of siderophore on cyanobacteria Microcystis aeruginosa FACHB-905 and Anabaena flos-aquae FACHB-245 were studied. Compared to the control, almost all the treated groups showed decrease in growth rate and chlorophyll a, carotenoids, phycocyanin, soluble protein, microcystin content, which was attributed the low iron bioavailability in the culture medium. In the study, S. maltophilia 15 showed algicidal activities by secreting siderophore and could inhibit cyanobacterial growth, especially when iron bioavailabity is very low. The two cyanobacterial strains showed distinct demand for iron. It was deduced that in the freshwater the competition between bacteria and cyanobacteria existed for the low-bioavailable iron, which may relate to the replacement of dominant cyanobacteria.     

Influence of soil factors and cultural practice on biological control of sheath blight of rice with antagonistic bacteria

Fluorescent and nonfluorescent strains of bacteria isolated from rice rhizospheres on the International Rice Research Institute (IRRI) farm were evaluated for in vitro antibiosis towards the sheath blight (ShB) pathogen Rhizoctonia solani, and for suppression of ShB in detached rice leaves. Efficient strains were located on the basis of consistent performance in two laboratory tests. Among nine efficient strains, 3 strains were identified as Pseudomonas fluorescens, 5 strains were tentatively identified as Bacillus spp. and one strain was identified as Enterobacter. In three greenhouse tests lowland rice soils with optimum pH for rice growth (pH 5.5–6.5), acidic pH (pH 5.0) and boron toxicity were found more suitable for biological control of ShB and, less frequently, also yield increases than were alkaline (pH 6.9) and zinc-deficient soils. Bacterial treatments afforded significant ShB reductions in 3 field experiments, but no significant yield increases resulted. In direct-seeded rice best performances by bacterial treatments in terms of ShB suppression were 66 and 98% during DS 1988 and WS 1988, respectively, which were comparable to or better than the performance of validamycin (a fungicide routinely used for ShB control) which afforded 42 and 63% disease suppression, respectively, in the same experiments. Although bacterial treatments caused ShB reductions both in direct-seeded and transplanted rice crops, disease control was more pronounced in direct-seeded than in transplanted crops. These results indicate that carefully selected strains of bacterial antagonists have the potential for ShB suppression in rice at least in areas where direct-seeding is practised.     

Screening for potential <Emphasis Type="Italic">Striga hermonthica</Emphasis> fungal and bacterial biocontrol agents from suppressive soils in Western Kenya

Striga hermonthica is a hemiparasitic weed that causes huge grain yield losses to small-scale farmers in Africa. Effective biocontrol agents against S. hermonthica can sustainably mitigate these losses. This study characterized the biocontrol potential of culturable fungal and bacterial isolates from S. hermonthica suppressive soils of western Kenya. These isolates were screened for their ability to produce antibiotic compounds and extra cellular enzymes and also their ability to cause S. hermonthica seed decay. Genomic DNA of the selected bacterial and fungal isolates was extracted and partial characterization of 16S rRNA and 18S rRNA genes performed respectively. Analysis show that antibiosis and enzymatic properties of potential biocontrol isolates correlated positively. Isolate KY041696 recorded high antibiosis, enzymatic and seed decay values. This study also revealed that bioactive bacterial isolates belonged to Bacillus, Streptomyces and Rhizobium genera. In this study, no fungal isolate caused S. hermonthica seed decay. This study therefore provides baseline information on the potential biocontrol microbes against S. hermonthica in Western Kenya that could be exploited further in the management of the weed.     

Isolation screening and characterisation of local beneficial rhizobacteria based upon their ability to suppress the growth of Fusarium oxysporum f. sp. radicis-lycopersici and tomato foot and root rot

Local beneficial rhizobacteria were selected based upon their ability to control the fungus Fusarium oxysporum f. sp. radicis-lycopersici which causes crown and root rot of tomato. Seven out of 384 strains prevailed in multiple and dual cultures and were identified as Pseudomonas chlororaphis (one strain), Bacillus cereus (one strain), Serratia marcescens (three strains) and Serratia rubidaea (two strains), by sequencing the 16S rRNA or the 16S and 23S rRNA inter-spacer region. The seven selected rhizobacteria were tested for their biocontrol and growth-promoting effects in planta, and their antifungal properties in vitro. All strains significantly reduced disease severity under controlled conditions, in a gnotobiotic system and in pots. Moreover, one P. chlororaphis and one S. marcescens strain significantly decreased disease severity to the level of the healthy control under natural conditions in pots experiments. The inhibitory activity of bacterial liquid cultures' metabolites on the fungus was demonstrated for all strains in vitro, using filter paper, thin layer chromatography and microtiter bioassays. Genes encoding phenazines were tentatively detected by PCR in the P. chlororaphis strain and chitinase-encoding genes were detected in one S. rubidaea and all three S. marcescens strains. Production of phenazine-1-carboxamide and hydrogen cyanide was evidenced for the P. chlororaphis strain while protease activity and production of siderophore-like compounds was confirmed in all bacterial strains. Possible use of these strains as biological control agents and their impact on natural biocontrol of pathogens in soils is discussed.     

Hrp- Mutants of Pseudomonas solanacearum as Potential Biocontrol Agents of Tomato Bacterial Wilt

There have been many attempts to control bacterial wilt with antagonistic bacteria or spontaneous nonpathogenic mutants of Pseudomonas solanacearum that lack the ability to colonize the host, but they have met with limited success. Since a large gene cluster (hrp) is involved in the pathogenicity of P. solanacearum, we developed a biological control strategy using genetically engineered Hrp^- mutants of P. solanacearum. Three pathogenic strains collected in Guadeloupe (French West Indies) were rendered nonpathogenic by insertion of an ω-Km interposon within the hrp gene cluster of each strain. The resulting Hrp^- mutants were tested for their ability to control bacterial wilt in challenge inoculation experiments conducted either under growth chamber conditions or under greenhouse conditions in Guadeloupe. Compared with the colonization by a pathogenic strain which spread throughout the tomato plant, colonization by the mutants was restricted to the roots and the lower part of the stems. The mutants did not reach the fruit. Moreover, the presence of the mutants did not affect fruit production. When the plants were challenge inoculated with a pathogenic strain, the presence of Hrp^- mutants within the plants was correlated with a reduction in disease severity, although pathogenic bacteria colonized the stem tissue at a higher density than the nonpathogenic bacteria. Challenge inoculation experiments conducted under growth chamber conditions led, in some cases, to exclusion of the pathogenic strain from the aerial part of the plant, resulting in high protection rates. Furthermore, there was evidence that one of the pathogenic strains used for the challenge inoculations produced a bacteriocin that inhibited the in vitro growth of the nonpathogenic mutants.     

Phenazines and Other Redox-Active Antibiotics Promote Microbial Mineral Reduction

Natural products with important therapeutic properties are known to be produced by a variety of soil bacteria, yet the ecological function of these compounds is not well understood. Here we show that phenazines and other redox-active antibiotics can promote microbial mineral reduction. Pseudomonas chlororaphis PCL1391, a root isolate that produces phenazine-1-carboxamide (PCN), is able to reductively dissolve poorly crystalline iron and manganese oxides, whereas a strain carrying a mutation in one of the phenazine-biosynthetic genes (phzB) is not; the addition of purified PCN restores this ability to the mutant strain. The small amount of PCN produced relative to the large amount of ferric iron reduced in cultures of P. chlororaphis implies that PCN is recycled multiple times; moreover, poorly crystalline iron (hydr)oxide can be reduced abiotically by reduced PCN. This ability suggests that PCN functions as an electron shuttle rather than an iron chelator, a finding that is consistent with the observation that dissolved ferric iron is undetectable in culture fluids. Multiple phenazines and the glycopeptidic antibiotic bleomycin can also stimulate mineral reduction by the dissimilatory iron-reducing bacterium Shewanella oneidensis MR1. Because diverse bacterial strains that cannot grow on iron can reduce phenazines, and because thermodynamic calculations suggest that phenazines have lower redox potentials than those of poorly crystalline iron (hydr)oxides in a range of relevant environmental pH (5 to 9), we suggest that natural products like phenazines may promote microbial mineral reduction in the environment.     

Antagonistic bacteria of composted agro-industrial residues exhibit antibiosis against soil-borne fungal plant pathogens and protection of tomato plants from Fusarium oxysporum f.sp. radicis-lycopersici

Rhizospheric and root-associated/endophytic (RAE) bacteria were isolated from tomato plants grown in three suppressive compost-based plant growth media derived from the olive mill, winery and Agaricus bisporus production agro-industries. Forty-four (35 rhizospheric and 9 RAE) out of 329 bacterial strains showed in vitro antagonistic activity against at least one of the soil-borne fungal pathogens, Fusarium oxysporum f.sp. radicis-lycopersici (FORL), F. oxysporum f.sp. raphani, Phytophthora cinnamomi, P. nicotianae and Rhizoctonia solani. The high percentage of total isolates showing antagonistic properties (13%) and their common chitinase and β-glucanase activities indicate that the cell wall constituents of yeasts and macrofungi that proliferate in these compost media may have become a substrate that favours the establishment of antagonistic bacteria to soil-borne fungal pathogens. The selected bacterial strains were further evaluated for their suppressiveness to tomato crown and root rot disease caused by FORL. A total of six rhizospheric isolates, related to known members of the genera Bacillus, Lysinibacillus, Enterobacter and Serratia and one RAE associated with Alcaligenes faecalis subsp. were selected, showing statistically significant decrease of plant disease incidence. Inhibitory effects of extracellular products of the most effective rhizospheric biocontrol agent, Enterobacter sp. AR1.22, but not of the RAE Alcaligenes sp. AE1.16 were observed on the growth pattern of FORL. Furthermore, application of cell-free culture extracts, produced by Enterobacter sp. AR1.22, to tomato roots led to plant protection against FORL, indicating a mode of biological control action through antibiosis.     

A model of insect—pathogen dynamics in which a pathogenic bacterium can also reproduce saprophytically

We developed a model of the population dynamic interaction between an insect and a pathogenic bacterium motivated by study of Serratia entomophila, a commercially exploited pathogen of the New Zealand grass grub (Costelytra zealandica). The bacterium is able to reproduce saprophytically, though it competes for saprophytic substrates with non-pathogenic strains, which appear to be superior competitors, probably because they lack a plasmid that carries genes required for pathogenicity. The effect of saprophytism and competition on the invasion criterion (R₀), short-term dynamics and long-term dynamics are described. Saprophytism can reduce (possibly to zero) the host threshold at which the pathogen can invade, though this reduction is less when there is competition with non-pathogenic strains. In a model of short-term population dynamics designed to mimic the application of bacteria to a host epizootic, saprophytism enhances the reduction in host density, though again this is tempered by competition with non-pathogens. In the long term, a pathogen that can develop saprophytically can drive its host to extinction in the absence of competition with non-pathogens. When the latter are present, host extinction is prevented. The addition of saprophytic reproduction can stabilize an otherwise unstable host–pathogen model, but we were unable to find a stable equilibrium given the further addition of a wholly saprophytic bacterial strain. The model suggests that enhancing or selecting for saprophytic ability could be a way of improving biological control.