Suppression of gray leaf spot (blast) of perennial ryegrass turf by Pseudomonas aeruginosa from spent mushroom substrate

Bacteria isolated from spent mushroom substrate (SMS) were evaluated for the suppression of Pyricularia grisea, the causal agent of gray leaf spot of perennial ryegrass (Lolium perenne) turf. Thirty-two of 849 bacterial isolates (3.8%) from SMS significantly inhibited the mycelial growth of P. grisea in vitro. Six bacterial isolates that afforded maximum inhibition of P. grisea were also refractory to Rhizoctonia solani, Rhizoctonia cerealis, Sclerotinia homoeocarpa, and Fusarium culmorum. Each of the six isolates was identified as Pseudomonas aeruginosa by fatty acid profile analysis. The biocontrol activity of the bacterial isolates was not compromised by a prolonged exposure to UV radiation in vitro. In controlled-environment chamber experiments, all 32 bacterial isolates were tested for suppression of gray leaf spot on Pennfine perennial ryegrass when applied as seed treatment or foliar sprays. Foliar applications of the bacteria (10⁸ cfu/ml 0.1% carboxymethylcellulose), but not the seed treatment, significantly reduced disease severity and incidence. The three most efficient isolates from foliar application treatments, which were among the six bacterial isolates identified as P. aeruginosa, were further evaluated for suppression of gray leaf spot as a function of timing of application. The three isolates of P. aeruginosa suppressed gray leaf spot in perennial ryegrass in Cone-tainers when applied at 1, 3, and 7 days prior to inoculation with P. grisea both in controlled-environment chamber experiments, and in potted ryegrass plants maintained in the field. All application intervals, regardless of the bacterial isolate, provided significant reduction of gray leaf spot severity. Suppression of gray leaf spot by isolates of P. aeruginosa under controlled-environment chamber conditions was not different from that observed in potted ryegrass plants maintained in the field. In field experiments, an isolate of P. aeruginosa provided significant suppression of gray leaf spot when applied at 1, 7, and 14 days prior to inoculation with P. grisea. The bacterium proved effective against gray leaf spot of perennial ryegrass maintained as fairway and rough heights. These results indicate that P. aeruginosa may be a potential biocontrol agent for gray leaf spot of perennial ryegrass turf.     

Biological Control of Rice Blast byPseudomonas fluorescensStrainPf7–14: Evaluation of a Marker Gene and Formulations

Pseudomonas fluorescensstrainPf7–14 was evaluated for biological control of rice blast in field experiments. StrainPf7–14 was formulated in methylcellulose:talc (1:4) and applied to IR50 rice (Oryza sativa) seeds as a seed treatment and as foliar sprays in seedbed and field experiments. When applied as a seed treatment followed by three foliar applications, the strain provided a 68.5% suppression of rice blast in the seedbed experiment and a 59.6% suppression in the field experiment. The persistence and migration ofPf7–14 on the rice plant was studied with the aid oflacZYgenes inserted into the bacterium. In greenhouse experiments,Pf7–14gal was detected on rice roots at 10⁶to 10⁵cfu/g of root tissue for 110 days, the duration of the rice crop. Migration of the strain from the seeds to the leaves occurred only until the seedlings were 16 days old. WhenPf7–14 was applied to the rice plants by foliar sprays, 10⁴cfu of the bacterium per gram of leaf tissue was detected for the next 40 days. The limited migration of the bacterial biocontrol agent emphasizes the need for multiple foliar applications of the bacterium to sustain the bacterial population for effective suppression of rice blast.     

Host-plant selectivity of rhizobacteria in a crop/weed model system

Belowground microorganisms are known to influence plants' performance by altering the soil environment. Plant pathogens such as cyanide-producing strains of the rhizobacterium Pseudomonas may show strong host-plant selectivity. We analyzed interactions between different host plants and Pseudomonas strains and tested if these can be linked to the cyanide sensitivity of host plants, the cyanide production of bacterial strains or the plant identity from which strains had been isolated. Eight strains (four cyanide producing) were isolated from roots of four weed species and then re-inoculated on the four weed and two additional crop species. Bacterial strain composition varied strongly among the four weed species. Although all six plant species showed different reductions in root growth when cyanide was artificially applied to seedlings, they were generally not negatively affected by inoculation with cyanide-producing bacterial strains. We found a highly significant plant species x bacterial strain interaction. Partitioning this interaction into contrasts showed that it was entirely due to a strongly negative effect of a bacterial strain (Pseudomonas kilonensis/brassicacearum, isolated from Galium mollugo) on Echinochloa crus-galli. This exotic weed may not have become adapted to the bacterial strain isolated from a native weed. Our findings suggest that host-specific rhizobacteria hold some promise as biological weed-control agents.     

Plant Reactions to Inoculation of Roots with Fungi and Bacteria

The potential of 120 isolates of fungi and bacteria from plant rhizospheres to interfere with plant development and growth was studied in greenhouse experiments. The pure cultured isolates were obtained from plant roots in the field and applied as suspensions to the roots of eight test plant species. 10–20% of the isolates caused distinct symptoms on shoots, growth retardations without other symptoms or growth promotions. Responses of treated plants ranged from death of plants soon after treatment to up to about 40% higher shoot fresh weight than in control plants. Two bacterial isolates induced strong reactions in most of the plant species tested while other isolates showed a more or less pronounced specificity by giving reactions in only some of the plant species tested.     

Pseudomonads: major antagonistic endophytic bacteria to suppress bacterial wilt pathogen, <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis> in the eggplant (<Emphasis Type="Italic">Solanum melongena</Emphasis> L.)

Endophytic bacteria of eggplant, cucumber and groundnut were isolated from different locations of Goa, India. Based on in vitro screening, 28 bacterial isolates which effectively inhibited Ralstonia solanacearum, a bacterial wilt pathogen of the eggplant were characterized and identified. More than 50% of these isolates were Pseudomonas fluorescens in which a vast degree of variability was found to exist when biochemical characteristics were compared. In greenhouse experiments, the plants treated with Pseudomonas isolates (EB9, EB67), Enterobacter isolates (EB44, EB89) and Bacillus isolates (EC4, EC13) reduced the wilt incidence by more than 70%. All the selected isolates reduced damping off by more than 50% and improved the growth of seedlings in the nursery stage. Most of the selected antagonists produced an antibiotic, DAPG, which inhibited R. solanacearum under in vitro conditions and might have been responsible for reduced wilt incidence under in vivo conditions. Also production of siderophores and IAA in the culture medium by the antagonists was recorded, which could be involved in biocontrol and growth promotion in crop plants. From our study we conclude that Pseudomonas is the major antagonistic endophytic bacteria from eggplants which have the potential to be used as a biocontrol agent as well as plant growth-promoting rhizobacteria. Large scale field evaluation and detailed knowledge on antagonistic mechanism could provide an effective biocontrol solution for bacterial wilt of solanaceous crops.     

Screening of diazotrophic bacteria Azopirillum spp. for nitrogen fixation and auxin production in multiple field sites in southern Brazil

Isolation of microorganisms, screening for desirable characters and selection of efficient strains are important steps to optimize high crop yields and improve the sustainability of the ecosystem. The objective of this study was isolate and identify Azopirillum spp. with enhanced potential to promote plant growth among the natural bacterial population associated with rhizosphere soil, roots and stem of maize collected from five maize-growing regions within the southern state of Rio Grande do Sul in Brazil. Diazotrophic microorganisms were isolated using semi-solid N-free and solid selective media NFb. In order to select the most efficient isolates as candidates for plant growth promotion, the purified bacterial strains were studied for cell morphology, and Gram staining, streptomycin resistance, as well as screened for their potential for nitrogen fixation and auxin production under sterile conditions. Among 224 isolates obtained 121 were able to fix nitrogen and produce auxin. The 30 most promising isolates produced indole-3-acetic acid (IAA) ranging in concentration from 3.51 μg to 246.69 μg IAA mg⁻¹. Nitrogen fixation ranged from 15.43 μg to 95.21 μg of N mg protein⁻¹ day⁻¹ From the 30 most productive isolates, chromosomal DNA was extracted and a portion of the nifH gene was amplified and sequenced. Twenty-nine isolates were found to be similar to the Azospirillum genus and one isolate was found to be similar to Herbaspirillum seropedicae. These bacterial isolates revealed potential to increase crop productivity, however field crop experiments in Rio Grande do Sul climatic conditions should be done in order to formulate recommendations for their use as inoculants.     

Integrated biological control of bacterial speck and spot of tomato under field conditions using foliar biological control agents and plant growth-promoting rhizobacteria

Integration of foliar bacterial biological control agents and plant growth promoting rhizobacteria (PGPR) was investigated to determine whether biological control of bacterial speck of tomato, caused by Pseudomonas syringae pv. tomato, and bacterial spot of tomato, caused by Xanthomonas campestris pv. vesicatoria and Xanthomonas vesicatoria, could be improved. Three foliar biological control agents and two selected PGPR strains were employed in pairwise combinations. The foliar biological control agents had previously demonstrated moderate control of bacterial speck or bacterial spot when applied as foliar sprays. The PGPR strains were selected in this study based on their capacity to induce resistance against bacterial speck when applied as seed and soil treatments in the greenhouse. Field trials were conducted in Alabama, Florida, and California for evaluation of the efficacy in control of bacterial speck and in Alabama and Florida for control of bacterial spot. The foliar biological control agent P. syringae strain Cit7 was the most effective of the three foliar biological control agents, providing significant suppression of bacterial speck in all field trials and bacterial spot in two out of three field trials. When applied as a seed treatment and soil drench, PGPR strain Pseudomonas fluorescens 89B-61 significantly reduced foliar severity of bacterial speck in the field trial in California and in three of six disease ratings in the field trials in Alabama. PGPR strains 89B-61 and Bacillus pumilus SE34 both provided significant suppression of bacterial spot in the two field trials conducted in Alabama. Combined use of foliar biological control agent Cit7 and PGPR strain 89B-61 provided significant control of bacterial speck and spot of tomato in each trial. In one field trial, control was enhanced significantly with combined biological control agents compared to single agent inoculations. These results suggest that some PGPR strains may induce plant resistance under field conditions, providing effective suppression of bacterial speck and spot of tomato, and that there may be some benefit to the integration of rhizosphere-applied PGPR and foliar-applied biological control agents.     

<Emphasis Type="Italic">Nimbya alternantherae</Emphasis> a potential biocontrol agent for alligatorweed, <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis>

Alligatorweed, (Alternanthera philoxeroides (Mart.) Griseb.), an aquatic and wetland plant native to South America, is an aggressive weed in many parts of the world. Its ability to compete with other native plants and to impede waterways has made it a serious threat to aquatic ecosystems. Although biological control with insects has been fairly successful in aquatic habitats, there is a need for additional agents to manage the weed in upland sites. Accordingly, in a survey in Brazil in 1997 a fungus, Nimbya (=Alternaria) alternantherae (Holcomb and Antonopoulus) Simmons and Alcorn, was discovered and confirmed to be highly damaging to alligatorweed. Studies were conducted to determine the potential of this fungus for controlling this weed. Several isolates from Brazil, USA, and Puerto Rico were compared and no differences in virulence were observed, although a lower dew requirement was demonstrated for the Brazilian isolates. Conidia were more effective than mycelial suspension, and inoculum concentrations of 1×10⁵ and 1×10⁶ conidia per ml provided significant levels of control of the weed in greenhouse and field experiments, respectively. In a host-range study, N. alternantherae infected 6 plant species from a total of 42 species belonging to 23 families. N. alternantherae has the potential to be an effective mycoherbicide for alligatorweed.     

Isolation and characterization of endophytic bacteria from soybean (Glycine max) grown in soil treated with glyphosate herbicide

Endophytic bacteria are ubiquitous in most plant species influencing the host fitness by disease suppression, contaminant degradation, and plant growth promotion. This endophytic bacterial community may be affected by crop management such as the use of chemical compounds. For instance, application of glyphosate herbicide is common mainly due to the use of glyphosate-resistant transgenic plants. In this case, the bacterial equilibrium in plant–endophyte interaction could be shifted because some microbial groups are able to use glyphosate as a source of energy and nutrients, whereas this herbicide may be toxic to other groups. Therefore, the aim of this work was to study cultivable and noncultivable endophytic bacterial populations from soybean (Glycine max) plants cultivated in soil with and without glyphosate application (pre-planting). The cultivable endophytic bacterial community recovered from soybean leaves, stems, and roots included Acinetobacter calcoaceticus, A. junii, Burkholderiasp., B. gladioli, Enterobacter sakazaki, Klebsiella pneumoniae, Pseudomonas oryzihabitans, P. straminea, Ralstonia pickettii,and Sphingomonassp. The DGGE (Denaturing Gradient Gel Electrophoresis) analysis from soybean roots revealed some groups not observed by isolation that were exclusive for plants cultivated in soil with pre-planting glyphosate application, such as Herbaspirillum sp., and other groups in plants that were cultivated in soil without glyphosate, such as Xanthomonas sp. and Stenotrophomonas maltophilia. Furthermore, only two bacterial species were recovered from soybean plants by glyphosate enrichment isolation. They were Pseudomonas oryzihabitans and Burkholderia gladioliwhich showed different sensibility profiles to the glyphosate. These results suggest that the application at pre-planting of the glyphosate herbicide may interfere with the endophytic bacterial communitys equilibrium. This community is composed of different species with the capacity for plant growth promotion and biological control that may be affected. However, the evaluation of this treatment in plant production should be carried out by long-term experiments in field conditions.     

Factors associated with detrimental effects of rhizobacteria on plant growth

Some of the factors interfering with the specific response of young common bean plants to two rhizosphere fluorescent pseudomonads were studied. These two bacterial strains produced symptoms in foliar plant parts and reduced yield in beans and several other plant species when inoculated on roots. Sensitivity in the plants subjected to bacterial application was highest at early growth stages (up to ten days old plants) giving typical symptoms for each strain in first expanding leaves and typically stunted plant growth. Symptoms to some extent also appeared on plants treated at the age of 2–3 weeks and the fresh shoot weight was affected also in such plants. Deleterious effects of the tested bacteria were found to be related to the level of inoculum (cfu/ml) used and presence or absence of certain nutrients in the bacterial suspensions used for inoculation. The two tested strains differed in nutritional requirements for affecting plant growth. One strain needed sucrose only. The other tested strain required peptone or yeast extract in addition to sucrose to induce typical symptoms and significant yield reductions. Supply of peptone as the only nutrient source to the bacterial suspensions eliminated the deleterious effects of both bacterial strains as tested on beans under nonsterile conditions. The two strains are assumed to affect test plants by different modes of action.     

Contribution of soil-borne bacteria to the rotation effect in corn

Few efforts have been directed at understanding how the rhizosphere microbiology of continuous corn may effect crop yields. This relationship may explain, in part, the decreases in yield associated with continuous corn as compared to the corn in rotation with a second crop. This study was conducted to determine the importance of soil-borne microorganisms to yield declines in long term continuous corn. Continuous corn (Zea mays L.) or rotated corn-soybean (Glycine max L.) field plots, established in 1975, under either fall plowing or no-till tillage treatments were used. Treatments consisted of methyl bromide applied at 48.8 g m⁻² 3 days prior to planting in all four combinations. Total plant samples from both the fumigated and non-fumigated areas were collected 14 days after planting. Rhizosphere bacteria were recovered and tested for their ability to impact plant growth. Bacterial assessments were made in a test tube bioassay where germinated corn was transported in to agar containing a bacterial isolate. In the first year of the study a highly significant interaction of fumigation and rotation was indicated. With fumigation continuous corn yields were similar to that of rotated corn-bean. Rotated corn yields were less affected by fumigation. In the second year, the effects were similar but less significant. Over 130 bacterial isolates were tested for their effect on plant growth. Approximately 22% were able to inhibit plant growth. Of these, 72% were from the continuous corn system. Clearly, the interaction of rotation and yield is at a microbiological level. The suggestion that microorganisms similar to those isolated are responsible for controlling early plant growth in the continuous corn system is indicated.     

Transgenic Potatoes Expressing a Novel Cationic Peptide are Resistant to Late Blight and Pink Rot

Potato is the world's largest non-cereal crop. Potato late blight is a pandemic, foliar wasting potato disease caused by Phytophthora infestans, which has become highly virulent, fungicide resistant, and widely disseminated. Similarly, fungicide resistant isolates of Phytophthora erythroseptica, which causes pink rot, have also become an economic scourge of potato tubers. Thus, an alternate, cost effective strategy for disease control has become an international imperative. Here we describe a strategy for engineering potato plants exhibiting strong protection against these exceptionally virulent pathogens without deleterious effects on plant yield or vigor. The small, naturally occurring antimicrobial cationic peptide, temporin A, was N-terminally modified (MsrA3) and expressed in potato plants. MsrA3 conveyed strong resistance to late blight and pink rot phytopathogens in addition to the bacterial pathogen Erwinia carotovora. Transgenic tubers remained disease-free during storage for more than 2 years. These results provide a timely, sustainable, effective, and environmentally friendly means of control of potato diseases while simultaneously preventing storage losses.     

Fitness consequences of infection of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> with its natural bacterial pathogen <Emphasis Type="Italic">Pseudomonas viridiflava</Emphasis>

Variation in plant resistance to pathogen infection is commonly observed in interactions between wild plants and their foliar pathogens. Models of host–pathogen interactions indicate that a large cost of infection is generally necessary to maintain this variation, yet there is limited evidence that foliar pathogens cause detectable fitness reductions in wild host plants. Most published work has focused on fungal pathogens. Pseudomonas viridiflava, a common bacterial pathogen of the annual weed Arabidopsis thaliana across its range, comprises two distinct genetic clades that cause disease symptoms of different severity. Here we measured the extent of infection of wild A. thaliana populations in the Midwest, USA, and examined the effect on seed production, in field and growth-chamber experiments, of experimental inoculation with isolates from the two clades. We found infection with P. viridiflava varied from 0 to 56% in Midwest A. thaliana populations, with the possibility of several leaves per plant infected later in the growing season. In the growth chambers, experimental inoculation reduced seed set by averages of 15 and 11% for clades A and B, respectively. In the field experiment, only clade A affected plant fitness significantly, reducing seed set by an average of 38%. Underlying these average effects we observed both negative and positive effects of infection, and variation in both fitness among plant genotypes and sensitivity to environmental conditions.     

The effects of ultraviolet-B radiation on loblolly pine

Summary Depletion of stratospheric ozone and the resulting increase in ultraviolet-B (UV-B) radiation may negatively impact the productivity of terrestrial ecosystems. This concern has led to a number of studies that report the influence of supplementing UV-B radiation on plant growth and development. However, only two of these field studies have included tree species and both were singleseason experiments. In this study, loblolly pine (Pinus taeda L.) from seven seed sources was grown under natural and supplemental levels of UV-B radiation. Irradiation treatments were continued for three seasons on plants from four of the seven groups and for 1 year only for three groups. The supplemental irradiances simulated those that would be anticipated with stratospheric ozone reductions of 16% and 25% over Beltsville, Md. The effects of UV-B radiation during the 1st year on plant growth varied among the seed sources. The growth of plants from two of the seven seed sources tested showed significant reductions following a single irradiation season and plants from one group tended to be larger under increased UV-B radiation. However, after 3 years of supplemental irradiation, plant biomass was reduced in all four groups by 12–20% at the highest simulated ozone depletion. These results suggest that the effects of UV-B radiation may accumulate in trees and that increased UV-B radiation could significantly reduce the growth of loblolly pine over its lifetime. However, they also point to a need for multiple season research in any analysis of potential consequences of global change on the long-term growth of trees.     

Risk to non-target plants from Charidotis auroguttata (Chrysomelidae: Coleoptera), a potential biocontrol agent for cat’s claw creeper Macfadyena unguis-cati (Bignoniaceae) in Australia

The potential of the leaf beetle Charidotis auroguttata as a biocontrol agent for cat’s claw creeper Macfadyena unguis-cati (Bignoniaceae), an environmental weed in Australia, and risk to non-target plants was evaluated under quarantine conditions. In no-choice tests, C. auroguttata adults and larvae fed on many plant species across different families, but egg to adult development occurred only on the target weed. However, when neonate larvae from the target weed were transferred onto Myoporum boninense australe (Myoporaceae), a non-target native plant, 11.7% completed development, as compared to 95% of larvae that completed development on the target weed. Larval development on this non-target species also took twice as long as on the target weed. No larvae completed development on other test plants. In choice tests, leaf area consumption by adults and larvae was significantly more on the target weed than on other plants, and oviposition occurred only on the target weed. In the no-choice demography trials, adults laid eggs from the second week after emergence on the target weed, with an average of 0.286 eggs/female/day, resulting in an 18-fold increase in the adult population over 16 weeks. On My. boninense australe adult survival remained high, but oviposition commenced only from the 10th week after emergence with an average of 0.023 eggs/female/day, and none of the eggs developed into adults. In the choice demography trials, oviposition on the target weed was evident from the fourth week onwards, while on the non-target plant oviposition commenced only from the 14th week. Only 10% of total adults and 11.3% of total eggs were found on the non-target plant, and none of these eggs developed into adults. Although the biocontrol agent can ‘spill-over’ from the target weed to the non-target native plant and cause adult feeding damage, the non-target plant could not sustain a viable insect population on its own. This agent was not approved for field release in Australia due to perceived risk to non-target species.     

Endophytic Bacteria Induce Growth Promotion and Wilt Disease Suppression in Oilseed Rape and Tomato

To determine whether bacteria isolated from within plant tissue can have plant growth-promotion potential and provide biological control against soilborne diseases, seeds and young plants of oilseed rape (Brassica napus L. cv. Casino) and tomato (Lycopersicon lycopersicum L. cv. Dansk export) were inoculated with individual bacterial isolates or mixtures of bacteria that originated from symptomless oilseed rape, wild and cultivated. They were isolated after surface sterilization of living roots and stems. The effects of these isolates on plant growth and soilborne diseases for oilseed rape and tomato were evaluated in greenhouse experiments. We found isolates that not only significantly improved seed germination, seedling length, and plant growth of oilseed rape and tomato but also, when used for seed treatment, significantly reduced disease symptoms caused by their vascular wilt pathogens Verticillium dahliae Kleb and Fusarium oxysporum f. sp. lycopersici (Sacc.), respectively.     

Endophytic fungi occurring in fennel, lettuce, chicory, and celery — commercial crops in southern Italy

The occurrence of endophytic fungi in fennel, lettuce, chicory, and celery crops was investigated in southern Italy. A total of 186 symptomless plants was randomly collected and sampled at the stage of commercial ripeness. Fungal species of Acremonium, Alternaria, Fusarium, and Plectosporium were detected in all four crops; Plectosporium tabacinum was the most common in all crop species and surveyed sites. The effect of eight endophytic isolates (five belonging to Plectosporium tabacinum and three to three species of Acremonium) inoculated on lettuce plants grown in gnotobiosis was assessed by recording plant height, root length and dry weight, collar diameter, root necrosis, and leaf yellowing. P. tabacinum and three species of Acremonium, inoculated on gnotobiotically grown lettuce plants, showed pathogenic activity that varied with the fungal isolate. Lettuce plants inoculated with the isolates Ak of Acremonium kiliense, Ac of Acremonium cucurbitacearum, and P35 of P. tabacinum showed an increased root growth, compared to the non-inoculated control. The high frequency of P. tabacinum isolation recorded in lettuce plants collected in Bari and Metaponto, and in fennel plants from Foggia agricultural districts, suggests a relationship not only between a crop species and P. tabacinum, but also between the occurrence of the endophyte and the crop rotation history of the soil.     

Plant colonization and environmental fate of the biocontrol fungus Phoma macrostoma

Several isolates of the fungus Phoma macrostoma demonstrated bioherbicidal activity against dandelion seedlings when applied to soil. Weed control ranged from 36 to 100% depending on the isolates and the doses applied. Using microbiological and molecular genetic techniques, the ability of these isolates to colonize target, and nontarget plants and to disperse and persist in soil were determined. PCR primers highly specific to the biocontrol isolates of P. macrostoma, were used to detect the isolates at rates of application between 4 and 1000 g/m². Based on the results from representative isolates tested, it was concluded that P. macrostoma colonized root tissues of both resistant and susceptible crop species and a susceptible weed species grown in treated soil, and the frequency of fungal isolation declined with time. It was occasionally detected on untreated plant tissues, which may have resulted from either natural occurrences on seed, or contamination of soil. The biocontrol fungus appeared to have limited mobility in the soil since it was not often detected away from the area where it was placed. It persisted in the soil at detectable levels for up to 4 months, but then its presence declined with time. One year post application, P. macrostoma was either not present or significantly reduced in both soil and plant samples depending on the year of sampling. The results suggested that the isolates of P. macrostoma used for biological weed control would have minimal environmental impact due to its ubiquitous nature, limited mobility, and weak persistence over seasons.     

Isolation of endophytic endospore-forming bacteria from Theobroma cacao as potential biological control agents of cacao diseases

Sixty-nine endospore-forming bacterial endophytes consisting of 15 different species from five genera were isolated from leaves, pods, branches, and flower cushions of Theobroma cacao as potential biological control agents. Sixteen isolates had in vitro chitinase production. In antagonism studies against cacao pathogens, 42% inhibited Moniliophthora roreri, 33% inhibited Moniliophthora perniciosa, and 49% inhibited Phytophthora capsici. Twenty-five percent of isolates inhibited the growth of both Moniliophthora spp., while 22% of isolates inhibited the growth of all three pathogens. Isolates that were chitinolytic and tested negative on Bacillus cereus agar were tested with in planta studies. All 14 isolates colonized the phyllosphere and internal leaf tissue when introduced with Silwet L-77, regardless of the tissue of origin of the isolate. Eight isolates significantly inhibited P. capsici lesion formation (p = 0.05) in detached leaf assays when compared to untreated control leaves. ARISA with bacilli specific primers amplified 21 OTUs in field grown cacao leaves, while eubacteria specific primers amplified 58 OTUs. ARISA analysis of treated leaves demonstrated that inundative application of a single bacterial species did not cause a long-term shift of native bacterial communities. This research illustrates the presence of endospore-forming bacterial endophytes in cacao trees, their potential as antagonists of cacao pathogens, and that cacao harbors a range of bacterial endophytes.     

Intra- and Interspecific Variation in Plant Response to Inoculation with Deleterious Rhizosphere Pseudomonads

Accessions of wheat, spinach, lettuce and different Brassica species were tested in greenhouse experiments for reaction to inoculation with two isolates of growth-inhibitory rhizosphere bacteria. Seedlings grown in non-sterile soil were inoculated with bacterial suspension and shoot dry weight was measured after five weeks. Large differences were found between the plant species tested in their average sensitivity to each bacterial isolate, and in the majority of plant species, significant differences were also found between accessions in the response to one or both isolates. These findings suggest that, in addition to the variation between plant species, intraspecific variation in the reaction to deleterious bacteria is a common feature in plants. This supports the hypothesis that plant reaction to rhizosphere bacteria is under genetic control. The results further indicate specificity in the interactions between plants and bacterial isolates, both at the plant species level and at the accession level.