Search

Abstract: Three fluorescent pseudomonads, isolates CW2, WB15 and WB52, were tested for their antagonistic activities against Pythium ultimum and Rhizoctonia solani isolates. In vitro experiments revealed pronounced inhibition zones on PDA agar medium against P. ultimum and R. solani by the isolates CW2 and WB15, respectively. Escherichia coli strains transformed with plasmids from the fluorescent pseudomonads isolates caused larger inhibition zones against both pathogens compared to the wild type isolates. In greenhouse experiments, the most effective isolate against P. ultimum was CW2 and isolate WB52 was the most effective against R. solani. The transformed E. coli strains proved to be significantly (P <0.05) more effective in reducing pre-emergence damping-off in cucumber caused by P. ultimum than the wild type isolates, while their efficacy against damping-off incited by R. solani was similar to or less than that of the wild type fluorescent pseudomonads isolates. According to the results obtained by HPLC analysis, an isomer of 2,4-diacetylphloroglucinol (DAPG) was detected in culture filtrates of isolate CW2, while in the E. coli strain transformed with the CW2 plasmid, pyoluteorin and other phenazine derivatives were detected. In wild type isolates of WB15 or WB52 and in E. coli isolates transformed with plasmids from the fluorescent pseudomonads isolates, pyoluteorin and phenazine derivatives were detected. These results suggest that the expression of some genes might be blocked in the wild type isolates. [Copyright &y& Elsevier]

Abstract: Fluorescent pseudomonads are ubiquitous bacteria that are common inhabitants of the rhizosphere and are the most studied group within the genus Pseudomonas. Bacterial isolates (n =103) from the rhizosphere of wheat and common bean were assessed as potential biocontrol agents in this study. Fungal inhibition tests were performed by a plate assay in which each isolate was tested directly for the production of hydrogen cyanide, protease, siderophore and cellulase. Production of DAPG was verified by using an analytical high performance liquid chromatography assay (HPLC). Plant growth promotion was assessed in phytochamber trials and biocontrol activity was evaluated in greenhouse trials. In all, 52 bacterial isolates with antifungal activity against Rhizoctonia solani were found. Of the 52 isolates, 41 were selected according to their high efficiency in in vitro antagonism, which was shown as inhibition zones in the dual-culture assay. Six of the 41 rhizobacteria, including isolates UTPF7, UTPF13, UTPF18, UTPF22, UTPF27 and strain CHA0 produced HCN. Production of protease enzyme was detected for all isolates excluding UTPF30 isolate. Although some stains appeared not to produce any compound with affinity for ferric iron, other isolates produced prolific amounts, creating a large zone of orange (up to 160mm2, i.e., UTPF16). Seventeen of 41 isolates of fluorescent pseudomonads including strain CHAO produced different amounts of DAPG ranging from 0.6 to 11.4ng/108 cfu. A total of 39 isolates induced statistically significant effects on plant growth compared with the non-treated control for at least one parameter. The predominant influence observed was increased root length. No bacteria could completely protect the plant against R. solani, although all isolates significantly increased fresh weight as compared to the infested control in greenhouse trials. Pseudomonas fluorescens isolates UTPF16 and UTPF26 significantly (P <0.05) decreased the number of seedlings with damping-off symptoms in the means of the experiments. [Copyright &y& Elsevier]

Abstract: Control of septoria tritici blotch (STB) of wheat caused by the fungal pathogen Mycosphaerella graminicola is heavily reliant on fungicide application. Rapid emergence of fungal isolates resistant to Quoine outside Inhibitors (QoI) and with reduced sensitivity to triazole-based fungicides necessitates the development and adoption of alternative control strategies. A collection of bacteria originating from barley leaves and grain, oat chaff and wheat rhizospheres and leaves were screened for their ability to control STB. A total of seven bacteria were further assessed under controlled environmental conditions and these inhibited STB development by up to 92%. Three of these bacteria [Pseudomonas fluorescens (strains MKB21 and MKB91) and a Bacillus megaterium (strain MKB135)] were assessed for their abilities to control STB on adult wheat plants in small-scale field trials, conducted both in 2004 and 2006. In these trials only B. megaterium (strain MKB135) consistently retarded STB development (by up to 80%). Additional in vitro seedling studies showed that both B. megaterium cell wall components and its culture filtrate and P. fluorescens strain MKB91 culture filtrate were capable of inhibiting disease development (by 62, 36 and 52%, respectively). While none of the three bacteria directly inhibited fungal growth in dual culture plate assays, in dual liquid culture assays volatiles produced by these bacteria reduced fungal biomass production by ⩾43%. Thus the ability of the most promising bacterium, B. megaterium strain MKB135, to reduce STB severity may be the result of a combination of different mechanisms. [Copyright &y& Elsevier]

Abstract: Bacterial wilt caused by Ralstonia solanacearum (Smith) has become a severe problem mainly on potato and tomato in Ethiopia and no effective control measure is available yet. To explore possibilities for the development of biological control for the disease, 118 rhizobacteria, most of them collected from Ethiopia, were screened against an Ethiopian R. solanacearum strain. On the basis of in vitro screening, six strains (RP87, B2G, APF1, APF2, APF3, and APF4) with good inhibitory effect were selected for in planta testing in a greenhouse. In the greenhouse, soil and tomato seedlings were treated with the antagonists and their effects studied. The study showed that APF1 and B2G strains significantly reduced disease incidence and increased weight of tomato plants. Area under disease progress curves (AUDPC) was reduced by 60% and 56% in plants inoculated with APF1 and B2G strains, respectively. Plant dry weight increase in plants inoculated with APF1 and B2G strains was 96% and 75%, respectively. APF1 was found to be the most beneficial strain in disease suppression and also growth promotion resulting in 63% dry weight increase compared to untreated control. The study revealed that APF1 and B2G strains are promising strains whose effectiveness under field conditions and their mode of action should be investigated. [Copyright &y& Elsevier]

Striga hermonthica (Del.) Benth. is an obligate root-parasitic flowering plant that severely threatens cereal production in sub-Saharan Africa. A potential biological control option for reduction of crop yield–loss within the season of application is the use of soil-borne antagonists of S. hermonthica seed. This study was aimed at selecting soil-borne fluorescent pseudomonad strains capable of suppressing germination of S. hermonthica seeds and consequently reducing parasitism and damage to maize. An in vitro screening procedure was developed and was used to evaluate 460 fluorescent pseudomonad isolates from naturally suppressive soils. This resulted in the identification of 15 Pseudomonas fluorescens/P. putida isolates that significantly inhibited germination of S. hermonthica seeds. In a pot experiment using steam-sterilized soil, there was a significant reduction in the number of S. hermonthica plants on maize grown from seeds that were inoculated with any of the 15 bacterial isolates. Inoculation of maize seed with six of these isolates resulted not only in a reduced number of S. hermonthica plants, but also in an increased maize shoot biomass compared with the check. When soils inoculated with these bacterial isolates were left dried for 5 weeks after maize harvest and then planted with a second maize crop, no reduction in S. hermonthica parasitism was observed. This suggested that the bacteria did not persist in the soil after the first crop of maize. These results suggest that saprophytic fluorescent pseudomonads have potential for biological control of S. hermonthica in maize and that periodic application of bacteria, perhaps through seed treatment, may be necessary for sustained control. [Copyright &y& Elsevier]