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. 2002 Jun;68(6):2637-43.
doi: 10.1128/AEM.68.6.2637-2643.2002.

Protection of tomato seedlings against infection by Pseudomonas syringae pv. tomato by using the plant growth-promoting bacterium Azospirillum brasilense

Yoav Bashan  1 , Luz E De-Bashan
Affiliations

Protection of tomato seedlings against infection by Pseudomonas syringae pv. tomato by using the plant growth-promoting bacterium Azospirillum brasilense

Yoav Bashan et al. Appl Environ Microbiol. 2002 Jun.

Abstract

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10(7) versus 10(5) CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10(7) versus 10(6) CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10(5) to 10(6) CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10(8) CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.

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Figures

FIG. 1.
FIG. 1.
Sizes of P. syringae pv. tomato and A. brasilense populations in the rhizospheres and on the foliage of tomato plants when the bacteria were inoculated on separate plants. Points located on the same line and denoted by different lowercase letters differ significantly at a P of ≤0.05 by one-way ANOVA. Points which share the same time position and are denoted by different capital letters differ significantly at a P of ≤0.05 by Student's t test. Bars represent standard errors (SE); missing bars indicate that the SE is smaller than the point.
FIG. 2.
FIG. 2.
Sizes of P. syringae pv. tomato and A. brasilense populations in the rhizospheres of tomato plants (A) and their effects on the dry weights (dw) of tomato seedlings (B) when the bacteria were inoculated alone and together onto seeds. (A) Points located on the same line and denoted by different lowercase letters differ significantly at a P of ≤0.05 by one-way ANOVA. Points which share the same time position and are denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA. (B) Columns denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA. Bars represent SE; missing bars indicate that the SE is smaller than the point.
FIG. 3.
FIG. 3.
Sizes of P. syringae pv. tomato and A. brasilense populations (A), the development of bacterial leaf speck disease in tomato leaves (B), and the effect on dry weight of the plants (C) after inoculation of the bacteria onto leaves. (A and B) Points located on the same line and denoted by different lowercase letters differ significantly at a P of ≤0.05 by one-way ANOVA. Points which share the same time position and are denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA. (C) Columns denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA. Bars represent SE; missing bars indicate that the SE is smaller than the point.
FIG. 4.
FIG. 4.
Sizes of P. syringae pv. tomato and A. brasilense populations (A), development of bacterial speck disease in tomato leaves (B), and the effect on dry leaves of the plants (C) after leaf inoculations. (A and B) Points located on the same line and denoted by different lowercase letters differ significantly at a P of ≤0.05 by one-way ANOVA. Points which share the same time position and are denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA (A) and by Student's t test (B). (C) Columns denoted by different capital letters differ significantly at a P of ≤0.05 by one-way ANOVA. Bars represent SE; missing bars indicate that the SE is smaller than the point. For clarity, values for A. brasilense inoculated alone onto leaves are not presented, as similar information is available in other graphs; values for P. syringae pv. tomato supplemented with malic acid are also not presented, as these were almost identical to those from inoculation with P. syringae pv. tomato alone. For clarity, some letters indicating statistical significance in panel A were also eliminated.
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