Influence of soil temperature and matric potential on sugar beet seedling colonization and suppression of Pythium damping-off by the antagonistic bacteria Pseudomonas fluorescens and Bacillus subtilis

Pseudomonasfluorescens B5 and Bacillus subtilis MBI 600 colonized sugar beet seedlings at matric potentials of -7 x 10(3), -140 x 10(3), and -330 x 10(3) Pa and under five temperature regimes ranging from 7 to 35degreesC, with diurnal fluctuations of 5 to 22degreesC. No interaction between matric potential and temperature was observed. In situ bioluminescence indicated physiological activity of Pseudomonasfluorescens B5. Colonization of the root at greater than or equal to4 cm below the seed decreased at very low matric potential (-330 x 103 Pa). Total population size of Pseudomonasfluorescens B5 per seedling was significantly increased at -140 x 10(3) Pa. However, matric potential had no significant effect on the population density of Pseudomonas fluorescens per gram of root fresh weight and did not affect the distribution of the population down the root. Total population size per seedling and downward colonization by Pseudomonas fluorescens B5 were significantly reduced at high temperatures (25 to 35degreesC). Maximum colonization down the root occurred at intermediate temperature (15degreesC) at both matric potentials (-7 x 10(3) and -140 x 10(3) Pa). Addition of B. subtilis MBI 600 to the seed had no effect on rhizosphere populations of Pseudomonas fluorescens B5. Populations of B. subtilis MBI 600, which consisted largely of spores, were slightly reduced at lower matric potentials and were not affected by temperature. Survival and dry weight of plants in soils infested with Pythium spp. decreased with increasing soil temperature and matric potential, indicating an increase in disease pressure. However, there was no significant interaction between the two factors. At -330 x 103 Pa, soil dryness but not Pythium infection was the limiting factor for plant emergence. At temperatures of 7 to 25degreesC and matric potentials of -7 x 10(3) to -120 x 10(3) Pa, treatment with Pseudomonasfluorescens B5 increased plant survival and dry weight. At 7degreesC and -120 x 10(3) Pa, there was almost complete emergence of seeds treated with Pseudomonasfluorescens B5. Antagonistic activity of Pseudomonasfluorescens B5 decreased with increasing soil temperature and decreasing matric potential. At 25 to 35degreesC and -7 x 10(3) Pa, no effect was observed. In regimes with different day and night temperatures, the maximum (day) temperature was decisive for disease development and antagonistic activity. B. subtilis MBI 600 displayed no significant antagonistic effect against Pythium ultimum and did not influence the performance of Pseudomonas fluorescens B5 in combined inocula.