In this study we designed and tested a methodology to minimize the variation of soil matric potential due to changes in soil water content, thus achieving a close control over the water regime in a microcosm, used to study the biocontrol of Pythium sp. infection of seeds by antagonistic bacteria. The variation of volumetric water content and matric potential were monitored at different depths during an average experimental period of 14 days in soil, contained in replicate 1 315 cm(3) microcosms, each sown with 16 sugar beet (Beta vulgaris) seeds and placed in a phytotron. Several experiments with target soil matric potentials of -10, -100 and -300 kPa were performed. It was found possible to maintain a matric potential of about -10 kPa in soil with minimum water content gradients between different parts of the microcosm by watering uniformly at each depth. With all appropriate watering and sampling procedure it was also possible to control psi(m) between -150 and -50 kPa, with a target value around -100 kPa; and to control psi(m) the range between -250 and -375 kPa for a target value of -300 kPa. In all experiments spatial and temporal variability of matric potential was larger at the top than in the Middle and in the bottom of microcosms. Variability in matric potential between replicate microcosms within each experiments was attributed to slight variation in packing, soil texture and drying rate, initial VWC.

The effect of organic, low-input and conventional management practices on barley yield and disease incidence was assessed in field trials over two years. Conventional fertility management (based on mineral fertiliser applications) and conventional crop protection (based on chemosynthetic pesticides) significantly increased the yield of winter barley as compared to organic fertility and crop protection regimes. Severity of leaf blotch (Rhynchosporium secalis) was highest under organic fertility and crop protection management and was correlated inversely with yield. For mildew (Erysiphe graminis), an interaction between fertility management and crop protection was detected. Conventional crop protection reduced severity of the disease, only under conventional fertility management. Under organic fertility management, incidence of mildew was low and application of synthetic pesticides in “low input” production systems had no significant effect on disease severity.

The effect of initial inoculum density of the antagonistic bacterial strains Pseudomonas fluorescens B5 and Pseudomonas corrugata 2140 (103 to 108 CFU per seed pellet) on sugar beet seedling colonisation, in situ bioluminescence and antagonistic activity towards Pythium ultimum was investigated. Populations of the bacteria colonising sugar beet root systems approached an apparent carrying capacity of 105 to 106 CFU per plant after 12 d growth, irrespective of inoculum density. This meant an up to 320-fold population increase at low inoculum densities and a decrease at high densities. Population densities of both bacteria and their corresponding in situ bioluminescence (resulting from luciferase enzyme expression from the inserted luxAB genes) reached highest levels in the hypocotyl region and in the upper root region 0–20 mm below seed level (104–106 CFU/cm section, 101–103 RLU/cm section) and decreased with root depth. In situ bioluminescence, which indicates physiological activity, was measurable at lowest antagonist initial inoculum density (103 CFU per seed pellet) and did not increase significantly with increasing inoculum density. Bioluminescence was also significantly correlated with population density. For Pseudomonas fluorescens B5, the total population size per plant and downward colonisation of the root (below 40 mm depth) increased significantly with antagonist inoculum density applied to the seeds. For Pseudomonas corrugata 2140, no significant influence of initial inoculum density on root colonisation was observable. Survival and dry weight of sugar beet seedlings in Pythium infested soil increased significantly with increasing inoculum density of Pseudomonas fluorescens B5, whereas for Pseudomonas corrugata 2140, initial densities of 104 to 106 CFU per seed resulted in maximal survival of plants.

Pseudomonas fluorescens B5 and Bacillus subtilis MBI 600 colonized sugar beet seedlings at matric potentials of -7 × 103, -140 × 103, and -330 × 103 Pa and under five temperature regimes ranging from 7 to 35°C, with diurnal fluctuations of 5 to 22°C. No interaction between matric potential and temperature was observed. In situ bioluminescence indicated physiological activity of Pseudomonas fluorescens B5. Colonization of the root at ≥4 cm below the seed decreased at very low matric potential (-330 × 103 Pa). Total population size of Pseudomonas fluorescens B5 per seedling was significantly increased at -140 × 103 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 35°C). Maximum colonization down the root occurred at intermediate temperature (15°C) at both matric potentials (-7 × 103 and -140 × 103 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 × 103 Pa, soil dryness but not Pythium infection was the limiting factor for plant emergence. At temperatures of 7 to 25°C and matric potentials of -7 × 103 to 120 × 103 Pa, treatment with Pseudomonas fluorescens B5 increased plant survival and dry weight. At 7°C and -120 × 103 Pa, there was almost complete emergence of seeds treated with Pseudomonas fluorescens B5. Antagonistic activity of Pseudomonas fluorescens B5 decreased with increasing soil temperature and decreasing matric potential. At 25 to 35°C and -7 × 103 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.