INTERACTION BETWEEN STRAINS OF PSEUDOMONADS IN SUGAR-BEET SPERMOSPHERES AND THEIR RELATIONSHIP TO PERICARP COLONIZATION BY PYTHIUM-ULTIMUM IN SOIL

The growth and interaction of coinoculated strains of Pseudomonas spp. on sugar beet seed were monitored for 48 h after planting in natural soil maintained at -15 J/kg and 16 C. There was no interaction between and among strains in the spermosphere when they were inoculated together at approximately 10(4) cfu per seed. Strains capable of producing antibiotics and siderophores did not inhibit sensitive strains in the spermosphere. However, antagonism between strains often occurred when seeds were coinoculated with one strain at a high inoculum density (approximately 10(7) cfu per seed) and the other at a low density (approximately 10(4) cfu per seed). For example, growth of Pseudomonas fluorescens-putida ML5 was inhibited in the spermosphere when coinoculated with high densities of P. fluorescens A1 or P. putida GR12-2. In contrast, P. putida 332 and A1 were less affected under similar conditions. Although ML5 was inhibited by strains A1 and GR12-2 on King's medium B, the addition of 100 mu M FeCl3 or Fe-EDTA into soil did not affect growth of ML5 in the spermosphere. This suggested that siderophore production was not a mechanism that affected inhibition of ML5 in the spermosphere. Growth of rifampicin-resistant strains in the spermosphere was inhibited in the presence of near-isogenic wild type strains on seed when the two were inoculated at low and high inoculum densities, respectively. However, inhibition did not occur when the plasmid NAH7 was introduced into the low-inoculum rifampicin-resistant strain and sodium salicylate was added to the soil. The plasmid enabled the bacterium to use the substrate. A reduction to complete nullification of growth inhibition by addition of sodium salicylate in soil also was observed when the low-inoculum strain harboring the plasmid was coinoculated with different (nonisogenic) strains at high densities. These results suggest that competition for carbon was the primary factor affecting antagonism in the spermosphere. The effectiveness of ML5 in reducing pericarp colonization by Pythium ultimum was significantly reduced in dual strain inoculations with A1. Other combinations of strains did not suppress pericarp colonization by P. ultimum more effectively than did single strain inoculations.