POPULATION-DYNAMICS OF PSEUDOMONAS-CEPACIA IN THE PEA SPERMOSPHERE IN RELATION TO BIOCONTROL OF PYTHIUM

Pseudomonas cepacia strain AMMD applied to pea seeds reduced preemergence damping-off caused by Pythium ultimum and P. sylvaticum by 47% in growth chamber experiments. Protection by the biocontrol agent was as effective as seed treatment with metalaxyl at all temperatures tested (16, 20, 24, and 28 C). Under controlled soil matric potential conditions (-6 kPa) at 24 C, Pythium began to infect nontreated seeds within 12 hr, and 100% of the seeds were infected 30 hr after planting. Incidence of seed infection during the first 48 hr was reduced 44-60% by seed treatment with metalaxyl or P. cepacia strain AMMDR1, resistant to rifampicin. No significant difference was found between the effectiveness of the fungicide and the biocontrol agent. The incidence of seed infection by Pythium, and doubling time of the introduced strain in the pea spermosphere, were inversely proportional to the initial population density of P. cepacia strain AMMDR1 on the seeds. When applied at log 4.9 colony-forming units per seed, the doubling time of this strain during the first 24 hr after planting was 3.1 hr. The introduced strain represented an increasing proportion of the population of bacteria associated with the seed during this period, an indication of its competitive success in the spermosphere.