INVOLVEMENT OF ROOT-COLONIZING BACTERIA IN PEACH ORCHARD SOILS SUPPRESSIVE OF THE NEMATODE CRICONEMELLA-XENOPLAX

A field site suppressive to peach tree short life and the ring nematode Criconemella xenoplax was identified. Steam treatment of these soils eliminated suppressiveness, whereas small amounts of nonsteamed suppressive soil added to steamed soil inhibited C xenoplax multiplication. Chemical and physical analysis of the soil detected no significant differences between suppressive and nonsuppressive soils, supporting the hypothesis that a biological agent(s) is important in C xenoplax suppression. Among 290 randomly selected field strains of fluorescent pseudomonads from the rhizosphere of peach trees growing in suppressive soil, approximately 2.4% suppressed nematode population increases, 5.2% increased multiplication rates, and 92.4% had no effect in greenhouse bioassays. In root colonization tests, a suppressive strain exhibited the population dynamics typical for many root-colonizing pseudomonads. Bacterial populations increased to approximately 2 X 10(6) cells per gram of root and then declined to 5 X 10(2) cells per gram of root 12 wk later. Concomitantly, ring nematode population increases were significantly attenuated on peach seedlings receiving the bacterial treatment. These results suggest that rhizobacteria contribute to the antagonism of C xenoplax in the suppressive soils.