EFFECT OF ORGANIC-MATTER DECOMPOSITION LEVEL ON BACTERIAL SPECIES-DIVERSITY AND COMPOSITION IN RELATIONSHIP TO PYTHIUM DAMPING-OFF SEVERITY

Rhizosphere bacteria were isolated from root tip segments of cucumber seedlings grown in a suppressive, slightly decomposed light-colored pe at mix, a conducive, more decomposed dark-colored peat mix, and a supp ressive dark peat mix amended with composted hardwood bark. The bacter ia were identified by a gas chromatographic fatty acid methyl ester an alysis. The total number of taxa recovered from a single root tip segm ent ranged from 9 to 18. No single taxon predominated on all root tip segments harvested from any of the mixes. The highest relative populat ion density reached by a given taxon on any root tip segment was 45%. Hill's first and second diversity numbers, the modified Hill's ratio, and Hurlbert's rarefaction method, which were used as measures of spec ies diversity, indicated that the organic matter decomposition level o f the potting mixes did not affect bacterial species diversity. Bray-C urtis polar ordination and Dice resemblance functions, however, indica ted that the organic matter decomposition level of a mix significantly influenced the composition of bacterial species in the rhizosphere. P seudomonas spp. and other taxa capable of inducing suppression of pyth ium damping-off predominated in the suppressive mixes. These organisms were absent from the conducive mix, in which Arthrobacter and Bacillu s spp. predominated. Although effective bacterial biocontrol agents we re isolated from both the suppressive mixes and the conducive mix, the majority were isolated from the less decomposed suppressive mixes. Fi nally, the efficacy of strains was significantly greater in the slight ly decomposed light peat mix than in the decomposed dark peat mix. Nat ural disease suppression within these mixes was associated with the or ganic matter decomposition level and the bacterial species composition s of the mixes.