Populations of bacteria and actinomycetes associated with sclerotia ofPhymatotrichum omnivorum buried in Houston black clay

Abstract

Sclerotia ofPhymatotrichum omnivorum (Shear) Duggar (the causal agent of root rot of cotton) were produced in the laboratory and then buried at a depth of 45 cm at three sites in Texas situated on Houston black clay soils with various cropping histories. The sites included a native grassland prairie, a field in continuous cotton production, and a field in which cotton, corn, and sorghum were grown in rotation. Samples of sclerotia were retrieved monthly over a 12 month period. Populations of bacteria and actinomycetes were enumerated using dilution-plate techniques and isolates were screened (in vitro) for their ability to produce substances inhibitory toP. omnivorum. The sclerotia supported large numbers of bacteria (including fluorescent pseudomonads) and actinomycetes. Numbers associated with sclerotia ranged from 106–109 cells per gram of sclerotia plus adherent soil and were 2–3 orders of magnitude greater than numbers from soil at the same depth but free of sclerotia. Bacteria and actinomycetes antagonistic toP. omnivorum were isolated from sclerotia buried at each of the three sites. Up to 26% of the isolates inhibited growth ofP. omnivorum.