Effect of a sudangrass green manure in two soil types on the activity, populations and community structure of soil microorganisms, populations of Verticillium dahliae in soil and on potato roots, and potato early dying, were evaluated in a field microplot experiment in the Columbia Basin. Potato cv. Russet Burbank was grown in identical green manure and fallow soil treatments transported from Idaho and Washington where sudangrass previously suppressed or enhanced early dying of potatoes, respectively. Incorporation of sudangrass increased total microbial activity (TMA) by 46.2 and 30.1% in the Idaho soil in 1996 and 1997, and by 43.0% in the Washington soil in 1996 only. Neither green manure or soil type, however, affected soil populations of Fusarium, total bacteria, or actinomycetes. Across soil type, fluorescent pseudomonad populations were unaffected or increased by 107% in 1996 and 1997, respectively. Although not repeated across years, Fusarium root populations were 19.7 and 28.3% higher in sudangrass treated soil from Washington in 1996 and Idaho in 1997, respectively, but the proportion of Fusarium species were similar across soil types. Bacterial rhizosphere communities, based on sole-carbon-source utilization patterns on Biolog GN microplates, did not differ among the soil types or green manure treatments. Across soil type, V. dahliae soil and root populations were unaffected in 1996, but were lower by 20.4 and 41.2% in Idaho sudangrass soil treatments in 1997. Apical stem populations of V. dahliae and disease severity, however, did not differ among the treatments. Although tuber yield in Washington soil was 31.5% higher than Idaho soil in 1996, yield was not affected by a sudangrass green manure. Because suppression of early dying of potato observed in Idaho was not replicated in transported soil in the Columbia Basin, the macroenvironment may interact with the sudangrass green manure to regulate the effect, or lack of effect on disease. TMA is not an indicator of disease suppressive ability of a soil following sudangrass as activity increased despite a lack of effect on disease. Based on this study, there is no evidence for differences in microbial populations or communities between the Idaho and Washington soils that could explain the suppression or enhancement of early dying of potato by a sudangrass green manure. / Graduation date: 1998