Effects of Soil Fumigation and Cover Crops on Potential Pathogens, Microbial Activity, Nitrogen Availability, and Seedling Quality in Conifer Nurseries

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Abstract
In three forest tree nurseries of the Pacific Northwest, fall fumigation with chloropicrin or dazomet led to an immediate increase in total microbial respiration and nitrogen availability, but levels were comparable to those in unfumigated plots when tree seedlings were sown the next spring. Population densities of Fusarium spp. and Pythium spp. in fumigated plots were reduced dramatically and stayed significantly below those in unfumigated plots throughout the 2-yr crop-cycle. Grass or legume cover crops increased pathogen population densities over those in bare, fallow plots before fumigation, and the trend continued throughout the crop cycle in unfumigated plots. At the end of the study, population densities of Fusarium in fallow, unfumigated plots were comparable to those in cover-cropped, fumigated plots. Seedling mortality was lower, and surviving seedlings were larger and more uniform on fallow and fumigated plots than on cover-cropped or unfumigated plots.