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Effect of Soil Type and Soil Matric Potential on Infection of Tobacco by Phytophthora parasitica var. nicotianae. J. R. Sidebottom, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; H. D. Shew, assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 75:1439-1443. Accepted for publication 22 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1439.

Black shank development was observed in greenhouse-grown tobacco seedlings growing in soil material collected from 12 soil series. When infested with similar levels of Phytophthora parasitica var. nicotianae, the soils varied in conduciveness to disease development; seedling infection ranged from 8 to 92%. Disease incidence was correlated with soil texture, drainage class of the parent soil, fumigation, and nutrient status. In four of the soil materials maintained at four controlled matric potential regimes, infection of tobacco seedlings was reduced in each soil material at - 40 millibars (mb) compared to higher potentials. Disease suppression at - 40 mb was attributed to restricted zoospore release and dispersal. Infection was greatest in each soil material when plants at - 40 mb were exposed to a single 24-hr saturation period (- 40 mb +S) 1 wk into a 3-wk test. Infection was lower in two disease-suppressive soil materials at - 10, - 20, and - 40 mb + S than in two disease-conducive soil materials. Propagule survival over a 6-mo period in the different soil materials was not correlated with disease conduciveness even though survival was lower in some untreated compared to fumigated soils. The nature of disease suppression appeared to be a combination of microbial, physical, and chemical factors.