VIEW ARTICLE

Ecology and Epidemiology

Effects of Soil Texture and Matric Potential on Sporangium Production 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 75:1435-1438. Accepted for publication 22 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1435.

Mycelial mats of Phytophthora parasitica var. nicotianae grown on nylon mesh in 5% V-8 juice broth were incubated in the sand fraction of a Wagram fine sandy loam at constant matric potentials ranging from 0 to - 15 bars. After 2 days of incubation, few to no sporangia were produced on mats at 0 millibars (mb) and at - 10 mb, production was erratic at - 20 and at - 30 mb, while > 140 sporangia per square millimeter were produced between - 40 mb and - 15 bars. Sporangium production also was observed in five soils or their sand fractions maintained at constant matric potentials between +2 mb (flooded) and - 40 mb. Soil texture was a critical factor in determining the highest matric potential at which sporangia were consistently produced; - 40 mb in the sand fraction of a fine sandy loam, - 20 mb in the sand fraction of a sandy loam, and 0 mb in a coarse sand. Sporangium production was abundant in Grantham silt loam, a soil material conducive to development of tobacco black shank, but were inhibited over a range of matric potentials in two soil materials (Duplin sandy loam and Coxville sandy loam) characterized as suppressive to black shank. Sporangium production was inhibited in untreated (compared to pasteurized) Norfolk sandy loam even though the soil was conducive to disease development. Soil pasteurization did not influence sporangium production in the Grantham, Duplin, or Coxville soil materials. The effects of matric potential on sporangium production by P.p. var. nicotianae were modified by soil texture, soil pasteurization, and unknown factors contributing to suppression of black shank disease in several soils.