VIEW ARTICLE

Ecology and Epidemiology

Chemical and Physical Soil Characteristics Related to Lysis of Oospores of Pythium ultimum. Peiyuan Qian, Research associate, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37901-1071, Permanent address: Instructor in Microbiology, Department of Plant Protection, Shanghai Agricultural College, Shanghai, People’s Republic of China; Leander F. Johnson, Professor, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37901-1071. Phytopathology 77:1062-1066. Accepted for publication 19 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1062.

Thick-walled oospores of Pythium ultimum in agar films on glass slides were buried in soil collected from 11 cultivated fields in western Tennessee. The slides were retrieved after 21, 42, 63, and 84 days of incubation, and oospore morphology was determined. Mean oospore conditions for all 11 soils after 84 days were: 6% thick-walled, 39% thin-walled, and 55% lysed. Four percent of the lysed oospores had germinated before lysis. Significant differences in rates of conversion from thick to thin walls and lysis of thin-walled oospores occurred among the soils. Lysis of oospores was correlated positively with pH, C:N ratio, percent organic matter, and available P and negatively with percent clay. Rates of conversion from thick to thin walls were not significantly correlated with any of these soil variables. Significant correlations of thin-walled or lysed oospores were not found with concentrations of exchangeable Ca, Mg, or K, or with extractable Fe or Mn. From stepwise regression analysis, soil conditions that favored lysis were a high pH, along with high N and P contents. In greenhouse experiments, lysis was increased in an acid soil with a low level of available P by adding calcium hydroxide or calcium phosphate.