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Ecology and Epidemiology

Effects of Soil Moisture and Temperature on Phomopsis Seed Decay of Soybean in Relation to Host and Pathogen Growth Rates. M. L. Gleason, Department of Plant Pathology, University of Kentucky, Lexington 40546, Current address: Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011; R. S. Ferriss, Department of Plant Pathology, University of Kentucky, Lexington 40546. Phytopathology 77:1152-1157. Accepted for publication 9 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1152.

Experiments were conducted in which soybean seeds were incubated for 3 days in pasteurized soil under specific temperature and moisture conditions followed by 18 days at soil temperature and moisture near optimum for emergence. In the temperature-moisture experiment, seeds from a seed lot with 100% incidence of seed coat infection after artificial inoculation with Phomopsis longicolla and from a corresponding control lot with < 1% infection were incubated at initial soil matric potentials of 0.008 and 15 bars combined with temperatures ranging from 12 to 36 C. Final relative emergence (inoculated lot emergence/control lot emergence) was lowest at 15 bars and 18 or 24 C. Performance of the inoculated and control lots did not differ significantly for any temperature at 0.008 bars. In the moisture experiment, seeds from a lot with 45% natural infection and from a control lot with < 1% infection were incubated at initial soil matric potentials ranging from 0.008 to 51 bars at 2127 C. Relative emergence decreased with decreasing matric potential. Results of both experiments were compared with independent variables composed of measures of germination and emergence rates of the control lot under the same soil moisture and temperature conditions and the growth rate of P. longicolla at different temperatures on potato-dextrose agar osmotically adjusted with KC1. For the moisture experiment, final relative emergence was significantly (P< 0.05) correlated with times to 50% germination (Gt50) or emergence (EMt50) and the coefficient of velocity of emergence but not with pathogen growth rate (PGR). These results indicate that host growth was probably the controlling factor in determining the outcome of the host-pathogen interaction. For the temperature-moisture experiment, relative emergence was not significantly correlated with any independent variable, but disease incidence (DI = 1 [relative emergence]) was significantly correlated with a number of independent variables. In general, DI was more highly correlated with independent variables containing Gt50 than EMt50, and the variable with the highest correlation involved the summation of values of PGR multiplied by Gt50 for the period before germination. These results indicate that DI may be related to Gt50 both because Gt50 is a measure of the duration of the host-pathogen interaction and because the inverse of Gt50 is a measure of host response during that period.