Maize grains were exposed to 16-20 combinations of soil moisture and soil temperature (1-30 degrees C.) for varying periods of time. Grains germinated readily in the wettest soils at 30 and 20 degrees C., but germination was inhibited at the lower levels of soil moisture, and inhibited or delayed at the lower temperatures. Grains which failed to germinate in the soil were transferred to damp sand at 20 degrees C. and the residual viability of the samples was determined. Mortality of grain was highest at combinations of temperature and soil moisture which just did not allow of germination. Soil conditions even more unfavourable for germination gave greater residual viability in the samples. The viability of dormant grains was more rapidly destroyed at high than at low temperatures although the reduction in viability continued longer at low temperatures, probably because of the slower drying of the soil. This mortality is interpreted in terms of pathogenic activity, and it is shown that mortality at high temperatures (20-35 degrees C.) is explicable on this general hypothesis. It is tentatively suggested that temperature and soil moisture conditions interact in selecting out and favouring the activity of specific populations of pathogens in the soil.
