Abstract
Both direct and indirect effects of the environment, and the possible role of the crop in modifying the environment of the weeds, are considered. High humidities and high temperatures in general increase susceptibility, and temperature also may have important morphogenetic effects. Low light intensity before, and high intensity after, spraying appear to increase susceptibility. Light also may affect leaf morphogenesis. Soil condition also may affect susceptibility, particularly soil residues of certain herbicides, and crop competition may contribute to weed mortality. The relationship of stage of growth at spraying to susceptibility is also discussed.