Many candidate mycoherbicides have shown promise in the laboratory or greenhouse, but most have been ineffective in the field. Factors limiting mycoherbicide efficiency include temperature and humidity. Results from this thesis indicate that solar radiation has both a damaging effect(reduction in germination)limiting efficacy and a photomorphogenic effect(appressorium induction)increasing efficacy. The study has also shown significant interaction between temperature and solar radiation on the survival of conidia of potential mycoherbistats. Therefore, solar radiation should be considered as third major component of the environment that should be considered when trying to produce mycoherbistats. With the findings presented in this thesis and further research on disease development under different conditions, in combination with the formulation of conidia in suitable UV protectants, a computer simulation modelling the conditions leading to epidemics caused by C.orbiculare, D.avenacea and R.alismatis could be constructed. It may be possible to manipulate fungal application time in order to expose conidia to doses of solar radiation that are not harmful to conidium germination and which stimulate appressorium formation. However, additional protection may be needed. / Doctor of Philosophy (PhD)