Abstract: Disease development is the cumulative effect of various factors on the host andpathogen whereby either a-biotic effects such as climate or beneficial microorganisms may affectthe severity of the disease. Oidium neolycopersici is the causal agent of tomato powdery mildew;it causes powdery white lesions on the leaf surface, petioles and the calyx. Severe infections leadto marked reduction in fruit size and quality and to leaf wilt. A slight change in temperatures mayallow either more severe disease (22 to 24°C change) or reduction in disease severity (26 to28°C). Beneficial microorganisms can provide sufficient control, but they are sensitive toenvironmental conditions. A better understanding of the influence of environmental factors onpathogen – biocontrol agent interaction can help in improving their efficacy. The effect ofmicroclimate on the development and survival of two powdery mildew antagonists wasevaluated. When applied on detached leaves and incubated for 7-14 days at differenttemperatures, RH, and disease levels, the bacterium (B2) survival was poorer at hightemperatures and low RH compared to the yeast (Y13). The microorganisms survived well at 10-15°C and high RH. Tomato plants were grown in a net house with climate regimes of high andlow RH, and two disease levels were induced. Plants were sprayed weekly with the twomicroorganisms. The bacterium survived better under high RH as compared with lower RH andon leaves with powdery mildew as compared with symptomless leaves. The yeast was lessaffected by microclimate conditions, and survived well for 14 days even with a singleapplication. We can conclude that the yeast survives better under different microclimateconditions. In the present study, various abiotic factors were found to affect different aspects ofthe tomato powdery mildew disease cycle. It was demonstrated that a slight change inmicroclimate conditions can affect the outcome of the interaction of plants with a pathogen, andof the plant-pathogen relationship with populations of control agents. The effects of climatechange may be different in different plant-pathogen systems. Nevertheless, it is clear that sucheffects will occur and that adaptive measures need to be developed in order to respond to theseexpected changes.