ABSTRACTMicroencapsulation is an important technique for protecting the viability and activity of microorganisms under adverse environmental conditions. To improve biological control, controlled-release microcapsules ofTrichoderma asperellumwere prepared and embedded in combinations of the biodegradable wall materials sodium alginate (SA). The microcapsules were evaluated for their ability to control cucumber powdery mildew in the greenhouse. The results showed that the highest encapsulation efficiency of 95% was obtained by applying 1% SA and 4% calcium chloride. The microcapsules provided good, controlled release and UV resistance, and could be stored for a long time. The greenhouse experiment revealed that theT. asperellummicrocapsules had a maximal biocontrol efficiency of 76% against cucumber powdery mildew. In summary, embeddingT. asperellumin microcapsules is a promising technique to improve the survivability ofT. asperellumconidia. TheT. asperellummicrocapsules exerted significant biocontrol efficiency against cucumber powdery mildew.IMPORTANCETrichoderma asperellumis widely found in plant roots and soil and has been used for the biocontrol of various plant pathogens; however, the control efficiency ofT. asperellumis usually unstable in field trials. To improve the control efficiency ofT. asperellum, in the present study,T. asperellummicrocapsules were prepared using sodium alginate as wall material to reduce the effects of temperature, UV irradiation, and other environmental factors on its activity, and to significantly improve its biocontrol efficiency on cucumber powdery mildew. Microcapsules can prolong the shelf life of microbial pesticides. This study provides a new way to prepare a biocontrol agent against cucumber powdery mildew with high efficiency.