Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most economically important disease of sugar beets worldwide. Leaf spots formed by the pathogen coalesce to form large necrotic regions which can kill entire leaves. The regrowing of leaves necessary to maintain photosynthesis comes at the expense of sugar stored in the root, which reduces yield. In order to combat this disease, it is important to understand how infection and necrosis are perpetrated at a molecular level. C. beticola uses an array of effector molecules, including proteins and secondary metabolites, to cause disease. While a few C. beticola effectors have been characterized, there are many more effectors yet to be identified. Recent culture filtrate analysis has suggested the existence of multiple unidentified potential protein effectors. Among these candidate effector proteins is gene 05663, which has been predicted to be an apoplastic effector due to its small size and secretion signal. Using PEG-mediated transformation, we have successfully created five ∆05663 mutants. By comparison to the wild-type strain and three ectopic strains, ∆05663 mutants present delayed symptoms in inoculated sugar beets, suggesting that gene 05663 plays an integral role in the disease-causing process. Inoculation results, protein characterization data, and the potential role this gene plays in virulence will be presented.