Abstract: Sugar beet and safflower are sometimes rotated or grown side by side in the Sidney, MT region of the Lower Yellowstone River Basin (LYRB). Cercospora beticola and C. carthami infect sugar beet (Beta vulgaris) and safflower (Carthamus tinctorius) respectively. C. beticola is ubiquitous in sugar beet, but C. carthami has not been reported in LYRB. Observations of unusual leaf spots on safflower in Sidney led to investigation and subsequent identification of safflower as a host of C. beticola. We describe a comparative structural study of progression of C. beticola infection and disease development in both sugar beet and safflower. The two crops were manually infected with two isolates of C. beticola (C2 and Sid1). Gradual development of the pathogen on the leaf surface and disease symptoms were investigated with scanning electron microscope operated at a variable pressure mode. Some specimens were sputter coated with gold to obtain higher resolution images. Lesions in sugar beet and safflower showed a substantial amount of hyphal mass. A number of stomatal apertures in lesion areas of both host plants and in splits in sugar beet lesions clearly showed protruding hyphae, indicating presence of internalized hyphae after establishment of infection. Substantial hyphal mass developed eventually and covered the lesions of both host plants. Assay of the symptoms by PCR provided evidence for C. beticola in the lesions, thus confirming it as the causal agent of the leaf spot of both sugar beet and safflower.