ARS | Publication request: COMPARATIVE STRUCTURAL STUDY OF LEAF SPOT DISEASE OF SAFFLOWER AND SUGAR BEET BY CERCOSPORA BETICOLA

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 2006
Publication Date: February 27, 2007
Repository URL: http://hdl.handle.net/10113/50372
Citation: Lartey, R.T., Lenssen, A.W., Evans, R.G., Ghoshroy, S. 2007. Comparative structural study of leaf spot disease of safflower and sugar beet by cercospora beticola. Plant Pathology Journal. 6(1):37-43.

Interpretive Summary: Sugar beet and safflower are major crops in the Sidney, MT region of the Lower Yellowstone River Basin where they are sometimes rotated and occasionally may even be seen growing side by side. The fungal pathogen Cercospora beticola causes leaf spot of sugar beet while another species of Cercospora, C. carthami is known to infect safflower, but in the old world. In Sidney, incidence of C. beticola is quite frequent in sugar beet, but C. carthami has never reported. Observations of strange leaf spots on safflower in Sidney led to an investigation and eventual identification of safflower as a host of C. beticola. Because of importance of the two crops and abnormality the disease, we decided to follow up and present here a comparative structural study of sequence of C. beticola infection and disease development in the two crops. Sugar beet and safflower were manually infected with isolates C2 and Sid1 of C. beticola. Development of the pathogen on the leaf surface and disease symptoms were investigated with scanning electron microscope. We observed entry of the pathogen through stomatal openings of safflower. Several stomatal apertures in lesion areas of both host plants and in splits in sugar beet leaf lesions clearly showed protruding hyphae. These observations indicate the presence of internalized hyphae after establishment of infection and reemergence of the pathogen through the observed openings. Substantial hyphal mass developed eventually and covered the lesions of both host plants. Assay of the symptoms by PCR provided additional evidence for occurrence of C. beticola in the lesions, thus confirming the pathogen as the causal agent of the leaf spot of both sugar beet and safflower.

Technical 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.