Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803
University of Arkansas, Cooperative Extension Service, P.O. Box 357, Lonoke 72086
Calhoun Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Calhoun 71225
Blackberry (Rubus sp.) cane samples (cultivar Navaho) showing disease symptoms were collected from a commercial grower in Texarkana, AR, and sent for diagnosis to the plant disease clinic in Lonoke in June, 1997. Cane symptoms consisted of stem cracking, tissue discoloration beneath the bark, and the presence of an orange, velvetlike growth that was identified as sporangiophores and sporangia of the parasitic green alga Cephaleuros virescens Kunze. Zoospores were released when sporangia were placed in water drops. The alga was isolated from infected canes on potato dextrose agar, but pathogenicity tests were not attempted because pathogenicity has never been demonstrated successfully, nor have zoospores been produced in culture. C. virescens also was found infecting cultivated blackberry at the Louisiana Calhoun Research Station, where it occurred more commonly on thornless than on thorned type cultivars. This parasitic alga occurs commonly along the U.S. Gulf Coast and has been recorded on 287 plant species and cultivars, including 80 that are subject to stem infections (1,2). The occurrence of C. virescens in Arkansas extends its known northernmost range from 32.5°N to 33.5°N in the U.S. This is the first report on the occurrence of C. virescens in Arkansas and the first report of its occurrence on cultivated, commercially grown blackberry.
References: (1) G. E. Holcomb. Plant Dis. 70:1080, 1986. (2) R. B. Marlatt and S. A. Alfieri, Jr. Plant Dis. 65:520, 1981.