First Report of Rhizosis Caused by Ceratocystis radicicola on Date Palms in South Africa.

During late summer (February to March) of 1997, a sudden loss of large date palms (Phoenix dactylifera L. 'Medjool') and adjacent suckers was observed in the Pofadder area of South Africa. The palms were planted in sandy soil, with flood irrigation from the adjacent Orange River. Initial symptoms included wilting of leaves, chlorosis of young leaves, and root necrosis. Ceratocystis radicicola (D.E. Bliss) C. Moreau (synamorph Ceratostomella radicicola; anamorph Chalara) was consistently isolated from necrotic roots and lower crown portions of diseased palms. Dark colored spores were observed in root sections. Pathogenicity studies were performed in a shadehouse on 5-year-old Medjool clones. Two experiments were conducted in which Medjool plants were inoculated with 2-week-old C. radicicola cultures grown on corn meal agar (CMA). In the first experiment, a 7-mm wound was made at the leaf base of 15 plants and inoculated with an agar plug of C. radicicola culture. The same number of control plants was inoculated with CMA. Wounds were sealed with Parafilm, and lesions were measured after 3 months. In the second experiment, conidial suspensions and pieces of mycelia from five C. radicicola cultures were harvested in 1.5 liters of water, and 100 ml of inoculum was added to each of 15 plants. Control plants (15 total) each received 100 ml of sterile water. Plants were examined after 3 months. In the first experiment, all leaves inoculated at the base with C. radicicola became necrotic at the point of inoculation, wilted rapidly, and died. In the second experiment, roots showed severe necrosis resembling rhizosis observed in the field. In each experiment, only three plants died, and C. radicicola was successfully isolated from all inoculated plants. No symptoms developed on control plants, and C. radicicola could not be isolated from them. Rhizosis of date palms caused by C. radicicola has been described in the United States (1) and differs from the disease of date palm stems, leaves, buds, and inflorescences caused by C. paradoxa (Dade) C. Moreau. This is the first report of rhizosis on date palms in South Africa. Reference: (1) D. E. Bliss. Phytopathology 31:1123, 1941.