A New Race of Pseudomonas syringae pv. tabaci on Tobacco in Zimbabwe.

Two forms of Pseudomonas syringae pv. tabaci cause wildfire (Tox⁺) and angular leaf spot (Tox^-) diseases of tobacco in Zimbabwe. Two races of the pathogen (races 0 and 1) occur in Zimbabwe (4). Two groups of cultivars are available: one with resistance to race 0 only, tbe second with resistance to races 0 and 1 (4). P. syringae pv. tabaci was first observed on cultivars with resistance to races 0 and 1 in 1993, and infection on these cultivars is now widespread. The bacteria isolated from the infected plants were gram-negative rods and produced fluorescent pigment on King's medium B. Levan-oxidase-potato rot-arginine dihydrolase-tobacco hypersensitivity (LOPAT) tests indicated that the pathogen was a Group 1a pseudomonad (2). Pathogenicity and race designation for 58 isolates of the pathogen were determined on 8-week-old, greenhouse-raised tobacco plants. Inoculated plants were kept in controlled environment units (13/11 h photoperiod, 30 ± 2°C, relative humidity >75%) for 10 days. Pathogenicity was determined on cv. Kutsaga E1, which has no known resistance to any races of P. syringae pv. tabaci (4), and isolates from typical wildfire and angular leaf spot lesions were used for race designation by inoculating six plants of each indicator tobacco genotype. Genotypes included Nicotiana tabacum cv. Kutsaga E1 (susceptible to races 0, 1, and 2), which was used as the indicator genotype for race 0, N. longiflora (race 0 resistance) (1), N. tabacum cv. Kutsaga Mammoth 10 (resistance to race 0 derived from N. longiflora), N. rustica var. brasilea (resistance to races 0 and 1) (3,4), a breeding line, WZ 3-2-1-1, and cv. Kutsaga 35 (both resistant to races 0 and 1 with resistance derived from N. rustica var. brasilea). Six spots, three on either side of the midrib, were inoculated with an artist's airbrush (Aero-pro 251) operated at 250 kPa. The inoculum, (approximately 1 × 10⁶ CFU/ml) suspended in a quarter-strength Ringer's solution was applied to one side of the midrib and Ringer's solution only to the other side, which served as control. Thirty-eight Tox⁺ isolates and 20 Tox^- isolates were tested in a series of experiments in randomized complete blocks with four replications per treatment. Resistance to wildfire was characterized by localized chlorosis or whitish-tan hypersensitive lesions and susceptibility by necrotic lesions (>2 mm) surrounded by large chlorotic halos. Resistance to angular leaf spot was characterized by hypersensitive lesions or absence of symptoms and susceptibility by presence of symptoms (4). Sixty-six percent of the Tox⁺ isolates and 70% of the Tox^- isolates successfully infected cultivars with known resistance to races 0 and 1, and were therefore designated race 2 (1). All other isolates were race 1. This is the first report of P. syringae pv. tabaci Tox⁺ and Tox^- race 2 in Zimbabwe. References: (1) K. K. Knoche et al. Phytopathology 77:1364, 1987. (2). R. A. Lelliott and D. E. Stead. Methods in Plant Pathology. Vol. 2. 1987. (3). J. R. Stavely and H. A. Skoog. Proc. Am. Phytopathol. Soc. 3:231 (Abstr.), 1976. (4). J. J. Woodend and E. Mudzengerere. Euphytica 64:149, 1992.