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Bacterial Leaf Spot of Sunflower in South Africa. D. Fourie, Grain Crops Institute, ARC, Private Bag X1251, Potchefstroom, 2520, South Africa . A. Viljoen, Grain Crops Institute, ARC, Private Bag X1251, Potchefstroom, 2520, South Africa; and J. J. Serfontein, Plant Protection Research Institute, ARC, Private Bag X134, Pretoria, 0001, South Africa. Plant Dis. 80:1430. Accepted for publication 9 October 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1430B.

A previously unreported leaf spot disease was widespread on all commercial varieties of sunflower {Helianthus annuus L.) grown on about 600,000 ha in South Africa during the 1995 to 1996 season. The dark brown, necrotic spots were similar to Alternaria or Septoria lesions, but were smaller (1 to 2 mm in diameter) and angular. Lesions sometimes coalesced and made leaves dry and brittle. Gram negative, rod-shaped bacteria that produced fluorescent pigment on King's medium B were recovered from watersoaked margins of young lesions on leaves, stems, and petioles. LOPAT (levan-oxidase-potato rot-arginine dihydrolase-tobacco hypersensitivity) tests indicated that the bacterium belonged to Group I of the phytopathogenic pseudomonads. The bacterium was identified by reactions on Biolog GN MicroPlates as Pseudomonas syringae pv. helianthi with a similarity of 0.847 to strain descriptions in the Mi-croLog database 3.50 (Biolog Inc., Hayward, CA). Pathogenicity of nine strains was verified on leaves of 2-week-old, greenhouse-grown sunflower seedlings spray inoculated with aqueous bacterial suspensions containing 108 CFU/ml. Leaves and stems of 8-week-old sunflower plants were inoculated with equivalent suspensions by multiple needle puncture (1). Twelve plants were inoculated with each strain. Sterile water was substituted in mock inoculations of control plants. Test plants were maintained in a humid chamber (+95% relative humidity) at 19 10C for 2 days, then returned to the greenhouse at 25C day/18C night for 10 days, during which symptoms developed. P. syringae pv. helianthi was consistently recovered from characteristic watersoaked lesions that developed on inoculated plants. No symptoms developed on control plants. The pathogen was distinguished from P. syringae pv. mellea, which is levan negative, from P. syringae pv. aptata, which liquifies gelatin and is unable to utilize L-tartrate, and from P. syringae pv. tagetis, which does not utilize erythritol or lactate. This is the first report of sunflower bacterial leaf spot caused by P. syringae pv. helianthi in South Africa. The extensive occurrence of the disease could have been enhanced by frequent rainfall and humid conditions during the growing season

Reference: (1)C. F. Andrus. Phytopathology 38:757, 1948.