First Report of Oakleaf Hydrangea Bacterial Leaf Spot Caused by a Pathovar of Xanthomonas campestris. . W. Uddin, Department of Plant Pathology, University of Georgia, Athens, Georgia 30602. S. M. McCarter, Department of Plant Pathology, University of Georgia, Athens, Georgia 30602, and R. D. Gitaitis, Coastal Plain Experiment Station, Tifton, Georgia 31793. Plant Dis. 80:599. Accepted for publication 9 March 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0599B.
A severe angular leaf spot disease occurred on container-grown oak-leaf hydrangea (Hydrangea quercifolia W. Bartram) plants in a commercial nursery in Georgia during a warm and humid period in June of 1995. Initial symptoms on young, unfolding leaves appeared as irregular watersoaked spots (1 to 4 mm in diameter) that became angular and turned purple or dark brown as they enlarged. Coalescing lesions sometimes caused complete necrosis of older leaves. Streak-plate isolations made from expanding lesions on nutrient yeast agar and yeast dextrose calcium carbonate agar (YDC) yielded nearly pure cultures of a Gram negative rod-shaped bacterium typical of a xanthomonad. Carbon source utilization (Biolog) tests and fatty acid analysis confirmed the baclerium as a pathovar of Xanthomonas campestris: highest similarity was with X. campestris pv. carotae. Similarity index was 0.58 for Biolog and 0.29 for fatty acid. The second-best match was with X. campestris pv. vitians (0.12 for Biolog and 0.25 for fatty acid). Pathogenicity was confirmed by spray inoculating wounded (slight sand abrasion) and nonwounded plants with a 108 CFU/ml suspension prepared from YDC cultures, covering the plants with polyethylene bags for 72 h in a growth chamber at 25°C, and holding them in the same chamber to observe symptom development. Water-soaking was observed on both nonwounded and wounded plants in 3 lo 5 days after inoculation, and angular leaf spots typical of those observed in the nursery appeared in 7 days. The bacterium was readily reisolated from the inoculated plants and its identity confirmed.