First Report of Rhododendron Leaf Spot Caused by Pseudomonas cichorii . W. Uddin, Department of Plant Pathology, University of Georgia, Athens 30602 . S. M. McCarter, Department of Plant Pathology, University of Georgia, Athens 30602. Plant Dis. 80:960. Accepted for publication 30 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0960A.
A severe leaf spot disease developed on container-grown rhododendron (Rhododendron catawbiense Michx.) cv. Alburn plants in a commercial nursery in Georgia during humid and warm weather in September 1995. Symptoms first appeared as 1- to 5-mm-diameter, irregular, water-soaked lesions on young expanding leaves. Lesions later enlarged, and became reddish brown (o dark brown and angular. On older leaves, lesions coalesced, causing large areas of necrosis. Streak-plate isolations from 12 water-soaked young expanding lesions on King's medium B (KMB) yielded a gram-negative, rod-shaped, bacterium with colonies that fluoresced on exposure to UV365 light. The bacterium was positive for oxidase and negative for arginine dihydrolase. Biolog characterization showed 0.96 similarity to database values for Pseudomonas cichorii. Pathogenicity was confirmed by spray-inoculating five sand-wounded and five nonwounded rhododendron plants with a 108 CFU/ml aqueous suspension of bacteria from 24-h KMB cultures. Inoculated plants and three water-sprayed controls were covered with polyethylene bags for 72 h in a growth chamber at 25°C and a 12-h per day fluorescent light period. Bags were removed, and plants maintained in the chamber to observe symptoms. Water-soaked spots developed on nonwounded and wounded plants in 3 to 5 days, and angular leaf spots similar to those observed in the nursery developed in 7 days. No symptoms developed on controls. The bacterium was reisolated from the inoculated plants and characterized as identical to the original strain. The disease was widespread and severe among thousands of rhododendron plants in a nursery.