Four o'clock (Mirabilis jalapa) is an ornamental used in California as a landscape plant. In 2000 and 2001, four o'clock transplants produced in commercial greenhouses in coastal California were severely affected by a foliar disease. Initial symptoms were small (2 to 5 mm diameter), angular, water-soaked leaf spots that often developed first on leaf edges. Spots enlarged, rapidly turned brown to dark brown, coalesced, and caused transplants to take on a blighted appearance. When leaf spots were macerated and streaked onto sucrose-peptone agar, a yellow, mucoid bacterium was consistently isolated. Based on morphological, physiological, and biochemical characteristics, we identified the bacterium as Xanthomonas campestris (2). Furthermore, Biolog GN profiles identified five Mirabilis strains as X. campestris pv. campestris with similarity values of 79.7 to 87.6%. Fatty acid analyses identified them as the same pathovar with similarity values of 75.8 to 82.1%. These strains also used cellobiose and arabinose as carbon sources and produced acid from arabinose, which are characteristic features for this pathovar. Pathogenicity was demonstrated by growing inocula of six strains in nutrient broth shake cultures for 48 h and misting 30 ml of the broth cultures (106 CFU/ml) on sets of 12 potted four o'clock plants. Control plants were misted with sterile broth. After inoculation, plants were incubated in a dew chamber (100% relative humidity at 18°C) for 24 h then maintained in a greenhouse (24 to 26°C). After 7 to 10 days, leaf spots similar to those originally observed developed on all inoculated plants. Bacteria reisolated from these plants were characterized and found to be the same as those used for inoculation. Control plants remained symptomless. Because of the biochemical similarity between Mirabilis strains and the black rot pathogen of crucifers (X. campestris pv. campestris), we inoculated sets of six cauliflower (Brassica oleracea var. botrytis cv. White Magic) plants with five Mirabilis strains and two X. campestris pv. campestris strains using the same method. After 14 days, cauliflower plants inoculated with Mirabilis strains were symptomless, while plants inoculated with the black rot pathogen developed symptoms typical of this disease. X. campestris was only reisolated from the cauliflower plants inoculated with the crucifer strains. Because the four o'clock transplants from the commercial greenhouse were produced in close proximity to vegetable hosts of other Xanthomonas pathogens, we inoculated pepper (Capsicum annuum), tomato (Lycopersicon esculentum) and four o'clock transplants with four Mirabilis strains using the method described. The four o'clock strains failed to cause any disease on pepper and tomato but resulted in typical leaf spots on four o'clock. Bacteria reisolated from four o'clock plants and characterized were the same as the original strains. All inoculation experiments were repeated and results were the same. To our knowledge, this is the first report of X. campestris on four o'clock in California and the United States. Occurrence of this disease on direct-seeded plants in enclosed greenhouses provides circumstantial evidence that the pathogen might be seedborne. This pathogen may be related to the Xanthomonas sp. reported on four o'clock in India (1).
Reference: (1) J. C. Durgapal and B. M. Trivedi. Curr. Sci. 45:111, 1976 (2) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria, 3rd ed. The American Phytopathological Society, St. Paul, MN, 2001.
