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Xanthomonas Leaf Spot and Stem Canker on Blueberry in Nurseries in Florida

In June and July 1998, in nurseries in south Florida, lesions were observed on the foliage and stems of three southern high-bush cultivars of blueberry hybrids (Vaccinium coryumbosum). Plants were approximately 1.5 to 2 years of age (0.6 to 1 m high). Disease incidence ranged from 95 to 50%, depending on cultivar. Lesions on leaves were roughly round, 5 to 20 mm in diameter, reddish brown surrounded by a yellow halo, and frequently coalesced to form large blighted areas. Stem cankers were soft, dark brown-to-black, and often resulted in the death of the entire branch. Microscopic examination of free-hand sections through lesion margins revealed bacterial streaming. Isolation of bacteria on nutrient agar consistently recovered mucoid, yellow bacteria typical of a xanthomonad. A pure culture of the bacteria was gram negative, oxidase negative, nonfluorescent, and proteolytic and produced a hypersensitive reaction on tobacco. Strains had fatty acid profiles with 88.5% similarity to Xanthomonas campestris pv. maniotis and 84.6% similarity to X. campestris pv. fici. Koch's postulates were fulfilled by spray-inoculation of 18-month-old blueberry cv. Sharpblue with a bacterial suspension (1 × 10⁸ CFU/ml) in sterile water. Control plants were sprayed with sterile distilled water. Plants were covered with plastic for 24 h and kept in a growth chamber at 25°C with a 12-h photoperiod. Symptoms were reproduced on inoculated plants, and bacteria were reisolated from lesions. Control plants were asymptomatic. The disease was controlled by applications of copper compounds, increased plant spacing, rouging of infected plants, reduction of leaf wetness, and elimination of overhead irrigation.