Marginal Necrosis and Intercostal Leaf Spots of Cocoyam Infected by Xanthomonas campestris pv. dieffenbachiae. Ken Pohronezny, University of Florida, IFAS, Tropical Research and Education Center, Homestead 33031. Wilbur Dankers, Bruce Schaffer, Hector Valenzuela, and Michael A. Moss. University of Florida, IFAS, Tropical Research and Education Center, Homestead 33031. Plant Dis. 74:573-577. Accepted for publication 25 January 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0573.

Pathogenic bacteria from intercostal and marginal lesions of cocoyam (Xanthosoma sagittifolium [= X. caracu]) leaves in Florida were both identified as Xanthomonas campestris pv. dieffenbachiae based on physiological and biochemical tests. Inoculation of bacteria into intercostal or marginal tissue produced symptoms normally observed regardless of lesion type from which the strain was originally recovered. Numbers of intercostal and marginal lesions were independent of strain source. An average of 11 marginal lesions per leaf developed when actively guttating plants were mist-inoculated, but only 2.4 marginal lesions per leaf developed on nonguttating plants. Greater numbers of intercostal lesions (13.6 per leaf) were evident when plants were inoculated during midafternoon than when bacteria were sprayed on leaves early in the morning (0.025 lesions per leaf). Addition of a surfactant to the inoculum did not affect the numbers of either intercostal or marginal lesions.