Bacteria occurring in onion (Allium cepa L.) foliage in Puerto Rico

Abstract

Bacteria associated with foliar symptoms of onion (Allium cepa L.) were examined in the southern region of Puerto Rico from January through April 2004. Different symptoms were observed in onion foliage of cultivars 'Mercedes' and 'Excalibur' at Juana Díaz and Santa Isabel, Puerto Rico. Ellipsoidal sunken lesions with soft rot and disruption of tissue were the most common symptoms observed in onion foliage in field conditions. From a total of 39 bacterial strains isolated from diverse symptoms in onion foliage, 38% were isolated from soft rotting lesions. Ninety-two percent of the bacteria isolated from onion foliage was Gram negative. Pantoea spp. with 25%, was the most frequently isolated genus, followed by Pasteurella spp. and Serratia rubidae with 10% each. Fifty- six percent of the strains held plant pathogenic potential; these strains belong to the genera Acidovorax sp., Burkholderia sp., Clavibacter sp., Curtobacterium sp., Enterobacter sp., Pantoea spp., Pseudomonas spp., and Xanthomonas spp. Pathogenicity tests showed that seven out of eight tested bacterial strains evaluated under field conditions caused symptoms in onion foliage for both cultivars. Acidovorax avenae subsp. citrulli, Burkholderia glumae, Pantoea agglomerans, P. dispersa, Pseudomonas sp., Xanthomonas sp., and Xanthomonas-like sp. were pathogenic to leaf tissues. Clavibacter michiganensis was not pathogenic to leaf tissues. Other bacteria identified as associated with onion leaf tissue were Curtobacterium flaccumfaciens, Cytophaga sp., Enterobacter cloacae, Flavimonas oryzihabitans, Mannheimia haemolytica, Pantoea stewartii, Pasteurella anatis, P. bettyae, P. langaaensis, Photobacterium damselae, Pseudomonas syringae pv. aptata, Rhizobium radiobacter, Serratia rubidae, Sphingobacterium spiritivorum, Sphingomonas sanguinis, and an unknown strain. This paper is the first survey of bacteria associated with onion foliage in Puerto Rico. The role of non- phytopathogenic bacteria associated with the life cycle of onion under field conditions remains unknown.