Emergence of Acidovorax avenae subsp. citrulli as a Crop Threatening Disease of Watermelon and Melon

Acidovorax avenae subsp. citrulli (Pseudomonas pseudoalcaligenes subsp. citrulli), the causal agent of a watermelon seedling blight and fruit blotch (WFB), has emerged as a serious seedborne pathogen of watermelon and melons world-wide. The disease was first reported in 1965 in seedlings of several USDA Plant Introduction (PI) lines in Georgia. The pathogen was described 13 years later in diseased seedlings from two of the same PI seed sources in which Web and Goth had originally identified the disease. The disease remained known only in seedlings at the introduction station until Wall and Santos isolated a non-fluorescent pseudomonad from rotting fruit in Guam in 1987 and concluded it was the same organism. WFB was reported in Florida in 1989 and a year later emerged as a major economic disease of watermelon in the Southeast and Midwest USA. Since then, WFB has been reported in Turkey, Brazil, Australia, Japan, Korea, Thailand, and China infecting cantaloupe, melon, honeydew, pumpkin, and citron under field conditions. Because the original seedling strains were reported as giving a negative tobacco hypersensitive reaction and failed to rot fruit like the field disease strains, some have suggested that the originally described organism was not the same organism as that causing WFB. DNA fingerprinting of a collection of 121 strains using pulse field gel electrophoresis revealed considerable genetic diversity. All strains typed into two groups; over 80% were in Group I and the remaining, including the original type strain, in Group II. Control of the disease is based primarily on use of pathogen-free seed. Although attempts have been made to develop a routine seed assay to detect the organism in seeds, no reliable method is available. Contaminated seeds remain a problem for the seed industry.