Evaluation of Citrullus sp germ plasm for resistance to Acidovorax avenae subsp citrulli

In the greenhouse, 1,344 Citrullus spp. and Praecitrullus fistulosus accessions were screened for resistance to Acidovorax avenae subsp. citrulli. Seedlings were inoculated at the first true leaf stage by misting with a water suspension of a virulent strain of A. avenae subsp. citrulli originally isolated from commercial watermelon in Florida in 1989. Seedlings were considered resistant if less than 20% of the cotyledons were necrotic and there were no lesions on the true leaves 10 days after inoculation. Twelve accessions had individual seedlings that were resistant to A. avenae subsp. citrulli. Selfs of seven of these accessions were susceptible in greenhouse and field tests. Selfs were obtained from five accessions from Zimbabwe and Zambia that possessed a level of greenhouse and field resistance to A. avenae subsp. citrulli that could provide control of bacterial fruit blotch of watermelon. Based on field evaluations, plant introduction (PI) 482279 and PI 494817 were judged to contain plants with the best sources of resistance; however, PI 500303, PI 500331, and PI 482246 also had plants with high levels of resistance.