| Cucurbit bacterial fruit blotch caused by Acidovorax avenue subsp. citrulli is a significant threat to watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] production worldwide. Improved understanding the disease epidemiology of bacterial fruit blotch helped researchers come up with disease management strategies, including seed treatments to externally remove inoculum from seed coat, seed health testing assays and bactericide application. In the United States, watermelon production has moved to seedless cultivars, and critical requirements for seed germination make the production largely rely on transplant production in greenhouses to ensure a high germination rate. Unfortunately, the warm and humid greenhouse environment provides ideal conditions for the spread of bacterial fruit blotch. Despite the marginal success of current disease management strategies, they all have limitations. Resistant cultivars offer a solution to the problem, if genetic resistance can be identified. The objectives of this study were to 1) improve the resistance screening methodology in both field and greenhouse, 2) identify highly resistant germplasm to bacterial fruit blotch using the available Plant Introductions (PI) accessions in the USDA germplasm collection, 3) investigate the interaction between watermelon plants and bacterial fruit blotch via leaf surface morphological traits. Field evaluations based on foliar disease symptoms at the flowering stage were conducted at Clinton, NC in 2011 to 2013. The field experiment was a randomized complete block with 1,699 cultigens, 3 years, and 2 replications of single-plant plots. Disease rating was on a 0-9 scale when the disease was uniformly distributed across the field (0= no symptoms, 1-2 = trace, 3-4 = slight, 5-6 = moderate, 7-8 = severe, and 9 = dead). Also, watermelon leaf surface structure was studied using the scanning electron microscope. Statistical analysis on weekly ratings showed that significant differences exist in disease severity among accessions (P=0.05). Resistant accessions were identified based on low mean, low standard deviation over replications, and high number of observations (few missing plots). The scanning electron microscope results revealed the impact of trichome and stomata on disease symptoms caused by bacterial fruit blotch. |
