Screening for Bacterial Fruit Blotch Resistance in the USDA Watermelon Germplasm Collection

Cucurbit bacterial fruit blotch caused by Acidovorax avenue subsp. citrulli is a significant threat to watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] production worldwide. The disease can cause complete loss of marketable crop under ideal conditions. Since seed is the primary inoculum for bacterial fruit blotch in cucurbit crops, bacteria-free seed production has helped limiting outbreaks. Currently, the most effective strategy for managing bacterial fruit blotch has been excluding the pathogen from seeds, fruits, and transplants. However, efficacy of the external treatments varies because bacteria can be under the seed coat. Therefore, genetic resistance would be of great scientific and economic value. The overall objective of this study was to identify resistance to bacterial fruit blotch using the available Plant Introductions accessions in the USDA germplasm collection. Field evaluations based on foliar disease symptoms at the adult stage were conducted at Clinton, NC, in Summer 2011 to 2013. The experiment was a randomized complete block with 1689 cultigens, 3 years, and 2 replications of single-plant plots. Selected PIs were tested at the two-true leaf stage in a controlled environment in 2013 to confirm the resistance source. Studies are under way to improve the disease rating system by accounting for different growth stage and leaf type. Both field and greenhouse screening adopted a rating scale with 0–9 (0 = no symptoms, 1–2 = trace, 3–4 = slight, 5–6 = moderate, 7–8 = severe, and 9 = dead). Statistical analysis on weekly ratings showed that significant differences exist in disease severity among accessions (P = 0.05). The 10 most resistant and 2 most susceptible accessions were identified and then confirmed in retest experiments. Resistant germplasm will be made available for those interested in developing molecular markers and improved cultivars.