Screening for bacterial fruit blotch resistance in watermelon fruit

Bacterial fruit blotch (BFB) caused by Acidovorax citrulli is a serious threat to the watermelon industry. Currently, there are no commercial watermelon cultivars with resistance to BFB. In this 3-yr study, we conducted a screening of 1,452 Citrullus spp. cultigens (PIs and cultivars) for resistance to BFB at the immature fruit stage. However, due to high levels of missing data, many cultigens require additional testing to confirm resistance. Fruit BFB symptoms presented initially as raised bumps, then progressed to blotching, cracking, and internal necrosis. Resistant cultigens rarely proceeded beyond raised bumps at the infection site. Fruit resistance is significantly affected by environmental conditions and has low heritability. Among a subset of cultigens selected for high resistance and susceptibility during the first 2 yr of screening and retested using increased replications, PI 494819 [C. lanatus (Thunb.) Matsum. & Nakai., Zambia], PI 596659 (C. amarus Schrad., South Africa), PI 596670 (C. amarus, South Africa), PI 490384 [C. mucosospermus (Fursa) Fursa, Mali], and PI 596656 (C. amarus, South Africa) had average disease ratings of <1; significantly more resistance than the commercial check, Charleston Gray; low plant-to-plant variability within each PI; and infrequent incidence of advanced symptoms. These PIs are potential sources of fruit resistance for breeding programs as well as for research focused on understanding the underlying mechanisms and genetics.