Genetic Resources of Watermelon

As a result of many years of domestication and selection for desirable fruit quality, most of the modern dessert watermelon cultivars share a narrow genetic base. Africa is the center of origin and diversity of the genus Citrullus and is thus the focus of efforts to collect and conserve germplasm for enhancing dessert watermelons with resistance to diseases and pests. In addition to C. lanatus, accessions of several other species of Citrullus have been used as sources of disease and pest resistance. These are C. amarus (citron watermelon), which is native to southern Africa, C. mucosospermus (egusi watermelon) of sub-Saharan/western Africa origin, and C. colocynthis (colocynth) native to the deserts of northern Africa, the Middle East and Asia. Citrullus amarus, C. lanatus, and C. mucosospermus are readily intercrossed with one another and thus C. amarus and C. mucosospermus have at times been classified as subspecies or botanical varieties within C. lanatus. Genetic resources within Citrullus contain genes conferring resistance to a broad range of fungal diseases such as Fusarium wilt, anthracnose, gummy stem blight; oomycete diseases including Phytophthora capsici, powdery mildew, downy mildew; viruses such as the watermelon strain of Papaya ringspot virus, Zucchini yellow mosaic virus, and Squash vein yellowing virus (SqVYV); and insect pests such as root-knot nematodes, whiteflies, and mites. Watermelon germplasm collections are maintained in China, South Africa and Zimbabwe. The United States Department of Agriculture (USDA), Agricultural Research Service (ARS), National Plant Germplasm System (NPGS), maintains a large collection of watermelon and related Citrullus spp. germplasm. The USDA/ARS/NPGS, Germplasm Resources Information Network (GRIN) http://www.ars-grin.gov/npgs contains general information on accessions held within the USDA/NPGS collection.