The Present Status of Breeding and Germplasm Collection for Resistance to Viral Diseases of Cucurbits in Japan

Cucumber (Cucumis sativus L.) and melon (C. melo L.) are globally important crop species in the family cucurbit. However, pests and disease, including various viral diseases, affect cucumber and melon production in Japan. Choosing a resistant cultivar is one of the most effective methods for disease control. Many sources of viral resistance have been identified in cucurbits, and various resistant commercial cultivars have been bred. This review describes the present status of cucurbit breeding and sources of resistance to the four major viruses in Japan. Melon yellow spot virus (MYSV) and Cucurbit chlorotic yellows virus (CCYV) are devastating insecttransmitted viruses that were first reported in Japan. MYSV-resistant cucumber and CCYV-resistant melon were selected from germplasm collections. The resistance of cucumber accession 27028930 to MYSV is temperature dependent and intermediate; melon isolate (MYSV-S) inoculation resulted in no systemic infection of resistant cucumber plants at low temperature (20°C). The melon accession JP 138332 exhibits high resistance to CCYV. These accessions are considered promising sources for breeding resistant cultivars. Cucumber green mottle mosaic virus (CGMMV) and Melon necrotic spot virus (MNSV) are soil-borne viruses. The Korean melon accession ‘Chang Bougi’ is resistant to the SH isolate of CGMMV. The resistance of ‘Chang Bougi’ is controlled by two independent complementary recessive genes (cgmmv-1 and cgmmv-2), whereas resistance to MNSV is controlled by a single recessive gene (nsv) in melon. The nsv gene was introduced into commercial cultivars, and these cultivars have shown high resistance to MNSV. There is an increasing demand for new cultivars with resistance to additional viruses. To satisfy this demand and solve problems related to cucurbit viruses, core collections of each cucurbit crop are required for the identification of resistant germplasm, and marker-assisted selection will allow us to breed new cultivars rapidly and accurately.