Resistance of a Previously Screened Old Japanese Watermelon Cultivar to Diverse Anthracnose Strains and Its Inheritance

Anthracnose, caused by the fungal pathogen Colletotrichum orbiculare, is one of the most severe diseases in watermelon (Citrullus lanatus) cultivation in Japan. Genetically conferred host resistance is the best way to control it. To develop new cultivars with high resistance, identification of promising sources of resistance is important. We focused on an old Japanese cultivar, 'Tanso Teikosei' (NR28), with high resistance, preserved at the Nara Prefecture Agricultural Research and Development Center, evaluated its resistance to diverse C orbiculare strains distributed in Japan, and revealed the inheritance of its resistance. NR28 and two other accessions were resistant to the highly virulent MAFF 306737 strain. NR28 was also resistant to 16 other strains collected from various regions of Japan. Many strains caused severe symptoms and wilting in susceptible accessions, including the popular Japanese F-1 cultivar WF01. CAPS marker analysis indicated that NR28 and the two other resistant accessions were homozygous for the resistance allele of Cla001017, previously reported as the gene responsible for resistance to US race 1. Susceptible accessions were homozygous for the susceptibility allele at Cla001017. Segregation analyses using F-2 and BC1F1 lines derived from crosses between NR28 and a susceptible accession suggested that the dominant mutant allele of Cla001017 caused the high resistance of NR28. In contrast, NR28 and other resistant accessions homozygous for the resistance allele of Cla001017 were highly susceptible to two strains sampled in Iwate Prefecture. This suggests that these strains have overcome the Cla001017 resistance allele, as have strains of C orbiculare classified as US race 2. NR28 and the two other resistant accessions are promising materials for breeding resistance in diverse C. orbiculare strains distributed in Japan that have similar pathogenicity to US race 1. The CAPS marker that we designed in this study offers efficient analysis for watermelon breeding.