Inheritance of resistance to zucchini yellow mosaic virus and watermelon mosaic virus in watermelon.

High resistance to zucchini yellow mosaic virus-China strain (ZYMV-CH) and moderate resistance to watermelon mosaic virus (WMV) were found in a selection of PI 595203 (Citrullus lanatus var. lanatus), an Egusi type originally collected in Nigeria. Mixed inoculations showed primarily that these two viruses have no cross-protection. This fact may explain the high frequency of mixed infection often observed in commercial fields. When plants were inoculated with a mixture of the two viruses, the frequency of plants resistant to ZYMV was lower than expected, indicating that WMV infection may reduce the ability of a plant to resist ZYMV. We studied inheritance of resistance to ZYMV-CH and WMV, using crosses between a single-plant selection of PI 595203 and the ZYMV-susceptible watermelon inbreds 9811 and 98R. According to virus ratings of the susceptible parents, the resistant parent, and the F1, F2, and BC1 generations, resistance to ZYMV-CH was conferred by a single recessive gene, for which the symbol zym-CH is suggested. The high tolerance to WMV was controlled by at least two recessive genes.