Quantification of zucchini yellow mosaic virus in melon plants segregating for the major resistance Zym-1 locus

The melon accession PI414723 carries the Zym-1 resistance locus, which is used in breeding programs to control the disease caused by zucchini yellow mosaic virus (ZYMV). We investigated the effects of Zym-1 on the phenotype and on viral titers of F-2 plants derived from the cross between PI414723 and Vedrantais. For that, we used the simple sequence repeat marker CMAG36, which is linked to Zym-1, and a developed quantitative PCR (qPCR) protocol based on the coat protein of ZYMV. PI414723 and F1 remained symptomless 21 days after inoculation (dai), whereas Vedrantais displayed extreme susceptibility. At 8 dai ZYMV was not detected in the apical leaf of the resistant genotype but was in F-1 plants at very low levels. Most plants homozygous for resistance (R) or susceptibility (S) alleles at Zym-1 were either symptomless or presented extreme symptoms, respectively. However, a few opposite cases were observed in the two genotypes. Different from the R and F1 plants, the reactions of heterozygotes (H) were more diverse, indicating a partial dominance of Zym-1 at the phenotypic level. ZYMV was detected on R and H plants in equivalent titers, but about 59 times lower than in S plants. This study illustrates the major effects of Zym-1 on the phenotype and viral multiplication or movement. It indicates that in the heterozygous state, Zym-1 may interact with the other resistance genes of PI414723 in an epistatic manner.