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 F2 plants derived from the cross between PI414723 and Védrantais. 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 Védrantais displayed extreme susceptibility. At 8 dai ZYMV was not detected in the apical leaf of the resistant genotype but was in F1 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. [ABSTRACT FROM AUTHOR]