AbstractKey messageRose has 19 MLO genes. Of these, RhMLO1 and RhMLO2 were shown to be required for powdery mildew infection, which suggests their potential as susceptibility targets towards disease resistance.AbstractPowdery mildew, caused byPodosphaera pannosa, is one of the most serious and widespread fungal diseases for roses, especially in greenhouse-grown cut roses. It has been shown that certainMLO genes are involved in powdery mildew susceptibility and that loss of function in these genes in various crops leads to broad-spectrum, long-lasting resistance against this fungal disease. For this reason, theseMLOgenes are called susceptibility genes. We carried out a genome-wide identification of theMLOgene family in theRosa chinensisgenome, and screened for allelic variants among 22 accessions from seven differentRosaspecies using re-sequencing and transcriptome data. We identified 19MLOgenes in rose, of which four are candidate genes for functional homologs in clade V, which is the clade containing all dicotMLOsusceptibility genes. We detected a total of 198 different allelic variants in the set ofRosaspecies and accessions, corresponding to 5–15 different alleles for each of the genes. Some diploidRosaspecies shared alleles with tetraploid rose cultivars, consistent with the notion that diploid species have contributed to the formation of tetraploid roses. Among the fourRhMLOgenes in clade V, we demonstrated using expression study, virus-induced gene silencing as well as transient RNAi silencing that two of them,RhMLO1andRhMLO2, are required for infection byP. pannosaand suggest their potential as susceptibility targets for powdery mildew resistance breeding in rose.