Are tetraploid roses more resistant to powdery mildew compared to diploids?

In roses morphological differences between tetraploids and diploids are well documented. In tetraploids thicker and larger leaves, etc. are described. However, little is known about stress resistance in polyploids. In our research we compared powdery mildew resistance in five diploid rose genotypes and their tetraploid counterparts. Four diploids derived from a well characterized cross between the diploid cultivar ‘Yesterday’ and the species Rosa wichurana were chosen for polyploidisation. The diploids differed in resistance level. Another diploid species hybrid 95/13-39 with known powdery mildew resistance was added to the test. The five selected rose genotypes were polyploidised through a treatment with the antimitotic agent oryzalin in tissue culture. Both tetraploids and diploids were acclimatized in the greenhouse and after six months powdery mildew inoculations were done on detached leaves. Two pathotypes of powdery mildew, R-E and R-P, were used. Disease development was evaluated on detached leaves after standardized inoculation using a sedimentation tower. A disease index was calculated from the scored leaves. In general the tetraploids showed an increased resistance to powdery mildew when compared to the diploids. Differences in resistance are discussed for the diploids and tetraploids tested.