Podosphaera pannosa is one of the most common diseases that attacks both garden and greenhouse roses worldwide. It is also an important disease of Rosa damascena in Iran. Since Iran appears to be a center of genetic diversity for Damask rose, it is expected that powdery mildew resistance genes would be found. Therefore, a set of Damask roses from diverse regions of Iran (Fars, Isfahan, Kerman and East Azerbaijan provinces) were screened for powdery mildew resistance. The plants were screened for natural infection over two years (2006 and 2007) both in the field and in pots. The diseases index and factor were calculated for each genotype and severity of disease was compared at different scoring times. Screening in the field was not consistent, probably due to non-uniform pathogen distribution, but potted plants that were placed in favorable environmental conditions (half-shade with relatively high humidity) gave consistent ratings over replications. Significant differences in the rate of powdery mildew development were found between genotypes. The East Azerbaijan (colder with higher rainfall and humidity) and Kerman (warmer and dryer region) genotypes were recognized as the most resistant and susceptible genotypes respectively. This is the first report on resistance for powdery mildew in Damask roses. Results are promising for planning the future breeding programs on this valuable species.

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