Disease resistance screening in roses: different approaches lead to more knowledge

In plant breeding different approaches can be used to evaluate disease resistance. We have combined different types of bio-assays and screening methods to evaluate powdery mildew resistance in roses. The three methods evaluated were: 1) inoculation of detached leaves with an inoculation tower, 2) microscopic evaluation of resistance reactions in leaves and 3) scoring of disease symptoms in the field. The first method allows the inoculation with specific pathotypes in a standardized way and under controlled circumstances. Evaluation is done based on fungal development on the leaf. In the second method different types of plant reaction upon inoculation with different pathotypes can be assessed. The speed of reaction in the plant is important to generate an efficient resistance response. A third approach, the evaluation of powdery mildew resistance in the field or greenhouse, depends on favorable conditions for the disease. It is not possible to work with defined pathotypes of powdery mildew, but the evaluation of resistance in the intact plant is possible. We used the three methods on 90 genotypes in a diploid F1 population of ‘Yesterday’ x R. wichurana. Correlation for disease resistance between the different approaches was very weak. Nevertheless all three methods contribute to the evaluation of disease resistance in the plants. The combination of data on resistance towards specific pathotypes, specific resistance mechanisms observed in the leaves and field performance of the plants allows breeding towards better resistant plants.