Resistance breeding for powdery mildew (Podosphaera pannosa) and black spot (Diplocarpon rosae) in roses

In the past, priority in rose breeding was given to external plant quality like flower colour, size and shape, shoot field, thorniness and plant habit. Little attention was paid to resistance to fungal diseases. In garden roses most important fungal diseases are black spot (Diplocarpon rosae) and powdery mildew (Podosphaera pannosa). The aim of this research was to support a disease breeding programme for garden roses by gathering knowledge and the development of crossings- and selection methodology useful in practice. It was important to study the pathogens themselves, to evaluate bioassays, and to test possibilities of interploidy crosses and induced resistance. Knowledge on the pathogens is important in disease resistance screening. For both fungal diseases pathotypes differing in virulence towards different rose genotypes were found. Some powdery mildew isolates found on roses are virulent for Prunus avium too. These powdery mildew types can be recognised by a one base pair difference in the ITS sequence compared to powdery mildew isolates virulent only for rose or only for Prunus spp. To be able to select genotypes with enhanced disease resistance in breeding and selection, appropriate bio-assays are needed. To select parent plants with a good potential for disease resistance, inoculations are made in a standardised way on detached leaves. For powdery mildew an inoculation tower can be utilised. Water should be avoided when conidia of powdery mildew are used. For black spot, an inoculum suspension can be used on detached leaves in a lab test. Disease resistance should be regarded as one of the main characteristics in seedling selection and should be evaluated as soon as possible to be most efficient. Therefore artificial inoculations are made on young seedlings. For powdery mildew, inoculation plants in between the seedlings make infection spreads early and uniformly in the greenhouse. For black spot, greenhouse plants are inoculated by spraying a conidia suspension. Normally black spot does not occur in greenhouses. Artificial inoculation is possible when the plants are placed under a plastic tent to prolong the time of leaf wetness and when they are watered on top of the plants. A resistance screening for both diseases can be combined in the first growing season of the seedlings. The occurrence of powdery mildew on rose genotypes in the greenhouse is correlated with the results of later field evaluation. Field evaluations on the occurrence of black spot should be made in different years, since the pathogen needs time to spread uniformly in the field. Among rose cultivars, little resistance to fungal diseases is found. To enhance disease resistance, crosses with rose species are needed. Therefore interspecific hybridisations by interploidy crosses are necessary. A lot of rose species are diploid, whereas cultivars are often tetraploid. Crosses between di- and tetraploids yield triploid offspring. Many triploid roses can be used in crosses. In this way genetic material of diploid species can be introduced in tetraploid cultivars.Resistance in plants can be induced by plant growth promoting rhizobacteria and specific chemical compounds. In roses, resistance to powdery mildew is induced with BTH (salicylic acid analogue), an extract of the giant knotweed (Reynouteria sachalinensis) and different plant growth promoting rhizobacteria. Possibly this induced systemic resistance can be used to recognise genotypes with partial resistance. Results from this study have led to a more efficient selection for disease resistance roses. In this work selected rosegenotypes are currently evaluated for their potential as new cultivars.