GENETIC DIVERSITY OF DIPLOCARPON ROSAE: IMPLICATIONS ON PRACTICAL BREEDING

The risk of infection by a specific pathogen in a plant population mainly depends on the reproduction system and the gene flow of the pathogenic species. The hemibiotrophic pathogen Diplocarpon rosae has a predominant asexual reproduction and is distributed by splash water and direct contact. Using D. rosae specific SSRs, a low genetic diversity among fungal isolates was estimated in young rose plantations or in rose collections often treated with fungicides. In contrast, old and extensive collections of rose species and cultivars have up to three fold higher diversity of D. rosae. Hence, the pressure of selection for resistance on a three year old rose test field is low compared to samples from older rose gardens. This indicates that active management of D. rosae genetic diversity in rose test plots could significantly improve the selection of resistant roses in the field. Selection for black spot resistance in test fields of one or two years old roses is a common practice but causes little disease pressure on the tested genotypes. Leaves sampled from disease free plants of these fields showed up to 80% growth of D. rosae after inoculation with conidia suspensions in the lab which is in agreement with the level of natural infections in the field. In conclusion of these results we propose an early selection for black spot resistance using artificial inoculations. Experiments with artificial inoculations of rose seedlings showed that this could generate a more intensive, faster and reliable selection for black spot resistance in roses. This could lead to cost saving and more efficient breeding of D. rosae resistant rose cultivars.