Research

The RHS and the Royal National Rose Society assisted the University of Hertfordshire, CABI Bioscience, and the Central Science Laboratory of the Department of the Environment, Food and Rural Affairs in a research project on rose black spot disease.

This disease is caused by the fungus Diplocarpon rosae. Research has shown that there are several races of the rose black spot fungus Diplocarpon rosae and rose varieties differ in their susceptibility to these races. This helps to explain why newly released resistant varieties often succumb to the disease. Aspects of the biology and virulence of the pathogen were investigated, to improve methods of control.

Fungal races
The Society contributed to the project by providing samples of infected leaves from its Garden at Wisley, for isolation of the disease. The project used molecular techniques to investigate the differences between fungal races in more detail. Isolates of different races of the fungus were collected and a standard techniques used to inoculate the host plants. The races were analysed for culturing characteristics, DNA "fingerprints", and pathogenicity in the range of cultivars. This may lead to a better understanding of resistance, which will be helpful to rose breeders and will assist gardeners in selecting varieties.

Differential resistance
A plot of differential rose cultivars has been planted with a range of resistance to black spot. Some are uniformly resistant or susceptible, others are resistant to some races but not others.

Life cycle
Aspects of the life cycle of the fungus, in particular its survival over winter were examined. The spores of the fungus which survive in small lesions on the stems and buds, or on leaves which hang on over winter, are very important in initiating new infections next spring. The importance of the sexual (ascospore) stage is uncertain.

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