A DIGITAL IMAGE ANALYSIS SYSTEM FOR RESISTANCE EVALUATION IN RHODODENDRON

Cylindrocladium scoparium is one of the most important fungal pathogens of Rhododendron simsii. Successful control by breeding for resistance to this pathogen requires sensitive, practicable and reproducible screening methods. A research project aimed at developing effective screening methods for evaluation of plant resources for Cylindrocladium resistance in Rhododendron simsii will be presented. Bioassays with detached leaves and shoots were established. The responses of the genotypes to C. scoparium were estimated by symptom scoring with a digital image analysis system. Tests of young plants in the greenhouse were used to verify the results of the bioassays. All three screening methods were reproducible. Nevertheless, there only existed a correlation between the bioassays but not between the bioassays and the test of young plants. Tolerant genotypes (R60, R114 and R120) could be distinguished from highly susceptible genotypes within all tests. However, resistance against C. scoparium did not exist within the screened gene pool. The combination of the bioassay of shoots and the digital image analysis could be used as a well-adapted and non-destructive evaluation method of the host-pathogen-system Rhododendron-Cylindrocladium. Results of the bioassay of shoots should be verified by testing whole plants like rooted cuttings.