Chrysanthemum Breeding for Resistance to White Rust

A series of experiments were conducted to promote chrysanthemum breeding for resistance to white rust (Puccinia horiana P. Hennings), For testing the resistance to white rust, three plants of each species and cultivar were incubated in a box kept at 17'C under 100% relative humidity and the inoculation was effected by hanging leaves with telia actively producing sporidia from the ceiling of the box. The inoculum concentration was restricted to ca. 70 sporidia per 1mm2 leaf area. Reliable criteria for grading and rating inLection were established based principally on the extent of telium formation on the fully unfolded top leaves. White rust resistance of chrysanthernum cultivars could be evaluated by this testing method at an early stage of the plants, for their reactions to white rust at the young plant stage and the flowering stage were highly correlated. Variation in resistance depending on the fungal isolates was found in the tests of 40 cultivars inoculated with 6 typical isolates in Japan, suggesting physiological specialization in the pathogen. Among 6 isolates, some showed a wide range of pathogenicity and others medium and narrow ranges. Besids the existence of unstable cultivars showing various reactions to the fungal isolates, cultivars with stable resistance to all the isolates were also found.