Cultivation of the major flower bulb crops, e.g., lily, narcissus, gladiolus and tulip, is threatened by the soil-borne fungus Fusarium oxysporum. Fusarium infected bulb lots have lower yields and cause significant problems for bulb export and cut flower production. Besides cultivation practices and chemical protection, resistant cultivars can play an important role in preventing this disease. To breed Fusarium resistant cultivars, screening and selection tests have to be developed and genetic variation in the host and in the pathogen determined. Information about the inheritance can be helpful in selecting the parents with the best breeding value for Fusarium resistance. Clonal screening tests under standardized conditions (e.g., concerning temperature, inoculum concentration and duration of the test) are developed for lily, narcissus, gladiolus and tulip using artificially infested soil or bulbs. By using a clone of a genotype, levels of resistance can be determined accurately. Severity of the disease is mainly observed by using disease ratings or the percentage of diseased bulbs. Partial resistance to Fusarium is present in all four crops. In Narcissus and Gladiolus even species with absolute resistance were identified. Resistant species are used in interspecific breeding programmes with commercial cultivars. Variation in virulence of Fusarium was investigated for f.sp. lilii, f.sp. gladioli and f.sp. narcissi. In all these formae speciales indications for race-formation were observed. Comparison of different sizes of bulbs within genotypes showed that screening immature bulbs is possible. Tests of individual plants in the seedling stage also gave promising results for a more efficient selection system. The use of indirect selection by molecular markers was investigated in the lily-Fusarium interaction. Inheritance of resistance in all four crops was mainly of a polygenic nature. Breeding values of parents determined after diallel analysis correlated well with resistance levels determined in clonal tests.
- Genetic variation