Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2001
Publication Date: 10/25/2001
Citation: Wang, Min; Farnham, Mark W.; Thomas, C.E. 2001. Inheritance of true-leaf stage downy mildew resistance in broccoli. J. of American Society of Horticultural Science. 126(6):727-729
Interpretive Summary: Downy mildew is one of the most destructive diseases of broccoli and is caused by a fungus. Most broccoli producers spray fungicides to control this disease. Due to concerns about the negative effects of pesticides on the environment and as residues on vegetables, we have developed lines of broccoli that have natural resistance to downy mildew and do not require fungicide sprays to prevent leaf, head and stem infection. The goal of this research was to determine how resistance is inherited in these lines. This is necessary so that we can devise strategies for breeding this resistance into additional, and hopefully better varieties. To accomplish our goal we crossed a resistant variety with a susceptible variety, developed progenies from this cross, and evaluated the response of the progenies to artificial inoculation and infection by the downy mildew fungus. From these evaluations we determined that two independent genes confer downy mildew resistance in our resistant broccoli lines. We also determined that the resistance genes are dominant over the susceptible genes. This work is of interest to commercial and public breeders attempting to develop improved broccoli hybrids. The resistance studied in this work and knowledge of its inheritance will be used by the above breeders as they work to create superior hybrids with high levels of downy mildew resistance that producers can grow without using fungicides.
Technical Abstract: Downy mildew incited by the biotrophic fungal parasite, Peronospora parasitica (Pers. Fr.) Fr., is one of the most destructive diseases of broccoli (Brassica oleracea L., Italica Group) and other related crop species throughout the world. The cultivation of resistant cultivars is the most desirable control method because it provides a practical, long- term, and environmentally benign means of limiting damage from this disease. The commercial hybrid 'Everest' has been shown previously to contain a high level of downy mildew resistance. Doubled-haploid (DH) lines developed from that hybrid were also shown to exhibit a similar, high level of resistance at the 3- to 4-true leaf stage. To determine mode of inheritance of this true leaf resistance, the resistant DH line was crossed to a susceptible line (derived from 'Marathon') to make an F1 hybrid. Subsequently, F2 and backcross (BC) populations were developed from it. In naddition, a DH population of about 100 lines was developed from the same F used to create the F2 and BC. All populations were evaluated for response to artificial inoculation with P. parasitica at the 3- to 4-leaf stage. All F1 plants were resistant like the resistant parent and F2 populations segregated 9 resistant to 7 susceptible. BC populations using the resistant parent as the recurrent parent contained all resistant plants and the BC to the susceptible parent segregated 1 resistant to 3 susceptible. These results could be explained by a model with two complementary dominant genes. This model was confirmed using the DH population that segregated approximately 1:3, resistant to susceptible. Due to the dominant nature of this resistance, controlling genes should be easily incorporated into F1 hybrids and used commercially to prevent downy mildew.