Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2002
Publication Date: 4/20/2002
Citation: Giovannelli, Janel L.; Farnham, Mark W.; Wang, Min; Strand, Allan E. 2002. Sequence characterized amplified region markers linked to downy mildew resistance in roccoli. J. Amer. Soc. Hort. Sci. 127:597-601.
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 varieties of broccoli that have natural resistance to downy mildew and that do not require fungicide sprays to prevent leaf, head and stem infection. In orde to use these varieties more efficiently as a source of resistance, we set out to find a DNA marker or fingerprint that would identify resistant plants without having to perform costly and time-consuming disease testing. Using such a DNA fingerprint in conjunction with the resistant variety, one could develop even better varieties that have superior horticultural characteristics as well as disease resistance. To accomplish our goal, we crossed a resistant variety with a susceptible one, developed progenies from this cross, and then examined the DNA makeup of resistant and susceptible individuals. From these evaluations we identified two specific DNA fingerprints that nearly always identified resistant individuals. These fingerprints will prove very useful in identifying resistant plants when one is handling a large number of plants that includes a mixture of resistant and susceptible individuals. This work is of high value to commercial and public breeders attempting to develop improved broccoli varieties. Such breeders can use the fingerprints identified in our studies to create superior varieties with downy mildew resistance. In turn, producers will be able to grow such varieties without using fungicides.
Technical Abstract: Downy mildew, caused by the fungal parasite Peronospora parasitica (Pers. Fr.) Fr., is a destructive disease of Brassica oleracea L. crops, including broccoli (B. oleracea, Italica Group). As environmental concerns regarding pesticide usage increase, the development and deployment of downy mildew resistant broccoli cultivars is becoming a priority for breeders and producers. Our long-term goal is to identify genetic markers linked to downy mildew resistance genes to facilitate marker-assisted selection for resistance and the pyramiding of resistance genes into cultivars. The objectives of this study were to: 1) identify RAPD markers linked to a previously described single dominant gene for cotyledon-stage resistance in broccoli; 2) clone and sequence the linked RAPD markers; and 3) develop and evaluate SCAR markers as screening tools for identifying downy mildew resistance. Employing bulked segregant analysis to pool DNA samples from resistant and susceptible F2 plants led to the identification of eight linked RAPD markers following a screen of 848 decamer primers. Two of the linked RAPD fragments, UBC359-620 and OPM16-750 were converted to SCARs link in coupling to the resistance locus at 6.7 and 3.3 cM, respectively. These SCAR markers are among the first genetic markers found linked to a gene conferring cotyledon-stage downy mildew resistance in B. oleracea. The results of this work provide breeders with useful information and tools for the systematic development of resistant cultivars.