Resistant Broccoli’s DNA Used in Fight Against Downy Mildew

Agricultural Research Service scientists have taken an important step toward protecting the broccoli industry against downy mildew without relying on fungicides. They have found genetic markers that identify varieties of the vegetable that resist the disease.

According to research geneticist Mark W. Farnham, who spearheads the fight against the fungus-caused scourge at the U.S. Vegetable Laboratory in Charleston, S.C., the finding is of great value to breeders developing improved broccoli varieties.

The markers, consisting of genetic material called DNA, will allow downy-mildew- resistant broccoli to be identified without the use of costly and time-consuming disease tests--and can lead to more accurate methods of detecting resistant varieties, according to Farnham. The markers will prove very useful in singling out resistant plants from a large number that includes a mixture of resistant and susceptible ones.

Concerns over pesticide use first led to the development of broccoli varieties that have natural resistance to the disease. Farnham’s laboratory set out to find the related markers to use these varieties more efficiently as a source of resistance.

The markers can help Farnham reach one of his long-range research goals: pyramiding resistance genes. In pyramiding, scientists create varieties containing more than one type of resistance gene, which makes it harder for a pathogen to overcome the plant’s resistance.

Another goal of the research, which started in 1995, is to determine how resistance is inherited in one particular variety under study. This information is necessary so that researchers can devise strategies for breeding resistance from this variety into new and better ones, according to Farnham. The research can be applied to other cole crops such as cauliflower and cabbage that are within the same botanical family.

Downy mildew costs broccoli producers millions each year in lost profits. California is by far the leading U.S. producer of broccoli, although it is grown in almost all states. The industry boasted farm revenue averaging $484 million annually from 1996 to 1998. Americans consumed 2 billion pounds of broccoli in 1998, about 8 pounds per capita.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.