Agricultural Research Service (ARS) researchers and cooperators are introducing watermelon stock that may help breeders combat powdery mildew, a disease that threatens watermelon yields and quality in several states.

Recently, two races of powdery mildew have been reported on watermelon, and they appear to be geographically separate. Existing watermelon lines, which were thought resistant, were found to be susceptible. But ARS researchers and colleagues discovered the first documented resistance to Race 1 powdery mildew in an ARS germplasm collection.

The scientists first analyzed existing lines from the ARS Southern Regional Plant Introduction Station in Griffin, Ga., for resistance to Race 1 using field and growth chamber studies. They developed the new watermelon line, PI 525088-PMR, by repeatedly selecting the most resistant plants from the line PI 525088 (Citrullus lanatus var. lanatus).

According to Angela R. Davis, geneticist at the ARS South Central Agricultural Research Laboratory in Lane, Okla., watermelon has historically been resistant to powdery mildew, but the disease has become widespread during the past few years. A significant problem in Europe and Africa for about a decade, powdery mildew has emerged as a severe problem in some areas of the United States.

Powdery mildew appears as a dusty white or gray coating over leaf surfaces or other plant parts, and can be difficult to control. Davis conducted the research with Amnon Levi, an ARS geneticist with the U.S. Vegetable Laboratory in Charleston, S.C.; Todd C. Wehner of North Carolina State University in Raleigh; and Michael Pitrat of France’s National Institute for Agricultural Research.

The new watermelon line may be useful for introducing resistance to Race 1 powdery mildew (caused by the fungus Podosphaera xanthii, previously known as Sphaerotheca fulginea auct. p.p.) into commercial watermelon cultivars. Ultimately, it may also reduce the amount of fungicide needed to control the disease.

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