Black spot is a serious disease of roses in this country, except in semiarid areas with little summer rainfall. If roses are subject to infection, unless sprayed regularly to protect them from it, they'll give little pleasure.
Now there's hope that black-spot-free roses are on the horizon. Peter Semeniuk, research horticulturist at USDA Science and Education Administration, has developed three hybrid roses that are resistant to strains of the black spot fungus native to the East, South and Midwest.
Semeniuk is releasing budwood from the three cultivars (cultivated varieties) to interested commercial and amateur rose breeders.
While two of the cultivars will need further cross-breeding to produce an acceptable garden rose, the third, Spotless Pink, already has a well-shaped bud, a full bloom of 65 to 70 petals and semi-glossy dark-green foliage.
If it's judged acceptable by commercial breeders, the rose could reach the general market in three to four years; otherwise, it could take eight or more years before the offspring of Spotless Gold and Spotless Yellow are available to the public.
Semeniuk estimates that there may be as many of 50 to 100 strains of the blackspot fungus in the United States alone, but he chose seven strains that he had closely studied to test for resistance during his breeding program. The strains he maintained in the laboratory had been cultured from infected leaves from Beltsville; University Park, Pennsylvania; Ithaca, New York; Tifton, Georgia; Tyler, Texas; Ames, Iowa; and Delaware, ohio.
Because the new rose cultivars are resistant to these strains, there is a good possibility they will be resistant to other strains, he said. But we won't know for sure until after resistant roses are introduced and reports start coming in from growers around the country.
Polyantha roses, their hybrids and the floribundas are slightly more resistant than hybrid teas to black spot, according to specialists.
Many susceptible varieties can be grown without becoming infected because there's no source of infection nearby. The spores (germs) are spread by the wind.
The most susceptible varieties are the yellow, orange, flame, bi-colored and multi-colored kinds. However, in a wet summer when black spot is invariably more serious, few varieties escape some traces of infection.