Plant Pathology Infobytes
May 14, 1997
How To Control Black Spot Of Roses
Gardeners who want to keep their rose garden the envy of the neighborhood should plan to keep black spot fungus disease under control. Black spot is rose "public enemy number one" and commonly makes its appearance in the spring following warm, wet weather.
Symptoms of black spot begin on leaves as small, pin-head sized black or dark brown spots which are circular in shape. The spots increase in size, and infected leaves eventually turn yellow and fall from the plant.
Continued plant attack by black spot results in unattractive foliage as well as reduced size, number, and quality of blooms. Successive seasons of defoliation weakens plants and increases susceptibility to winter kill and other disorders.
Black spot usually occurs on the lower portion of the rose bush first and moves upward as the season progresses. Leaves are most susceptible to infection when young but can be invaded at any point of development during periods when showers are numerous.
Rose varieties vary in susceptibility to the black spot fungus. Grandiflora and floribunda types are generally more resistant than hybrid tea varieties, and gardeners who have hybrid teas in their planting should plan to carry out a full-season black spot control program. The following control guidelines are suggested.
Just how important is rose disease control? Any successful grower will tell you, "Don't even think about growing roses unless you are willing to devote special attention to the control of diseases and other pests." Yet, for individuals who are truly fond of roses, the pleasure of producing their own blooms will justify the time and expense required to keep disease problems to a minimum.
For additional information on blackspot and other rose diseases, check with us at your county Extension office.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.