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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.

  • When watering overhead, do so early in the day so that plants have a chance to thoroughly dry before nightfall. Since moisture on leaves is necessary for infection to take place, this cultural procedure is very important in cutting down on foliage infection.
  • A regular fungicide spray program throughout the growing season is generally necessary to control blackspot.
  • Most fungicides can safely be combined with insecticides and miticides and will perform better when small amount of spreader- sticker material is added to the spray solution. Dust formulations of rose fungicides generally don't provide the same level of control as liquids.
  • Regularity of spraying and thorough coverage are important to control. It's important to realize that fungicides protect plants from infection, and once the black spot fungus has invaded foliage, fungicide application is of limited value. For this reason, keep your plants covered with a protective fungicide at all times during the growing season
  • If only a few leaves show symptoms of black spot, remove and burn them. This will cut down on fungus spread to healthy foliage. Remove all diseased leaves from around the plants in the fall and destroy.
  • New plantings should be in areas with good air circulation to promote rapid drying of leaves. Also, avoid shady areas and dense plantings where high humidity problems are most likely to occur.

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.