Yellow Leaf Fungus on Knockout Roses

Knockout roses were bred to withstand diseases and insects. However, one fungus variety is able to slip by its defenses. Knockout roses with yellow leaves may be suffering from a fungal disease known as black spot.

Symptoms

Black spot causes circular black spots to appear on the leaves of knockout roses. The spots are 1/16th to 1/2 inch in diameter and surrounded by a yellow halo. The leaves will eventually turn yellow and drop.

Cause

Black spot is caused by a fungus that spreads by splashing water. The symptoms begin showing after the leaf has been wet for several hours. More serious infections occur after long periods of rainfall or bouts of high humidity.

Treatment

To treat this disease, remove any infected portions of the plant and dispose of them. Clean up any fallen debris around the base of the plant as well. Start any fungicides in the summer before any infection has set in. Spray plants twice a week during bouts of rainy weather. During drier weather, seven to 10 days in between spraying is ideal.

References

• University of Minnesota Extension: Rose Diseases
• Honolulu Rose Society: Diseases