Roses and Leaf Yellowing

• There are several diseases that commonly affect rose plants that can cause yellowing leaves. Probably the most common of these diseases is black spot, caused by the fungus Diplocarpon rosae. Climates that frequently see significant rainfall during the summer provide the ideal conditions for the fungus to infect the plant and sporulate, or reproduce by releasing fungal spores. If left untreated, the disease can be fatal to your rose plants.
  
  

Other Symptoms

• The black spots are circular and range in diameter from 1/16 inch to 1/2 inch, usually accompanied by a yellow halo encircling the black spot. As the infection grows, the entire leaf will turn yellow and eventually fall off the stem. Over successive seasons, you will also notice diminished blooming and poor stem health as a result of the disease.
  
  

Approaches to Control

• Never water the roses from the top of the plant down; always water at the base of the plant and during the daytime to allow for the sun to dry the rose completely. An overly wet rose leaf is the perfect breeding ground for the Diplocarpon rosae fungus. If your rose plant is already infected, prune away infected leaves and stems and discard them immediately.
  
  

Chemical Control

• As a last resort, a commercially available fungicide can be applied to the infected rose plant. Any fungicide that contains captan, chlorothalonil, mancozeb or thiophanate-methyl is effective at controlling black spot on roses. Whichever type of fungicide you use, always apply it during the summer before the rose leaves develop a coating that will protect the fungus from the fungicide. Spray the plants once every seven to 10 days, more so if the area in which you are growing the roses sees frequent rainfall or strong winds.