Black Spot of Roses Can Be a Challenge, Even for the Experienced Rose Grower

June 5, 2009

Editor's note: Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic is a leading diagnostic service that aids in the identification and diagnosis of plant diseases, insects and environmental disorders. The clinic is located on the campus of the College of Food, Agricultural, and Environmental Sciences in Columbus, Ohio.

We are launching a monthly article series to create more awareness of the facility's state-of-the-art technology and staff expertise by addressing potential horticultural problems for homeowners and garden enthusiasts. This is the first article in the series.

COLUMBUS, Ohio -- Roses are one of the more popular flowering shrubs grown in Ohio, but they can be plagued by a number of diseases that will rob plants of their vigor and beauty. One of the more persistent diseases now making an appearance is black spot.

Black spot of roses is a fungal, foliar disease that is widespread among most rose species and cultivars and thrives under wet, humid conditions throughout spring and summer.

Nancy Taylor, director of Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic, said that among the many diseases that can affect roses, black spot is one of the most difficult and challenging to control.

"The disease lurks on the lower foliage, not easily visible, so many rose growers don't realize they have a problem until they begin to see leaf loss," said Taylor. "It could be more of a challenge for inexperienced rose growers."

The disease first appears on the plant leaves as round to irregular black spots. Eventually the leaves yellow and drop from the plant. The result leads to less plant vigor, less blooming and greater susceptibility to other stresses. In some cases, said Taylor, the disease kills the plant.

"There are a number of options available to rose growers to help prevent or control the disease," said Taylor.

Taylor recommends that rose growers plant disease-resistant cultivars. Use of resistance is the most effective, least expensive, and most environmentally friendly tool a rose grower can employ, but keep in mind that resistance will vary due to differences in pathogenic races of the black spot fungus and geographic location, said Taylor.

Whether or not growers choose disease-resistant cultivars, a good management practice to follow is to keep foliage dry. "If you are irrigating roses, don't use oscillating sprinklers," said Taylor. "Fungal infections are favored by wet foliage."

Also, keep the rose bush free of disease-infested leaves, fruit and canes. At the end of the season, dispose of old leaves and prune out infested stems as black spot will overwinter on diseased leaves and canes and reappear the following spring under ideal conditions.

Although Taylor recommends an Integrated Pest Management (IPM) approach to disease management, in some cases, fungicide applications may be warranted.

"Most fungicides protect foliage from infection and need to be applied before or just at the onset of the disease and reapplied at regular intervals," said Taylor. She emphasizes that rose growers do their research to find the most effective, labeled fungicide before making any applications. Some fungicides available to home gardeners include: chlorothalonil (Daconil), triforine (Triforine, Funginex) and myclobutanil (Immunox).

Black spot of rose is just one of several common diseases that impact roses. Other diseases include powdery mildew, yellow mosaic virus, rust, stem cankers, witches broom and crown gall.

Rose growers spotting potential disease symptoms and are unsure of identification can submit samples to Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic for a nominal fee. To learn more, log on to http://ppdc.osu.edu.

For more information on black spot of roses, a list of resistant cultivars, and fungicide treatments, refer to the free Ohio State University Extension fact sheet, "Black Spot of Roses," at http:// ohioline.osu.edu/hyg-fact/3000/pdf/3072.pdf.

 

Author(s): 
Candace Pollock
Source(s): 
Nancy Taylor