Posted 12 July 2010. PMN Crop News.
Rose Lovers Beware: June Weather Fostering Rose Blackspot
Source: Kansas State University Press Release. www.ag.ksu.edu
Manhattan, Kansas (June 24, 2010)--Central U.S. landscapes began June with ideal conditions for a common rose disease - blackspot - which can totally defoliate susceptible varieties.
Kansas State University reports the fungal disease is most likely to be severe when:
• The relative humidity is 85 percent or higher,
• Air temperatures are in the 75- to 85-degree range, and
• The plant´s leaves are wet for six or more hours.
"Blackspot is why you should keep irrigation water off of the plants´ foliage. In fact, drip irrigation may be the best way to water roses," said Ward Upham, horticulturist with K-State Research and Extension. "The disease also is one reason roses need to be in a sunny area with good air movement. That kind of site limits the time the plants stay wet after a rainfall."
Blackspot infections usually start on lower leaves and work their way up. Newly expanding leaves are the most vulnerable, Upham said.
The disease´s most obvious symptom is dark, circular wounds with feathery-looking edges; they appear on leaves´ upper surface. Sometimes, however, young stems (canes) develop raised purple spots.
"Infected leaves often yellow between spots and may eventually drop. You need to remove them from the ground and throw them away, as well as prune out infected rose canes. That kind of sanitation helps minimize the amount of fungus or inoculum available for splashing water to spread around - this year or next," he said.
Upham recommends that gardeners avoid susceptible rose varieties unless they´re willing to use fungicide sprays. A list of blackspot- resistant varieties is on the Web at www.ppdl.purdue.edu/ppdl/weeklypics/3-22-04.html.
Protecting susceptible foliage during wet, humid weather requires spraying plants every 10 to 14 days. The fungicides K-State recommends for Kansas include such active ingredients as tebuconazole (e.g., Bayer Disease Control for Roses, Flowers and Shrubs), myclobutanil (Immunox), triforine (Funginex), thiophanate methyl (Fertilome Halt) and chlorothalonil (Broad Spectrum Fungicide, Garden Disease Control).