A common disease of roses is blackspot, a fungus disease that can
cause defoliation of susceptible plants. Look for dark, circular lesions
with feathery edges on the top surface of the leaves and raised purple
spots on young canes. Infected leaves will often yellow between spots
and eventually drop.
The infection usually starts on the lower leaves and works its way
up the plant. Blackspot is most severe under conditions of high relative
humidity (>85%), warm temperatures (75 to 85
degrees F) and six or more hours of leaf wetness. Newly expanding leaves
are most vulnerable to infection. The fungus can survive on fallen
leaves or canes and is disseminated primarily by
splashing water.
Cultural practices are the first line of defense.
1. Don’t plant susceptible roses unless you are willing to use
fungicide sprays. For a list of blackspot resistant varieties, go to:
http://www.ppdl.purdue.edu/
irrigation water off the foliage. Drip irrigation works well with roses.
3. Plant roses in sun in areas with good air movement to limit the
amount of time the foliage is wet.
4. Remove diseased leaves that have fallen and prune out infected
rose canes to minimize
inoculum.
If needed, protect foliage with a regular spray program (10- to a
14-day schedule) of effective fungicides. Recommended fungicides include
tebuconazole (Bayer Disease Control for Roses, Flowers and Shrubs),
myclobutanil (Immunox, Immunox Plus), triticonazole (Ortho Rose & Flower
Disease Control) and chlorothalonil (Broad Spectrum Fungicide, Garden
Disease Control, others). (Ward Upham)
