Extension Educator, Horticulture
By the time August rolls around plant diseases can be in full glory. Many diseases thrive in warm humid weather. Black spot of roses is no exception. Black spot fungus, as its name implies, causes nearly circular, black leaf spots with fringed margins. It is usually seen on the upper leaf surface but may also appear on the underside of leaves. The spots may merge to create large, irregular lesions on the leaves. A yellow margin may appear on the leaves then the leaves quickly drop. Left unchecked, roses may have few leaves at this time of year.
Black spot may also cause red spots and possibly some distortion of the flowers. The stems may have purplish red to black lesions. Roses with black spot look bare and straggly during the summer when other roses look lush. As Nancy Pataky, U of I Plant Clinic Director reports in the U of I Extension Home, Yard and Garden Pest newsletter the major concern with this disease is that it weakens the plant to infection by other pathogens.
Infected plants are often predisposed to environmental and site stress as well. Infected plants may not survive a severely cold winter. However, I've seen roses get this disease every year and they still seem to come back next year.
The most important management tool is selecting disease resistant varieties of roses. Rose cultivars vary from highly resistant to very susceptible to black spot. Because other diseases also bother roses, Pataky recommends looking for roses that are resistant to black spot, powdery mildew, and possibly rust. Some good disease resistant roses include: Nearly Wild (pink shrub); Jens Munk (pink shrub); 'Albo Plena' rugosa rose (white shrub); Henry Hudson (light pink shrub); David Thompson (red shrub); Charles Albanel (red shrub); John Cabot (red climber); William Baffin (dark pink climber); J.P. Connell (yellow shrub); and Louis Jolliet (pink climber). Keep in mind under warm humid weather even the disease resistant cultivars may get some black spot but infection will not be as severe as susceptible cultivars.
Management of this fungus should begin in the dormant season when plants are pruned of old cankers and winter-killed stems. Remove this material from the site because the causal fungus may overwinter on those tissues as well as on old leaves. At the end of the growing season all old leaves should be removed from the site.
Pataky states chemical options are very effective (and time consuming) in controlling the fungus. Sprays should begin as soon as the disease is seen. Generally, sprays are continued on a weekly basis per label instructions until hot, dry weather occurs. Then sprays can be limited to just after each rain until weather is cool again. Yes, this is a high-maintenance disease for rose growers. Many fungicides are labeled for black spot. Possibilities include chlorothalonil sold as Daconil or triforine sold as Funginex.
Since the black spot fungus infects when the leaf surface is wet, prune surrounding vegetation to allow better air movement in the garden. This helps plants dry more quickly and reduces the time that the fungus can infect the plant. For the same reasons, avoid overhead watering where possible.
Good sanitation, refraining from overhead irrigation and selecting disease resistant varieties can go a long way in keeping the black spot fungus in check.
Be sure to drop off your extra produce at the Schnucks Grocery Store on Mattis Avenue in Champaign or on Cunningham Avenue in Urbana for the "Plant a Row for the Hungry" Program. Look for the vans every Saturday from 10:00 am -1:00 pm until September 23. All produce goes to the Eastern Illinois Foodbank.