Kelly Jackson
Christian County Extension Office
Black Spot on Rose
Black spot is the most common cause of defoliation of landscape roses in Kentucky. Black spot is a problem both in greenhouse production and outdoor plantings. Susceptible landscape roses must be sprayed frequently with fungicides to keep the disease under control. Roses in the landscape are subject to this disease annually.
Black spot appears as nearly circular black spots with feathery or irregular margins on the upper leaf surface. Individual spots are small ranging in size from 1/10 to 1/2 inch, but several spots may coalesce to produce larger irregular lesions. A yellow halo may form around the spots. Eventually the entire leaf turns yellow and drops off. Defoliation weakens plants and reduces flower production and quality. Canes may also become infected. Lesions appear as raised purple-red irregular blotches, later becoming blackened and blistered.
Black spot is caused by the fungal organism, Diplocarpon rosae. Spores are spread to the highly susceptible, young, unfolding leaves in spring by splashing water. Wet conditions of at least 7 hours with temperatures between 60° to 80° F are required for infection. Therefore, the disease is most serious during times of high rainfall and high humidity. Symptoms appear in as little as three days or up to two weeks after infection, depending on weather conditions.
A preventative program for black spot should begin in the fall with a thorough sanitation program. Proper sanitary practices are essential, because the black spot fungus overwinters on infected plant material. Gather and destroy fallen leaves. Remove old infected canes by cutting them back as severely as possible without cutting too close to the graft.
During the growing season, overhead irrigation, which prolongs leaf wetness, should be avoided. If plants are overhead irrigated, watering should be done in the morning rather than the afternoon so that leaves dry quickly.
Fungicides registered for black spot control should be applied preventatively to susceptible roses starting in spring before the new leaves become spotted. Continue the sprays at 7 to 12 day intervals until fall. During wet seasons, use the shorter intervals between sprays; longer intervals may be adequate during dry weather. Products containing copper and/or chlorothalonil as the active ingredient may help.
The most effective way to prevent black spot is to plant roses that have resistance to the disease. Most roses get black spot to some degree, but roses that have been bred for resistance to this and another common disease of roses will require less maintenance than those that are known to be susceptible. A few reported highly resistant cultivars are: Coronado, Simplicity, Grand Opera, Carefree Beauty, Bonica, and Fortyniner.
