Black Spots on a Rose Bush

Identification

• The Diplocarpon rosae fungus causes black spot on rose bushes. The fungus spends the winter on infected canes and in fallen leaves on the ground. It emerges during rainy, warm spring weather when temperatures rise above 75 degrees F and humidity levels are high. The pathogen releases spores that travel by splashing water to infect new foliage. The spores germinate on wet leaves. Black spot infection occurs when the foliage stays damp for longer than six hours.

Effects

• Rose bushes develop rounded black spots on their upper surfaces. The spots, which are surrounded by feathery yellow halos, range in size from 0.0625 inches to half an inch around. The spots grow together to form large, dark lesions. The diseased foliage turns yellow and falls from the plant. Rose bushes may eventually lose all of their leaves. Depending on the cultivar, some flowers develop a pale or washed-out appearance. Year-old infected canes form purplish-red lesions. The lesions eventually turn black as the disease progresses. Diseased plants are susceptible to secondary infections.

Diseases Similar to Black Spot

• Roses are susceptible to several other foliar diseases that cause dark-colored spots on roses. The Sphaceloma rosarum fungus causes circular brown, purple or reddish spots on rose foliage; the middle of these spots fall out, leaving the foliage with a tattered appearance. Other fungal pathogens such as Cercospora cause purplish spots on leaves that eventually turn gray or tan. Diseased leaves turn yellow and fall from the rose bush, resulting in defoliation. According to F.L. Pfleger and S.L. Gould at the University of Minnesota, you can distinguish black spot from other infections because of its consistent black spots and feathery margins.

Prevention and Control

• Remove and destroy diseased leaf litter and prune out diseased canes to limit the number of spores that can infect new growth during the spring. Plant your roses in spots that have full sunlight and proper air circulation so that moist leaves dry quickly. Do not water plants late at night, and water plants from below rather than above to avoid getting the foliage wet. Choose resistant cultivars whenever possible. Apply a preventive fungicide during the summer and spray plants every week to 10 days until frost.