black spot on roses - Knowledgebase Question

margate, Fl
Question by humanist
July 2, 2009
I use an ortho anti disease control preparation every few days but the black spot comes back, sometimes the next day. I keep the bushes well ventilated by thinning them in the center and low part of the bush. I remove ruined leaves ASAP. SOUTH FLORIDA so there's plenty of sun.

Answer from NGA
July 2, 2009


I'm sure it's very frustrating to do everything right to control black spot and still have your roses continue to develop the disease. Some roses simply are more prone to the disease and if you've done everything in your power to control the disease and are not successful, you may want to replace your roses with more resistant varieties.

Black spot is caused by Diplocarpon rosae, a fungus that overwinters on old diseased leaves and infected canes. Leaves are most susceptible while expanding. After fungal spores land on leaves, it takes at least 9 hours of leaf wetness for the spore to infect. Fungal fruiting bodies form in 11-30 days. A new crop of spores is produced and spreads to healthy portions of the plant by splashing rain or irrigation water. Spores do not survive in soil but do survive on all infected plant debris. The natural genetic variability of the fungus means roses found resistant in one location may be susceptible in another location due to the presence of different fungal strains. Also, resistant roses may become susceptible after a few years due to changes in the local fungal population. The symptoms of black spot begin with circular black spots, frequently with fringed margins. Yellowing and defoliation are common in susceptible cultivars. In wet weather, spots may become very severe and run together, making large irregular spots. Control practices include avoiding dense plantings (good air circulation all around the plant is important). Avoid overhead watering. Rake up and remove all leaves at the end of the season. You can try removing the yellowing leaves as soon as you notice them, but I suspect the canes are also infected.

If you love roses I'm afraid you'll have to put up with the preventative spray schedule - or replace your roses with some of the newer, more disease resistant cultivars.

Best wishes with your landscape!

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