The Q&A Archives: black spots

Question: My rose bush I've bought on sale has a lot of black spots on the leaves.I 'd like to know what is that. Can I do anything about that now or should I wait until spring?

Answer: I love a bargain, too! It wouldn't matter whether you paid top dollar for your rose or if you bought it on sale - lots and lots of rose cultivars are susceptible to a disease called black spot. 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 might also be infected. A copper-based fungicide should stop the disease in its tracks. Or, you might try pruning your plants to remove any suspect wood and foliage. If you use a preventative spray in May and June of next year, your rose should not develop black spot in July. Try putting your rose on a regularly scheduled spray schedule. If it still develops black spot, regardless of treatment, you might consider replacing it with a more resistant cultivar. Best wishes with your roses.

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