The Q&A Archives: Rose Tree

Question: I have a question in regard's to my rose tree. I live in
> Wisconsin and have owned a rose tree for about 6 years now. I
> bury it in the winter and dig it up in spring and repot it. It
> get's beautiful roses on it but halfway through the summer the
> leaves turn yellow and get big black spots on them and they fall
> right off. So, I am left with a rose tree with beautiful roses
> and no leaves...I use a rose fertilizer that helps it grow and
> also keeps the bugs off of it. Do you have any suggestions?
> Stumped in Wis. Thank-you

Answer: The symptoms you describe sounds typical of 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 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.

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