The Q&A Archives: rose problem

Question: I have Knockout roses that have got some kind of disease. Lower leaves are turning yellow with some black spot. It has been very dry here in no. va I have watered them. Yhankyou Rolf

Answer: Black spot can be difficult to control, but here are some suggestions. Some varieties of rose are more susceptible to black spot than others, but if the weather conditions are right, even Knockout Roses can develop black spot. In my experience, too, some years are simply worse than others when it comes to black spot. Hygiene is important to help reduce reinfection: clean up and remove and destroy every infected leaf, especially in the late fall. Then, top off your mulch with a fresh layer to prevent it splashing back up onto the plants next season. Cultural practices can also help. The roses should be in full sun. Make sure the plants are spaced far enough apart to allow for good air circulation, do not handle them when they are wet, and do not water the foliage, especially if you water in the evening. Also be sure you are not overwatering and thus causing extra humidity around the plants. Finally, you can try treating it with a baking soda solution. To make the solution, mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil spray with a gallon of water. The oil is slightly fungicidal and acts as a spreader-sticker, helping the baking soda coat the leaf and cling to the surface longer. The solution is a contact protectant so it stops fungi from attacking plant tissue, but doesn't cure the problem... Apply it as soon as the symptoms appear, repeating about every 2 weeks. Be sure to get under the leaves too. I hope this helps!

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