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Ask Dr. Hort: Out, out, black spot on roses, and mold on crape myrtles

Winter is the best time to prune roses.

Winter is the best time to prune roses.

Out, out, black spot — and sooty mold

Q: I am having a constant problem with black spot on the leaves of my rose bushes. I have been spraying them with Garden Safe Fungicide 3 every two weeks. I still can't seem to stop the spread of the black spot. I have three rose bushes, next to each other, and they all have it. They produce beautiful roses, and I hate to see this happen. What can I do to get rid of the black spot for good?

Also, I seem to have a powdery mildew on the leaves of my crape myrtle. I have sprayed them several times, but it doesn't look like it is getting any better.

Any help would be greatly appreciated.

Dean Nitsos, Tierra Verde

A: Many cultural practices need to be employed together to successfully control black spot, a fungus disease. Make sure no irrigation is hitting the leaves. Collect and discard any fallen leaves for they will reinfect the plant. There needs to be good air circulation in, around and through the bush.

Some major pruning should be implemented to open up the plant to more of a vase shape. Winter is the perfect time to prune roses. The website will assist you with information on pruning with your spray program. Type in the search field what you want to do, such as "pruning roses," and you will find step-by-step instructions.

Your crape myrtle, Lagerstroemia indica, shows sooty mold, the black covering over the leaves, not powdery mildew, which is white. Sooty mold follows an infestation of sucking insects such as aphids, scales or white fly. These insects secrete "honey dew," which the sooty mold fungus feeds on. As the leaves fall this winter, collect and discard them. Use the same website to find instruction on how to rectify the problem before new leaves appear next spring.

Tomato plants need six to eight hours of sun

Q: We purchased two tomato plants and put them in large buckets with handles, as you suggested in one of your recent columns. We have them on our side screened porch, which faces the west and gets sun in the midmornings and afternoons. One of them had two small tomatoes on it and they are growing and doing fine, but there are no more blossoms showing up.

Can we keep them on the side screened porch through the winter and cover them if needed, or do they need to be outside so the bees, moths or whatever can get to them?

Gertrude McWilliams, Valrico

A: Tomatoes need six and preferably eight hours of sunlight each day to support flowers and fruits. With the days getting shorter, a western exposure would be pushing the light limit to set flowers. Bees are wonderful helpers in pollinating your tomatoes. On the patio, rub the flowers' noses to each other to transfer pollen. Make sure your fertilizer is one for vegetables. Too much nitrogen (the first number), compared to phosphorus or potassium will cause your plant to grow wonderfully, but it won't want to flower. Soluble bloom busters such as 12-48-16 or 10-30-20 work well. Protect your plants from the cold. There is plenty of tomato season left.

Need help? Dr. Hort (Greg Charles) answers questions about garden problems. Email him at or mail questions to HomeLink, Features Department, St. Petersburg Times, P.O. Box 1121, St. Petersburg, FL 33731. Describe problem in full, and include your name, city of residence and contact information. If possible, include a good-quality photo. Fuzzy ones won't do. Photos cannot be returned.

Ask Dr. Hort: Out, out, black spot on roses, and mold on crape myrtles 12/10/11 [Last modified: Saturday, December 10, 2011 3:30am]
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