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Ask Dr. Hort: Knock out black spot on rose bushes

Knock out black spot on rose bushes

Q: I live in a condo complex here in Clearwater and neighbors have planted about eight knockout rose bushes. These bushes are in a spot that is in the shade until about 1 p.m. each day and then receives sun the rest of the afternoon. The bushes have acquired black spot on the leaves and have become quite leggy. What do we need to use to get rid of the black spot and how far back should they be trimmed to encourage them to bush out more?

Also, we have an avocado tree that had branches brushing the roof of our carport and the neighbors cut off the offending branches and now want to top the tree. Is it possible or not a good idea to top an avocado tree?

Shirley Garrett, Clearwater

A: Knockout roses, if planted in full sun, are practically bulletproof when it comes to insects and diseases. It sounds to me like another case of "the wrong plant in the wrong place," hence the black spot problem. Cut the plants down to about knee high, which should eliminate all of the black spotted leaves, in fact, probably all of the leaves. Make sure to rake up all of the fallen leaves and discard them. At the first sign of spotting this spring, spray the plants weekly with a mix of 1 tablespoon baking soda, 1 tablespoon neem oil and a squirt of dish soap in 1 gallon of water. Stay vigilant and your problem should be lessened. If black spot persists, it's time to change the plants.

About your avocado: Never top a tree. You will end up with bigger problems down the road. The height of a tree can be reduced through a procedure called "drop crotching," which won't harm the tree and reduce its height. It would be best to have an ISA-certified arborist take a look and give you a quote.

She'd like to bring beauty, color back to red bay tree

Q: My red bay tree is approximately 10 years old and has been in my yard for about six years. I bought it from Wilcox Nursery in Largo. Until this spring, it flourished and became a shade tree. This past spring, there was yellowing of the leaves and some were falling. I went back to Wilcox Nursery and then to the agricultural extension service with a sample branch and the person at the extension center said it looked fine and recommended Florida's Finest Palm/Ornamental/Citrus 8-0-12. I applied this as directed, but see no improvement, and the leaves still fall. The yellowing has now crept almost to the top of the tree.

Gail Meyers, Seminole

A: Hopefully it is nutritional as you were told at the extension service, or a problem associated with too much or too little water. Keep adjusting these cultural conditions to see if new buds pop with new leaves.

If the fruits of your labor don't set new leaves, your tree could have a terminal problem named laurel wilt, a fungus spread by a new ambrosia beetle that, unlike most of the ambrosia beetle family, attacks healthy trees. Red bay, Persea borbonia, and avocado, Persea americana, are cousins and both are highly susceptible to the disease, for which there is no cure. The major symptoms are a soft wilt of the leaves, then a reddening or purpling usually, yellowing on occasion. However, as the leaves eventually turn brown, they stay on the tree and not fall, unlike your leaf drop.

Another possibility is a root disease from a change in water regimens, which would then cause yellowing followed by leaf loss. Unfortunately this scenario would also be fatal. Hopefully it is a cultural problem. Keep trying!

Need help? Dr. Hort (Greg Charles) answers questions about garden problems. Email him at drhort@tampabay.rr.com or mail questions to HomeLink, Features Department, Tampa Bay 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: Knock out black spot on rose bushes 01/14/12 [Last modified: Saturday, January 14, 2012 3:31am]

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