Black Tulip magnolias are small hybrid trees with purplish-black tulip-like flowers that grow up to 6 inches in diameter. As trees, these magnolias reach a height of 15 to 30 feet, but they can also be grown as hedges, and they do well in containers, too. While they tend to be free of most pests, they do suffer from a few problems.
Magnolia scales grow to about 1/2 inch in diameter. Adult females are pinkish-orange or brown and are usually covered with a white wax coating. Immature scales are dark grey and have a reddish brown ridge down their back. Scales feed by sucking the sap out of leaves, and they excrete a sticky, sweet substance called honeydew, which encourages the growth of black sooty mould. Natural enemies, like lady beetles, parasitic wasps and predatory mites, usually keep them in control. If it's necessary to use insecticides, they must target the crawler stage of this pest since other stages are protected by a waxy coating that keeps sprays away from them. Don't over-fertilise your magnolia, as too much fertiliser will increase scale problems. Infested branches should be pruned and destroyed.
The magnolia leafminer -- also known as the sassafras weevil, the tuliptree leafminer or the yellow poplar leafminer -- is a small black weevil that lays its eggs inside magnolia leaves. Grub-like larvae are white with dark heads. They burrow through leaves as they feed, making blotch mines between the upper and the lower surfaces of the leaves. Most trees can tolerate some leafminer activity, but heavy infestations can leave a tree susceptible to disease or attack by other insects. Leafminers are difficult to control because they're surrounded by leaf tissue. Parasitic wasps usually keep their population in check. Help your Black Tulip tree recover from damage by keeping it watered and fertilised so it remains healthy and doesn't become stressed. Prune infested branches and destroy them.
Gray mould, or botrytis blight, is a fungal disease that causes flowers to develop brown spots, beginning with the inside or middle petals first. Irregular brown spots can show up on the leaves as well, and a grey mould appears when the humidity is high. The fungus that causes botrytis lives on dead plant parts and in the soil and infects trees through wounds or when diseased tissue comes in contact with healthy tissue. Control botrytis by cleaning up diseased plant parts and destroying them. Botrytis is spread by splashing water, and the fungus needs a wet surface to germinate. Keep leaves dry and don't water the magnolia from overhead. Avoid injuring your tree. Fungicides can be used as a preventive.
Although verticillium wilt begins in the roots, symptoms usually aren't noticed until the upper part of the tree becomes diseased. Above-ground symptoms include branch dieback and leaves that wilt or curl, turn yellow and fall off the tree. In addition, the sapwood becomes discoloured and turns light to dark green. No cures exist for this disease. Magnolia trees that die from it should be removed, along with as much of the root as possible, and plants susceptible to verticillium wilt shouldn't be planted in their place for at least three years.
Magnolia trees aren't tolerant of cold weather, and spring frosts can kill or damage flowers. In addition, their soft wood makes them prone to limb breakage during ice storms. Trees also easily damaged by mowers because they have thin bark.
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- Clemson Cooperative Extension; Magnolia; Debbie Shaughnessy; November 2006
- University of Illinois Extension HortAnswers: Fungal Disease - Gray Mold
- University of Illinois Extension HortAnswers: Fungal Disease -- Verticillium Wilt
- University of Kentucky College of Agriculture; Magnolia Scale; Lee Townsend; March 2005
- University of Florida Extension; Leafminers on Ornamental Plants; Eileen A. Buss
- University of Connecticut: Magnolia x soulangiana