magnolia problem - Ask Extension
my magnolia tree has black trunks. I sprayed with horticulture oil six weeks ago. Should I spray it again? Will it die? What other treatment should I ...
Knowledgebase
magnolia problem #926568
Asked March 23, 2026, 1:24 PM EDT
my magnolia tree has black trunks. I sprayed with horticulture oil six weeks ago. Should I spray it again? Will it die? What other treatment should I do?
Erie County Ohio
Expert Response
Hi Barbara,
Most likely your magnolia has magnolia scale, which are insects that attach to the branches and suck the tree's sap. They excrete a sticky substance called honeydew that drips onto the branches and leaves, attracting a fungus called sooty black mold -- hence the black color.
The mold is not usually a significant problem in and of itself, other than being unattractive. But the insects can weaken the tree and cause damage or stunted growth. In heavy infestations, they can even kill magnolia trees.
Horticultural oil is a good treatment for magnolia scale, but the timing is a little tricky. The best time to apply it is from late July to the end of September, which the eggs are newly hatched and the oil can kill the first-instar insects called crawlers. Because the oil works only if it comes into direct contact with the bugs, it's important to apply the oil multiple times throughout that period and to wet the branches and leaves thoroughly each time.
While dormant oil can be effective on magnolia scale if applied in March, it can damage flower buds. I wouldn't apply it once the buds have begun to swell.
I'm including links to more information about magnolia scale, including some other treatment options. One of those involves applying a soil drench of imidacloprid, but please note that imidacloprid is highly toxic to bees. While applying it as a drench reduces the chance that bees will come into contact with it, it should not be applied until the tree has stopped blooming. Unless the tree is in poor health, I would suggest instead trying the horticultural oil in late summer.
With any insecticide, including horticultural oils, it is essential to read and carefully follow the label instructions.
Best of luck to you!
More info:
https://bygl.osu.edu/node/1803
https://ohioline.osu.edu/factsheet/ENT-61
Most likely your magnolia has magnolia scale, which are insects that attach to the branches and suck the tree's sap. They excrete a sticky substance called honeydew that drips onto the branches and leaves, attracting a fungus called sooty black mold -- hence the black color.
The mold is not usually a significant problem in and of itself, other than being unattractive. But the insects can weaken the tree and cause damage or stunted growth. In heavy infestations, they can even kill magnolia trees.
Horticultural oil is a good treatment for magnolia scale, but the timing is a little tricky. The best time to apply it is from late July to the end of September, which the eggs are newly hatched and the oil can kill the first-instar insects called crawlers. Because the oil works only if it comes into direct contact with the bugs, it's important to apply the oil multiple times throughout that period and to wet the branches and leaves thoroughly each time.
While dormant oil can be effective on magnolia scale if applied in March, it can damage flower buds. I wouldn't apply it once the buds have begun to swell.
I'm including links to more information about magnolia scale, including some other treatment options. One of those involves applying a soil drench of imidacloprid, but please note that imidacloprid is highly toxic to bees. While applying it as a drench reduces the chance that bees will come into contact with it, it should not be applied until the tree has stopped blooming. Unless the tree is in poor health, I would suggest instead trying the horticultural oil in late summer.
With any insecticide, including horticultural oils, it is essential to read and carefully follow the label instructions.
Best of luck to you!
More info:
https://bygl.osu.edu/node/1803
https://ohioline.osu.edu/factsheet/ENT-61
