Disease on our magnolia tree

Asked October 27, 2017, 12:35 PM EDT

Our 12-year-old magnolia tree shows diseased branches (see attached photos taken on Oct. 19). What is it suffering from? Magnolia scale disease? And, most importantly, what do you recommend we do? Many thanks.

Rice County Minnesota magnolia plant pests magnolia scale horticulture

4 Responses

Thank you for the question. Injury from magnolia scale is from nymphs that are under the protection of their mother’s covering or waxy coating. Since these nymphs are under this brown covering which limits the effectiveness of pesticides, timing is of the essence. The object is to kill the nymphs (crawlers) as they crawl out from under this protective shell usually in early to late fall (Aug-Sept). The key is to monitor them with a hand lens and when you first see the crawlers you will know it is time to act. The usual treatment is the use of horticultural oil which is available at most nurseries or box stores. It's too late to catch the nymphs now but if you monitor the tree closely in the spring, you should be able to apply the oil with good success. If only a few branches are affected, you can can prune them out and skip the chemical treatments but continue to monitor next spring for any you may have missed.

Another option would be to use a chemical insecticide. Imidacloprid is a systemic insecticide applied as a soil drench. This insecticide is taken up by the roots and transported throughout the plant where it is ingested by the scale insect. Application can be done in late April through early May or August through September. Read the label carefully to minimize damage to beneficial insects.

Chemical insecticides available are Sevin (carbaryl), Bayer Advanced Power Force Multi-Insect Killer (cyfluthrin), Bayer Advanced Garden Tree & Shrub Insect Control (imidacloprid), insecticidal soap, BioNeem (neem), and malathion. All are registered for control of magnolia scale crawlers. Depending on the insecticide, you may need to make repeat applications at the intervals recommended on the product label. Always read the label of any insecticide and follow their instructions.
Here are links to excellent publications on Magnolia scale:
http://www.extension.umn.edu/garden/insects/find/scales/

Thank you for contacting Extension.

I neglected to respond to one of your main questions: Is this magnolia scale?
Yes, it is!

Very helpful! Thank you. Sadly, the magnolia scale spread onto our entire (big) tree, so it sounds like we'll need to use a chemical insecticide, right?
Just for clarification for the other scenario (smaller spread to be fixed with horticultural oil): does the "monitoring" of the tree in the spring mean locating nymphs emerging from their brown coverings? Do nymphs have then two crawling out seasons per year: one in the spring and one in Aug-Sept? Thanks.

Good questions and sorry to be confusing. To clarify, there is only one nymph emergence time each year, usually in August-September. Monitoring in the spring refers to evaluating your tree, looking for the extent of the scale (easier to see without leaves) and applying a horticultural oil product in an effort to suffocate the nymphs before they hatch. The oil will also help control the nymphs once they hatch so it can be used again in the fall. Follow the application directions on the product of your choice.
If the scale is widespread, you will probably have to apply the oil in the spring and the fall and use a residual pesticides that contain pyrethroids, acephate, or carbaryl.