We are just moving in to a house in Warren Park here in Toronto, about 2 blocks E of the Humber River. The house faces S, and out front is a mature magnolia. About 3 days ago, I saw that there was something sticky on the leaves, and dropping down below on to the periwinkle covering the ground below. The weather has been extremely hot, and we’ve had heavy rain at night for the past couple days. The air is hot and humid. Today I saw that there are white blotches along most stems and branches, and I suspected it was scale. I’ve searched online, and confirmed it is scale. The whole tree is covered. Is the tree savable? The magnolia measures about 10-12 feet high. I have several more photos than the one I am submitting here, but it seems this platform only allows me to upload one photo.
You have correctly identified the problem with your magnolia tree: Magnolia scale (Neolecanium cornuparvum). According to The Morton Arboretum, “Soft scales, like Magnolia scale, feed directly in the tissues that move sap (phloem). They can remove large quantities of sap, stressing the host tree.” Although it sounds like your tree has a serious infestation, there are actions you can take to manage it. The Morton Arboretum (located in the U.S.) mentions systemic chemical controls that are not allowable under Ontario’s Cosmetic Pesticide Ban. Toronto Master Gardeners do not promote the use of chemicals that do not appear on the Allowable List.
Below is part of an in-depth response to a previous question about Magnolia scale:
The Toronto Master Gardeners’ website has a large amount of information on magnolia scale. At the top of the home page, you will see the heading Resources. Go there and fill in “magnolia scale” in the ‘find it here’ and the website will take you to questions and answers on scale. https://www.torontomastergardeners.ca/resources/
Late August and September is the time when scale eggs hatch and the nymphs begin crawling. At that time, you can treat the tree with dormant or horticultural oil. This is available at most nurseries. Using a sprayer and following the directions, you may be able to do this yourself (although you say that your tree is very tall). The oil smothers the nymphs, but has no effect on the adults which are covered in a waxy coating. Caution, the oil can burn the leaves.
ONNursery crops blog (from OMAFRA, the Ontario Ministry of Agriculture and Food) advises that a fall dormant oil spray (with emphasis on contacting undersides of twigs) can significantly reduce the nymph population in October. In fact, many horticulturalists say that fall dormant oil applications are the best way to manage this pest issue.
In early spring, before the leaves bud, horticultural oil can also be used effectively. By following the same directions, and being aware that the day and night temperatures must be above freezing, and no rain or high winds are in the forecast, spraying the overwintering nymphs will smother them. As there are no leaves yet, the risk of burn is minimal.
Another method which is less messy can be done now (mid summer). Use a strong blast of water from your hose to knock the magnolia scale off. At this point in their life cycle, they don’t have legs, and can’t crawl back into the tree. Take care to aim the spry to hit the bark, and avoid blowing off the leaves and flower buds. You may want to cover the area below the tree with plastic sheet to collect the scale and protect the plants below.
Another, albeit tedious, solution is to wear gloves, and perhaps a raincoat, and pick off the scale off the leaves.
Finally, if the infestation is large or the tree is big enough that you cannot do the job yourself, it may be the time to call in a professional arborist. You can find a certified arborist close to you on the Landscape Ontario site: https://landscapeontario.com/
Here is a link to our Ornamental Shrubs for Various Light Conditions: A Toronto Master Gardeners Guide, which will also be helpful: https://www.torontomastergardeners.ca/gardeningguides/ornamental-shrubs-for-various-light-conditions-a-toronto-master-gardeners-guide/
Good luck with your magnolia and I hope it brings you spring blooms for many years to come.
June 23, 2024