Magnolia tree with darkening bark

Asked May 15, 2018, 12:00 PM EDT

Hi, I have a magnolia tree that last year started getting darkening bark so now it is almost black in places. This year there were very few flowers and the tree is struggling. I have sprayed weed killer under the canopy in the past and maybe this is the problem but I did not spray a lot. Do you know what may be causing this? Thanks, Mike Beltz 651-246-9981

Hennepin County Minnesota

1 Response

You have what appears to be Magnolia Scale which has been taking hold in Minnesota over the past few years. Magnolia Scale is caused by an insect that matures in late July/early August. Crawlers hatch in late August or early September. Crawlers settle on the young twigs where they overwinter. There is only one generation per year. Sooty mold is an associated problem that darkens the branches. It is very difficult to treat but left untreated, it is likely to kill your tree. One of the difficulties is that different options have been used but abandoned as those options have been banned as too dangerous to the environment. Additionally, timing and the method used is very important. At this time, spray treatment is not recommended because there are other plants growing and the insecticide can harm or kill other non-targeted insects. Horticultural oils applied in autumn can smother overwintering nymphs. Spraying weed killer under a tree is never a good idea. It may have weakened the tree and allowed the scale to take hold Your best bet is to contact a certified arborist of which there are many good ones in the metro area to assess the problem and provide the correct treatment. Some treatments can only be applied by certified arborists. It won't be an easy fix and may actually take several years to clear up. Even if you do get it cleared up, other Magnolias may reinfect your tree so at that point you may want to consider a yearly prevention plan. Good-luck!