Several Landscape Professionals are reporting significant canopy dieback in maple trees this year.
I have received reports of significant branch dieback all across southern Ontario. Most notably we are seeing dieback on Norway maple (Acer platanoides and its cultivars) but we are also seeing dieback on sugar maple, silver maple, silver-red Freemannii hybrids, as well as Manitoba maples. It is also evident on honeylocusts and I’ve also had a few reports of mortality on less hardy species such as yellowood (Cladastris lutea). Remember how late everything was this spring, remember pointing at some trees and wondering if they were still alive?
Once it started getting hot and stopped raining every other day in June, soils had a chance to dry out a bit. The reduced moisture availability for root uptake uncovered a lot of branch mortality in the canopy. Partially killed branches were able to provide just enough water for leaf emergence and expansion, many crashed at that point while others held on until soils dried up a few weeks ago.
We see branch dieback on deciduous trees after every winter, especially on Norway maples. Let’s not forget how temperatures dipped down to near -15 the third week of November in 2018, were trees acclimated for winter dormancy by then? Given the amount of dieback on landscape and nursery trees, it doesn’t seem like it. After winters where temperatures dip down early, and at times are colder than the average (if I can even use that word), we see a lot more necrosis of the tissue just under the bark (the phloem, cambium and newest xylem, see photos above). Of course, this is the major conductive tissue of the tree and naturally the tissue death will be reflected in the canopy viability.
Some Landscape Professionals are wondering if this could be a wilt disease, like from the soil-borne fungus, Verticillium. In order to diagnose Verticillium wilt, I will peel the tree down to the sapwood, to the layer where it naturally slips off. Typically, Verticillium infection leaves a greenish-black stain to the sapwood (xylem), causing it to plug and die and prevent much-needed water into the tissue beyond the infection. Similar to Dutch elm disease.
Verticillium is a soil-borne fungus that makes a happy home in crops such as potatoes, beans and strawberries. Because it is soil-borne, it moves into the plant through the roots. It will continue to move into the root system up into the stem and then the canopy. That means that any discoloration that is not continuous into the root system, is not Verticillium. It can persist in the soil for years and has been an issue on field grown trees from time to time. During my 20-year career, I can count the number of times I have come across it in nursery production on my left hand. I have yet to see a confirmed case in the landscape.
There are many reasons why the xylem or sapwood can be discoloured in trees. Resist diagnosing sapwood discolouration as Verticillium and take a symptomatic, live branch or stem to a Diagnostic Agricultural Lab. The lab analysis will be able to confirm whether Verticillium is present in the sapwood.
I spent some quality time peeling symptomatic branches with University of Guelph Arborist, Rob Shaw-Lukavsky, this week. We were both amazed how mostly-killed branches were able to support canopy up until a couple of weeks ago when leaves wilted and quickly dried onto the branches.
We were unable to find branches with typical staining of Verticillium infection, mostly were either dead or light coloured with no discolouration (see photo above). We did submit some “suspect” discoloured branches into the Pest Diagnostic Clinic for testing and all of the tissues came back negative for Verticillium.
It is my experience that when we see established landscape trees dying back over a wide geographic range all at the same time, there tends to be an abiotic or environmental/cultural reason. I know we had some very hot and dry conditions throughout southern Ontario in 2018 and several other years in the last decade. We also observed some early deep-freeze temperatures in November, and this winter. Coupled with other stresses in the urban landscape, like road salt runoff and other root zone issues. When you take a closer look and review site history, it’s not surprising that our trees are suffering.