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. In order to diagnose Verticillium wilt, I will peel the tree down to the sapwood, to the layer where it naturally slips off. Some Landscape Professionals are wondering if this could be a wilt disease, like from the soil-borne fungus, Verticillium. Of course, this is the major conductive tissue of the tree and naturally the tissue death will be reflected in the canopy viability. 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). 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. We see branch dieback on deciduous trees after every winter, especially on Norway maples. 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. The reduced moisture availability for root uptake uncovered a lot of branch mortality in the canopy. Once it started getting hot and stopped raining every other day in June, soils had a chance to dry out a bit. Remember how late everything was this spring, remember pointing at some trees and wondering if they were still alive? 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). 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. I have received reports of significant branch dieback all across southern Ontario. Several Landscape Professionals are reporting significant canopy dieback in maple trees this year.
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