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June is here, summer will arrive shortly and the tree canopy is full. There are many issues to watch for as that canopy matures and closes. The scourge of Emerald Ash Borer is still the big news of the landscape but don't let it overshadow the possibility of other diseases seriously impacting other species of trees. There are other similarly serious and devastating conditions to watch for. The American Elm, Ulmus americana, disappeared from the landscape and nearly disappeared from its native habitat decades ago due to a highly infectious disease, Dutch Elm Disease, spread by the Elm Bark Beetle. Another disease appeared around the same time with very similar physical symptoms: rapidly yellowing, wilting and browning of entire branches or full canopy of affected Elm trees. This disease, Elm Yellows, or Elm Phloem Necrosis, also is spread by an insect; in this case a leafhopper. Both diseases are highly infectious, result in the rapid collapse and death of the infected tree, and frequently spread from tree to tree (elm to elm) by naturally forming root grafts. So if the American Elm is essentially gone from the landscape, why worry; the diseases diminished with the diminishing of the number of American Elms, right? No.
One in particular, native to this part of the country, is Ulmus rubra, Red or Slippery Elm. In the passed week, I have seen this species in a natural or woodland suburban landscape, with the total collapse of the canopy. Several young but mature trees, self sown, were retained to provide shade for the shade plants beneath. the trees are in small clustered groups as one would find in a natural forest setting. One earlier, the trees were developing a full canopy and looked vigorous and healthy. Within that month, the canopy of those trees, and only those trees, is rapidly withering, turning brown, and dropping; and obviously spreading from tree to tree. There is no apparent population of either bark beetles or white banded leafhoppers, although infection does not require an invading swarm. At this point, for this landscape, identifying which of the diseases is present is more of an intellectual exercise than a practical one. Twig samples and bark scraping revealed no galleries typical of bark beetle infestation. No leaf hoppers were visible. Bark scraping suggested a possible "caramel" discoloration of the phloem - just under the bark and cambium but inconclusive.
So why the alarm and what is the point? Both of these diseases present very similar outer symptoms in the canopy. Both are very infectious and move quickly between trees of same species. Both diseases have little effective treatment to save the infected host tree(s), meaning rapid removal of the infected tree(s) and destruction of their roots is imperative to limit the spread to other trees in and beyond the affected and infected landscape. Be aware, look up, approach unusual conditions with a questioning mind. Know when it is appropriate to bring in an expert and or send samples for a confirmed diagnosis and treatment options.
Elm Phloem Necrosis
Dutch Elm Disease bark scraping
Elm Phloem Necrosis bark scraping