Choos­ing our bat­tles: Dutch elm dis­ease is a fight we can win

Manitoba Gardener Magazine - - Contents - By Ke­ri­enne La France

As the fight against our lat­est in­va­sive pest, emer­ald ash borer, is now be­ing waged in Winnipeg it may be easy to for­get about our older and more fa­mil­iar foe – Dutch elm dis­ease. Over 8,000 Winnipeg trees were marked for re­moval dur­ing the 2017 sur­veil­lance sea­son and we an­tic­i­pate the same for 2018.

Not to de­tract from the lat­est news of emer­ald ash borer, it’s still im­por­tant not to lose sight of the bat­tles we can win. In the case of Dutch elm dis­ease, all is not lost. It’s a dis­ease that can be suc­cess­fully man­aged, and elms can re­main a per­ma­nent fea­ture on our land­scape.

A quick re­view of DED

Dutch elm dis­ease (DED) is caused by an in­va­sive fun­gus from Asia. Elm bark bee­tles are un­wit­ting car­ri­ers of the sticky fun­gal spores as they travel from one elm tree to the next, and the fun­gus quickly spreads into the tree’s vas­cu­lar sys­tem, block­ing the vi­tal tis­sues that trans­port wa­ter and nu­tri­ents. The re­sult is sud­den wilt­ing and pre­ma­ture leaf drop in the canopy, and the dis­ease can kill a tree within a sin­gle sea­son.

Keep­ing up with the dis­ease

The dreaded “orange dot” has be­come an all-too-fa­mil­iar sight on our streets, but boule­vard trees ac­count for only a small frac­tion of all the elms we lose. In fact, 80 per cent of elms lost to DED are lo­cated on pri­vate prop­erty and these can be some of the most dif­fi­cult trees to re­move. Pri­vate trees can be wedged against homes, garages, sheds, fences, or have pow­er­lines over­head – in some cases, all of the above may ap­ply at once! These cases re­quire spe­cial at­ten­tion and co-or­di­na­tion with util­ity com­pa­nies, and are of­ten the trees that end up at the bot­tom of the re­moval sched­ule. Elms along river banks can also be a dif­fi­cult reach and are usu­ally only ac­ces­si­ble in the win­ter when re­moval crews can take ad­van­tage of the frozen rivers. To date, a to­tal of about 670 trees from 2015 and 2016 are await­ing re­moval in Winnipeg. It’s largely ac­cepted that these lin­ger­ing elms are re­spon­si­ble for the cre­ation of new DED “hot spots” the fol­low­ing year and con­trib­ute to the ris­ing in­fec­tion rate.

Turn­ing the tide

It has be­come clear that the rapid re­moval of in­fected elms is one of the best strate­gies for keep­ing the dis­ease un­der con­trol. It’s not fea­si­ble to re­move all 8,000 elms at once, but those trees which har­bour the most elm bark bee­tles would be top can­di­dates for a rapid re­moval pro­gram. The Univer­sity of Winnipeg is pre­par­ing for its sec­ond sum­mer of re­search to try and de­ter­mine just that – how to quickly iden­tify which trees should be re­moved first. In the mean­time, the city of Winnipeg is im­ple­ment­ing a three-year plan to get on top of the DED re­moval back­log, with the goal of get­ting the 2015, 2016 and about 60 per cent of the 2017 marked elms out by the end of 2018.

The bat­tle isn’t all about re­movals though – fungi­cide in­jec­tions have been used in Winnipeg for years, help­ing home­own­ers pre­serve high­value elms on their prop­er­ties. Tree plant­ing is an­other cru­cial part of the DED man­age­ment equa­tion. The city of Winnipeg plants about 2,000 trees per year, but it’s up to home­own­ers to plant trees on pri­vate prop­erty. We know that tree species di­ver­sity helps make our ur­ban canopy more re­sis­tant to DED and other pests. For­tu­nately, we have bet­ter tree se­lec­tion than ever be­fore. Hack­berry, buck­eye, but­ter­nut, alder, Amur cork tree (and more!) are show­ing up in pri­vate yards, break­ing up the elm and ash mono­cul­tures of decades past.

Are we still plant­ing elms? Of course! Our na­tive Amer­i­can elm is one of the hardi­est trees in our re­gion and grows ex­cep­tion­ally well in our ur­ban en­vi­ron­ment (gar­den­ers know they grow pretty much any­where – even eaves troughs). Not many trees can boast a life ex­pectancy of a hun­dred years. Elms can live for decades be­fore suc­cumb­ing to dis­ease. Many have been part of our neigh­bor­hoods for over a cen­tury and are still stand­ing de­spite the spread of DED.

It’s cru­cial that we don’t let down our guard and give up on our elms. To “win” the bat­tle with DED is not to com­pletely erad­i­cate it, but to man­age it know­ing that in­va­sive species are sim­ply part of our new re­al­ity, and ad­just­ing our ex­pec­ta­tions for our trees ac­cord­ingly.

The Univer­sity of Winnipeg dis­sects dis­eased elms to learn more about the bee­tles within.

The fa­mil­iar orange tag of a dis­eased elm.

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