Here’s why you shouldn’t prune oak trees this spring

It’s the time of year when Londoners are cleaning up the yard and tending to their trees, but there is one tree in particular that shouldn’t be touched: oak trees.Invasive oak wilt is being monitored in southwestern Ontario after first being found in the Niagara region in 2023 and also in Michigan. The fungus affects all oak species, but the red oak is most susceptible.”It’s this disease that oaks can get when they’re pruned in the warm months, and it has the ability to not only just kill the oak tree that it gets, but other oak trees that are in the surrounding area,” said Jason Karkidakis, an arborist and owner of Koala Tree in London.Jason Korkidakis is an arborist and owner of Koala Tree in London, Ont. (Travis Dolynny/CBC)LISTEN | London arborist Jason Karkidakis answers listeners questions on CBC London Morning:London Morning18:25Answering your tree questionsLondon Morning invited Koala Tree Arborist Jason Korkidakis to the show to answer your tree questions. Korkidakis covered replanting, ‘sleepy’ fruit trees and more. Oak wilt can be spread by sap beetles, which can infect the trees through open wounds from pruning and also through the tree’s roots.”Oaks will graft their roots, so that means they share resources over a long distance. So one oak tree is actually communicating and sharing resources with another one. So since it’s a vascular disease, it will go from one oak tree to another,” said Karkidakis.”The best thing people can do is don’t touch their oak trees until November through till March.”Symptoms of oak wilt include wilting leaves that discolour starting at the top of the tree. Leaves may drop prematurely toward the end of summer. A black stain can also form on the sapwood of the tree along with a fungal mat that may have a fruity scent.Oak wilt disease was discovered in Belle Isle, Michigan, in 2016. (Michigan Department of Natural Resources)Red oaks can die within weeks of becoming infected, while white oaks can live for years with the disease. There are some treatments available, but they must be implemented early.”Oak wilt has been around for about 15 years, so there are some injections and some different things that people are trying, but usually, you won’t know that the tree is infected until it’s past the point.”According to the City of London, oak trees make up about two per cent of the city’s canopy, totalling about 8,000 trees. Half of the oak population is red oak, while the other half is white oak.How to prevent the spread of oak wilt: Do not move firewood from one area or region to another and try to source firewood locally. Do not prune oaks from April to October as they are susceptible to contracting oak wilt from sap beetles. Paint any open wounds with a latex paint to protect the tree from sap beetles. If you find the disease on your tree, it must be reported to the Canadian Food Inspection Agency by email or the city at foresthealth@london.ca.Signs of oak wilt including wiltin leaves that discolour starting at the tips. (Submitted by Lauren Rogers)

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