Invasive Species Centre
treating trees within a few hundred feet of an affected tree. Treatment is repeated in two years. A biological control is also under study in the States, an endophytic bacterium isolated from surviving live oaks (Quercus fusiformis). About 20 per cent of samples showed laboratory inhibition of oak wilt. Results suggest hope for injection in stems as a potential control of oak wilt. But these are early studies, and there are currently no real-world applications for this treatment.
Sanitation, prevention and education are the keys to liv-
A stand decimated by oak wilt; insets show leaf discolouration patterns.
D.W. French
sometimes only after death in red oaks. The CFIA web site says, “In the red oak group, symptoms are characterized by a wilting and bronzing of the foliage, starting at the tree top and tips of branches, and spreading rapidly throughout the entire crown. Symptoms develop beginning in May and continue throughout the growing season. Individual leaves turn bronze progressively, from the tip to the base, sometimes leaving a small area of green tissue at the base around the midrib. Leaves in all stages of discolouration, including green leaves, are shed more or less continuously as the disease progresses. Some diffuse staining may be observed in the outermost sapwood, where the fungus has induced the tree to produce gums and tyloses which discolour the wood.” Suspect any oak that shows fall colouring in May or June. Expect some oak survival, especially in the white oak group, but remember Austin, Texas, lost 10,000 oaks in 1990. The University of Texas at Austin supports use of Alamo with the active ingredient propiconazole at 14.3 per cent, but other research suggests this only delays onset by a couple years, and does not stop transmission by root grafts. This is not legal in Canada, but Texas researchers believe they are able to slow the spread by
ing with oak wilt. All dying and dead trees must be removed immediately. All tree materials must be burned, chipped or covered in plastic for 60 days. Root grafts should be broken if possible. Because some oak roots can be deeper than 60 inches, the limit of most cost effective mechanisms, this will have varying levels of success. Soil fumigants are used successfully to kill connecting roots. The above-ground portion of trees is considered infectious for a year, and the below-ground portion for longer. Root trenching with a 60-in. plow is essential, as a thousand oaks can be interconnected. The size of a containment area is a function of soil type, but the primary line must be beyond the radius of all infected trees, including asymptomatic but infected trees. On average across the U.S., it is 15.2 to 18.3 meters ahead of active symptoms. The main concern with this treatment is to avoid destabilizing trees with too many root cuts. Most trees should be cut on only one side, with the trenching around a clump or a forest of trees, not an individual specimen. The biggest issue with oak wilt is to avoid all pruning during the flight season for any of the beetles known to transmit the disease. For southern Ontario, this can be from March right through the end of June, and some literature says into July. When there is storm damage, broken areas should be sprayed with a cheap water-based spray paint as fast as possible, ideally within 15 minutes. Professional pruning can follow at a much later time. Replanting with another oak can be done within two years, but don’t allow suckers to grow from the infected roots, as they will be diseased. What oak wilt will cost Canadians will depend on landscape worker speed at identifying and removing affected trees. It is not going to be easy to motivate clients to act quickly — but it must LT be done.
Pat Kerr is an Ontario-based freelance writer.
8 | JUNE 2018 | LANDSCAPE TRADES