Local science and nature
Red pile mite is pictured on a sugar maple leaf. (BOB BOWLES, Special to The Packet & Times)
THE SUSCEPTIBLE SUGAR MAPLE
A few weeks ago, I was contacted by friends who were friends of a couple who owned a large treed lot near the shores of Lake Simcoe in Oro-Medonte Township. They were concerned because their large sugar maple trees, which had been growing on their lot for some time, seemed to be dying and the leaves were turning brown in mid-summer. They wondered if I could visit the site and tell them what was killing their sugar maples.
They had brought me a couple of leaves with some insects on them and wondered if that could be the problem. I explained that, normally, trees produce chemicals to repel insects and fungi from attacking, but when they become stressed, they do not produce as much or any chemicals to protect them, so fungi and insects start to attack the tree. That usually brings in spiders and birds to feed on the insects, and woodpeckers often make holes in the bark to extract the insects under the bark. These are usually secondary symptoms of a tree under stress, but not the main cause of the tree dying. I told them I would have to visit the site to assess the health of the trees and why they were dying.
Sugar maples are wonderful native trees, but they don’t always do well as landscape trees in urban areas and backyards. Often, they are planted in disturbed soils and subjected to salt and air pollutants. Maple decline is a generic term that is used to describe a decline in a tree’s vigour. Leaf scorch is when the leaves turn brown at the edges and is usually caused when leaves lose water more rapidly than it can be replaced from the soil. The cause is too little water in the soil or a physical restriction in the roots. It can happen during drought conditions or if pavement or stonework covers the area around the tree and prevents rainwater from getting to the roots. Building a barrier wall in the soil will also restrict the roots from gathering water and nutrients. Soil erosion around the tree can also expose the roots and dries them out. Girdling roots will also cause leaves to turn brown or the loss of small limbs due to lack of moisture and nutrients and is caused when roots from other trees wrap around the roots or the base of the trees. Norway maple roots are particularly bad for root girdling and will wrap around and strangle nearby vegetation, including trees. Salt spray from passing automobiles on roads that have been treated with salt will sometimes dry out the tree and cause vegetation to turn brown in the summer. Nutrient imbalances and high pH levels in urban areas after construction also result in poor vigour from sugar maple in landscape conditions. Too much water in the soil can be just as bad for sugar maples as too little water. Sugar maples like to grow in dry, well-drained soil and will not tolerate too much water. Red maples are tolerant of moist soil conditions and silver maples can survive seasonal flooding, withstanding water for long periods, but the sugar maple is intolerant of great moisture. Any landscaping that changes the drainage can kill sugar maples. Soil compaction and paving around or near sugar maples usually result in the death of the trees. Adding soil or mulch to the base of the tree will cover the root crown or flare too deeply, and having the root crown and roots exposed is not good. The soil should just come even with the top of the root crown or flare. This is often changed as well as the grade in urban conditions.
So, when people tell me their landscaped sugar maples are dying, the first thing I look for is recent landscaping changes in grade, drainage, soil cover, soil compacting, paving or stonework restricting the tree roots.
Sugar maples can also die from verticillium wilt, which is caused by fungi (Verticillium albo-atrum or Verticillium dahliae) found in the soil. It kills all species of maples, but is common in Norway maples. The fungus gets into the xylem and blocks the movement of water and produces toxins that kill the tree. You can check for evidence of wilt in the branches by looking for a discoloured vascular system and dark-brown streaks in the sapwood. That is the second cause I check for in dying sugar maples in urban areas. Tar-spot fungus, Rhytisma acerinum, is another fungus that attacks the leaves of sugar maples, reducing the leaves’ photosynthetic ability, but is rarely serious to the life of the tree. Phyllosticta leaf spot, Phyllosticta minima, a fungus that overwinters on fallen leaves, causes circular brown spots with purple borders to appear on the leaves. That may reduce the tree’s vigour in cases of heavy infestations over several years. Hypoxylon canker, Hypoxylon mammatum, can sometimes attack the bark of sugar maples, but it is more of a problem with trembling aspen, which it often kills. Cytospora canker, Valsa sordida, is another fungus that will attack the bark of a sugar maple, but, again, it is mostly a problem in large-tooth and trembling aspen. Eutypella canker, Eutypella parasitica, is a fungus that attacks primarily the bark of sugar maples. It makes the tree susceptible to decay and wind breakage. The bulging deformation and black fruiting area is very evident on the trunk of infected trees. Nectria canker, Nectria galligena, is another fungus that will attack sugar maple, but it is more of a problem with aspens. Infected areas on the bark have a darker colour and a water-soaked appearance. Older cankers have target-shaped ridges with tiny, red, balloon-like fruiting bodies at the edge of the canker. Once you are sure the dying sugar maples are free of cankers, the next thing to check is insect infestations.
Insects that attack sugar maples are divided into different groups, including foliage (caterpillars, gall makes, leaf miners, leaf rollers, bark borers and root borers). Mites, birds and mammals can also attack sugar maples. Deer, mice, rabbits, squirrels and yellow-bellied sapsuckers often eat the bark and/or sap of sugar maple trees. Large caterpillars eating sugar maple leaves include the green-striped mapleworm, saddle prominent, cecropia moth, American dagger moth, forest tent caterpillar, gypsy moth and orange-humped mapleworm. Loopers include fall cankerworm, Bruce spanworm, linden looper and lesser maple spanworm. Leaf rollers and tiers include maple leaf roller, maple basswood leaf roller, lesser maple leaf roller, boxelder leaf roller, maple leaf tier, maple leaf cutter, maple trumpet skeletonizer, maple webworm, maple leaf blotch miner and maple petiole borer. Midges include ocellate gall midge and gouty vein midge. Mites include yellow spider mite, maple spindle gall mite and red pile mite. Carpenter ants, sugar maple borer, horntails and ambrosia beetles attack the bark. Scales like gloomy scale and maple mealybug also attack sugar maples. There are several insects, mites and other creatures that can attack and kill sugar maple trees.
Next week, I will tell you what I found when my friends and I visited the location where all the sugar males were dying in a backyard habitat in Oro-Medonte.
Bob Bowles is a naturalist and an ecologist who is interested in a healthy, natural environment in and around Orillia. He can be contacted at email@example.com.