Tree Diseases

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Tree Diseases

New tree diseases seem to be springing up every year. The rise of globalization and international travel has no doubt helped. It is said that invasive species is one of the biggest threats to biodiversity, though humans do their part to threaten it as well with hunting, logging and agricultural practices. World commerce enables invasive species migration as many insects love to hitch a ride in wooden packaging crates. The newcomers make themselves known at ports of entry, in shipments of nursery stock or produce, or pretty much anything else. The growing occurrence of natural disasters and forest fires contributes to the problem by wiping out entire regions.

Invasive species is an animal, insect, plant, or disease that has migrated from one area to another, invading native habitats and killing the species there. Because invasive species are not indigenous to the region they are invading, they can cause great economic and environmental harm. (Though it should be noted that not all non-native species are invasive). Trees provide habitat for some wildlife and can be killed when a disease or insect blocks or consumes tissues used for water and nutrient transport. Once an invasive species is established, they are incredibly difficult to eliminate as they reproduce quickly, spread widely, and tolerate all kinds of challenging conditions. Natural predators can help keep the balance of the habitat, but they are not always present, so learning tree insect management is key.

Tree protection zone
Tree protection zone

A qualified arborist can identify a tree’s vulnerabilities and any infestation or disease it is victim to. Site conditions always influence how resistant a tree can be to disease and other stressors, so not all trees will be impacted the same way. That said, the most common condition that fosters disease spread is excess moisture or damp weather. Therefore, anything that contributes to this should be looked at. Inadequate sunlight, poor on-site drainage or restricted air flow all play a factor, so an arborist will most likely make recommendations on improving site conditions.

Our arborists take a holistic approach to tree disease treatment. We look at both the tree and the surrounding environment for cause, extent of infection and any mitigating factors.  Timely action and proactive care are critical as many manageable infections can become unmanageable if they get serious enough, leading to unnecessary tree death. For those willing to act preventatively, we recommend pre-emptive foliar treatments targeted to the specific pathogens a tree is vulnerable to. If a tree infection is at its early stage, fungal or anti-bacterial applications, in many cases, can take care of it completely.

Our arborist does a visual inspection of the foliage, trunk and stems to diagnose a tree disease and recommend preventative treatments. Fertilization or soil treatments may also be suggested. We will design a customized tree disease treatment plan that prioritizes the least toxic methods where possible. Treatments can include trunk injections, twig and limb care, soil applications or foliar sprays. The products used, or how often they must be used and for how long, varies. Some insects help suppress pest populations and arborists can recognize those, too, as well as assist with tree insect management.


If a disease has progressed too far there may be no other option but to take a tree down. Chemicals can do it but must be handled with great care as they can do damage to the surrounding environment. Herbicides can be very effective, but there are numerous different types and their method of killing a tree varies. Keep in mind, however, that it is illegal to use herbicides for any purpose not stated on the label, so read labels very carefully.

Herbicides can be very effective on trees that have been cut down and continue to grow back – otherwise known as zombie trees. For standing trees, there are options. Selective herbicides are best as they target specific areas while leaving others unharmed. Some are applied to the soil, some to the foliage. If you want to attempt using them, we strongly advise not using them on a windy day, or near bodies of water. Always wear a protective mask and gloves and cover your limbs. Ensure children and pets are nowhere near you and use only as much as you need.

Natural Pesticides

Believe it or not, some trees respond to natural pesticides. Here are a few we’ve discovered:

  • Tomato leaf spray: Tomatine has insecticidal properties and is found in tomato plants. It is an effective alternative to chemicals. Mix two cups of chopped tomato leaves with five cups of water, steep over night and strain. The liquid can be used as a spray.
  • Soap! A soap and water spray may work. Mix a couple of teaspoons of mild liquid soap with five cups of water and spray.
  • Neem oil: highly effective. It interrupts the feeding habits of insects and causes hormonal disruption in them, interfering with their ability to grow and reproduce. This also acts as a fungicide. Mix two teaspoons of Neem oil with one teaspoon of mild liquid soap and five cups of water. Shake and spray.
  • Diatomaceous earth: this is made from fossilized algae called “diatoms.” It is a natural, but lethal to insects. It functions by absorbing the waxy substance from their exoskeleton, which basically draws all fluid from their body and kills them. You can find it in a garden store. Sprinkle in on soil or leaves and re-apply after rain as it needs to be dry to be effective.

Common Diseases

Leaf Spot Diseases

Leaf Spot Diseases generally fall under the umbrella term Anthracnose, which is applied to an array of fungal or bacterial diseases that affect a variety of tree species. Because it primarily affects foliage, it is common in trees that shed leaves annually such as Sycamore, Ash, Maple, Walnut, Dogwood and Oak – especially White Oak. However, not all tree species are affected the same way. For some it causes a cosmetic problem, which generally impacts only a portion of a tree’s foliage and does not threaten its overall health. For other species it can be deadly. The pathogen can spread throughout the leaf canopy, leading to the infection of twigs and branches and either partial or complete leaf loss for several seasons. If this happens stunted growth can result. Where severe disease persists for years, a weakened tree is rendered more vulnerable to other diseases and can be invaded by insect borers.

Because Anthracnose is a fungal infection, it generally comes on in the spring when wet weather prevails. It is evident in leaves and blossoms, but it can also disrupt or even destroy a tree’s vascular system. Once the vascular system is infected it will be visible in the bark, with dark, sunken lesions or indents and possibly lead to further problems. Limb dieback, shoot blight or cankers can develop.

Tree protection zone

Emerald Ash Borer

The Emerald Ash Borer (EAB) is a metallic green or emerald beetle, about a half an inch long, and it affects ash trees or Green Ash primarily. It is an invasive beetle native to Asia, believed to have been introduced to North America in a wooden shipping crate in Detroit in the 90s, rapidly spreading ever since. Adult beetles feed on the edges of the foliage, but the larvae are the real problem. There can be hundreds of thousands in a single tree, and they feed between the bark and the sapwood, degrading the tree by cutting off the flow of nutrients. It is the tree’s transportation system that conducts this task, while also drawing water from its roots, and it can be destroyed. And this, in turn, leads to ash tree mortality by girdling.

A tree infested with EAB can display a variety of symptoms such as crown dieback, bark deformities (vertical cracks and shoots growing out of the lower trunk), woodpecker feeding holes, yellow foliage or D-shaped exit holes. There is a diminished density of leaves and the bark over a gallery turns pinkish brown and dies. To make matters worse, birds and other animals feed on the larvae and damage the bark even further. Unfortunately, all signs and symptoms only appear when the population of this beetle in the tree has been well established.

Black Knot

A widespread fungal disease, Black Knot can mostly be found in Plum and Cherry trees, both fruiting and ornamental. The fungus is easy to spot with hard, uneven black galls that get intertwined with twigs and thin branches, though not in the early stages. The disease takes a while to develop and often isn’t visible until after a full season. If the fungus can be identified before the disease is well established – in the beginning a swelling of twigs can be seen – then measures can be taken to prevent its spread. If it is left to grow it will girdle branches, strangle any new growth and slowly kill a tree.

The disease cycle begins anew whenever spores are released from established knots during damp spring conditions. The spores travel to other parts of the tree and can even be carried to surrounding healthy trees by a breeze. Generally, spores germinate on stems beneath a thin film of moisture and are hidden by the leaves they eventually kill. Knots or galls develop, bark splits on the galls and the galls can appear green. By the following spring galls are hard and black. Humid weather encourages the disease to spread even quicker and at some point, dark black fungus will coil along stems and branches, easy to spot. If the disease reaches this stage, chemical treatments or pruning may not even be enough to save the tree, even if the pruner takes extraordinary care in not spreading the spores or leaving them behind.

Black Knot can be controlled by pruning out infected twigs and branches in the fall and winter. Cutting at least four inches below the knot is most efficient and tools should be sanitized with bleach afterward to reduce the spread. An infected tree is under immense stress so keeping it supplied with good fertilizers can really make a difference as well.

Diplodia Tip Blight

Diplodia Tip Blight is a fungal tree disease that affects primarily pines – Austrian Pines, Scotch Pines, Ponderosa Pines and Mugo Pines, and is particularly harmful to pines 25 years or older. Those planted on land with poor soil conditions are even more vulnerable and can suffer root loss and a lack of nutrition. Other pines can be susceptible to a lesser degree, along with cedars and firs. It targets new growth, turning it yellow, and leads to a loss of lower branches as shoots become infected. When the pathogen becomes visible a stunting and browning of the current year’s growth is evident with dieback at the branch tips and needles failing to elongate. Fruiting bodies grow at the base of needles and branches become girdled. If it progresses over consecutive years it will stunt the tree’s entire growth, and gradually move up toward the crown.

The good news is that proper care and maintenance can help prevent infection. Trees under stress are far more vulnerable to this disease so keeping them mulched and watered during dry weather can really help. Once the disease is established, treatments include removing or destroying infected cones and tips, as well as of dead or dying limbs. All tools used for pruning must be disinfected between cuts with rubbing alcohol or a 10% bleach solution. Application of technical materials or fungicides must be timed for when the buds first break, new shoots are half emerged and when shoots are fully developed to be effective.

Dutch Elm Disease

Dutch Elm Disease (DED) is a particularly aggressive fungal tree infection that attacks primarily American and Siberian Elms, as well as trees stressed by drought. American Elm is extremely susceptible though exceptions have been found in cultivars. Asian Elms have been used in breeding programs to develop hybrids as they show significant resistance to it.

DED is considered one of the most historically devastating diseases on this continent, as well as in Europe, as it has led to the destruction of millions of large, mature trees. Its introduction in the early part of the 20th century dramatically altered the character of urban forests and elm populations worldwide. The fungus colonizes a plant’s vascular system, inhibiting the transportation of water. It travels within the sapwood of the tree and often begins with yellowing and dieback on a single branch. As the infection spreads, foliage can wilt or turn yellow and dieback becomes evident on the crown. Root grafts or bark beetles carrying spores can cause the infection to spread to surrounding healthy trees. The Native Elm Bark Beetle and the European Elm Bark Beetle are both vectors of the fungus. They deposit spores as they bore into cambium and sapwood, laying eggs in galleries they created under the bark. Verticillium also targets Elm trees with brown streaks in the sapwood.

Prevention and treatment are critical. The infection can spread through the root system of closely planted trees, beetles can walk for miles in search of a host and the pathogen can remain alive in streaked wood for prolonged periods of time. It is a lethal disease that must be taken seriously. Systemic fungicide injections by a qualified arborist may be warranted, along with gathering specimens through careful sanitation pruning. These actions can help if branches are removed at the first sign of wilt and root grafts between diseased and healthy elms are cut. If a tree dies completely, it must be removed immediately.

Apple Scab

Apple Scab is a common fungus that attacks apple trees, though crab apples and apples vary widely in their response to it. A related strain of this fungus can affect Hawthorn, Pear and Mountain Ash trees. It targets leaves and fruit, causing brown or black leaf spots, lesions, or even purplish blotches, feeling rough to the touch. These blotches can vary in size and will grow. Its effect on fruit is essentially to deform or cause cracks in it. Both fruit and leaves can drop prematurely and the tree’s vigor is reduced, forcing it to draw on stored energy reserves to produce a second leaf flush. If the infection is severe a tree can become completely defoliated.

The pathogen produces two types of spores and is far more severe in wet seasons. Dry weather, by contrast, and disease resistant cultivars, help prevent its spread. Pruning helps to improve air circulation through the canopy. Raking leaves before the winter can help minimize apple scab in future and fungicides can be employed to prevent Apple Scab on healthy trees – though timing is critical. They must be applied at first leaf bud flush and repeated every 7 to 10 days May through June. Regular fertilization is also important.

Asian Longhorned Beetle

The Asian Longhorned Beetle (ALB) is native to China and the Korean peninsula. It was first discovered in North America at several ports in the early 1990s and in the Ontario area since 2003. While quick action led to its eradication, it re-emerged in 2013. It is considered lethal, able to cause more damage than the Dutch Elm, Chestnut Blight and Gypsy Moths combined.

In Canada, the ALB attacks mostly Maple but other tree species as well such as Poplar, Birch, Sycamore, Willow, Horse Chestnut, Blackberry, Mountain Ash, Cherry, and Elm. As Maple trees are important to our culture and economy, the loss of this species could be devastating. It could affect Canada’s multi-million-dollar maple syrup industry if eradication measures are not effective, and the loss of hardwoods could affect our forest industry.

The adults feed on the leaves and bark; this alone causes significant damage. Young larvae feed on green inner bark and cause scoring of the cambial layer. Mature larvae bore into the xylem of the tree, creating large, winding galleries in the inner wood. Signs of infestation include a leaking sap, large exit holes, yellowing leaves, premature leaf drop, branch dieback and oval-shaped egg pits dug on the trunk, branches or exposed roots. Accumulation of coarse sawdust on the branches or the base of the tree can indicate larval feeding.

Treatment options are limited. Several insecticides have been tested. Imidacloprid has demonstrated the most effectiveness. It can have some effect on adult beetles as they feed on small twigs and against young larvae as they feed beneath the bark.

Oak Wilt

Oak Wilt is a fungus that grows on the outer sapwood of Oak trees. It is lethal to Red Oak species but less so to the White Oak family. It is a vascular disease that obstructs the nutrient and water conducting system, causing branches to wilt. It can infect through the roots of a tree that are grafted between infected and healthy trees. It can also be spread by sap or bark feeding beetles, who carry spores.

Evidence of infection begins at the top of a tree where leaves wilt and turn brown. Bark cracks in the trunk and on large branches. Fungal mats under the bark can emit a fruity smell. In Red Oak, it will spread rapidly through the crown causing significant dieback. Most trees infected will die within a year. In White Oak, dieback is more localized in one or more branches, and a tree will take years to die, if at all. Leaves often remain on dead branches but reflect fall color changes, or they fall prematurely.

Early detection matters. It is important to avoid pruning Oak trees during growing season as beetles are very active then and are attracted to freshly cut wood and tree wounds. If you have to do it, use a tree wound sealant that might help deter them. In fact, if wounds occur during a storm or by some other means, a wound dressing should be applied as soon as possible. If you have an infected tree, roots connected to healthy Oaks should be severed by a qualified arborist and specialized equipment.

Gypsy Moth

The Gypsy Moth is native to Europe and was established in North America in the 1980s. The larvae or caterpillars feed on the crown foliage of a wide range of hardwood and sometimes softwood trees, and its effect is primarily defoliation. It impacts mostly Oak, Maple, Alder, Aspen, Willow, Poplar, Apple, Hawthorn, Elm and Linden trees. During an outbreak the density of caterpillars can be so high that a tree can be completely stripped of its leaves by mid-summer. Defoliation often leads to the dieback of twigs and branches, but it also renders a tree more vulnerable to other diseases and pests as a tree has to draw on all its energy reserves to grow back its leaves.

Fortunately, a Gypsy Moth infection is rarely lethal, and outbreaks generally decline after two to four years because natural enemies like white-footed mice, certain birds or predatory insects and mites will attack their eggs, larvae, and cocoons. They can also decline because of the two diseases that affect Gypsy Moth populations themselves. One is the NPV virus and the other is a fungus. NPV causes high levels of mortality during an outbreak. Caterpillars killed by NPV typically hang limply from a trunk or a branch in an upside-down V-shape. The cadavers then liquefy and disintegrate. If you try to handle one, you will be greeted by a foul odor. The fungus, on the other hand, is native to Japan and eats insects. It was introduced in Michigan over a hundred years ago as a biological control for Gypsy Moth.

Today, a number of measures can be taken to try to control Gypsy Moth. It is important to check outdoor furniture, trailers, camping equipment and firewood on a regular basis during the spring and summer months for egg masses, larvae, or caterpillars. If you find any, wear gloves when handling as their hair can cause skin irritation. If you find eggs, scrape the fuzzy, tan-colored masses off the surface they are affixed to and destroy them immediately. You can do this by crushing their eggs or submerging them in a bucket of water, bleach and soap for two days. Following that period, discard the solution and egg mixture. A few other tips:

  • Use only approved insecticides to manage young larvae.
  • Pheromone traps can be used to time both the emergence & flight of adult males.
  • A folded burlap cloth wrapped around a tree provides the large larvae shelter. You can use this to collect them from those shelters and destroy them.
  • When inspecting firewood, be very careful & never move your firewood.

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Cedar Apple Rust

Cedar Apple Rust is a fungus that infects Cedar, Juniper and Apple trees and is common in Ontario, especially in the Ottawa Valley, or to the east of Lake Ontario. It is unique in that it requires two host threes to complete its life cycle. It must alternate between Apple, Cedar and Juniper trees, which is why it must be handled by treating all three tree species in the area at once, even if they show no symptoms.

The fungus is also unique in how it presents, though it varies depending on the tree. In a CBC piece published recently, a reporter described it as “clumps of gummy bears” growing out of the trunk and foliage. They were a bright orange, with bumps “almost like a walnut” with “orange tentacles”.

Other symptoms are more common. They begin appearing on the upper surface of apple leaves shortly after bloom. Small, pale-yellow spots will show on the upper surface and fruit and eventually, small black spots appear in the center of lesions. Those lesions grow large and more orange-colored, often with a red margin, as more black or brown spots appear. Sometimes orange spots appear on fruit or leaves. If the tree continues to be under stress, it will lose leaves and fruit prematurely.

This fungus spreads when spores are blown by the wind and rapidly multiply during wet periods. As the season progresses, those same spores start spreading from tree to tree, beginning new infections. Sometimes those trees are as far as a mile away.

Luckily, Cedar Apple Rust is not deadly and the harm it brings is mostly cosmetic. That said, because it can spread so easily it is a good idea to apply fungicide spray during the growing season, as well as surrounding Cedar and Juniper trees. This can be done weekly. Some have said that you can also use a copper solution on Junipers (about two ounces to a gallon of water) at least four times between August and late October. It will also make a difference if you rake up fallen leaves and other debris under trees and remove galls from infected Junipers.

Fire Blight

Fire Blight is a bacterial infection caused by bacteria and it affects many trees including Apple, Pear, Hawthorn, Crab Apple, Mountain Ash Cotoneasters, Spirea, Pyracantha and Firethorn. It is a highly destructive disease and difficult to control. Weather plays a significant role in terms of how it presents or how severe the infection becomes, more so than location. Like a fungal infection, it proliferates in warm moist conditions.

The first symptoms you see are small oozing patches of dead bark called cankers that appear on branches, twigs or the trunk. Twig shoots become crooked while leaves and fruit still attached to branches wilt, shrivel and darken, giving a tree a haunting, fire-scorched appearance. Milky-white to amber droplets of bacterial ooze may exude from areas of infection. Insects, wind and rain then transport the bacteria to open blossoms. The bacteria can also enter through tree wounds from insect infestations, pruning or other injuries. Cambial tissue is killed in the region of the canker and will be brown or black instead of green. Blossoms make be discoloured or water-soaked and fruit may develop small, dark and shrivelled lesions. Trees are extremely vulnerable to this disease right after rain or heavy storms, when leaf tissue may have subtle physical damage. While symptoms appear during wet weather, large cankered limbs may not die back until dry weather in the middle of summer. As the disease advances it begins to block a tree’s vascular system and eventually kill it if measures are not taken.

To control or prevent Fire Blight, spraying the tree with a liquid copper fungicide can reduce or eliminate bacterial spores. Be careful with pruning as too much of it will encourage more succulent growth, which the infection loves. Prune only during dormancy (in dry weather) and prune beyond the last visible signs of disease. Disinfect pruning tools between cuts with 10% household bleach after each branch is pruned as you can actually spread the disease from branch to branch if you do not. Spray bacterial products at early bloom, full bloom and petal fall during warm, humid weather.

Given the grave threat that Fire Blight presents, an integrated, organized approach is imperative. Pruning must be well timed, as well as fertilization based on soil analysis using only slow-release nitrogen products. (Too much nitrogen poses the same problem as too much pruning). Make sure you follow directions of foliar bactericides carefully. A soil-injected treatment will reduce shoot blight, but it also must be properly timed, between when the buds first appear and when petals fall.

Dothistroma Needle Blight

Dothistroma Needle Blight is a fungus that affects pine trees: Austrian Pine, Bishop’s Pine, Jeffrey Pine, Maritime Pine, Monterey Pine, Ponderosa Pine, Shore Pine, Shortleaf Pine and Western White Pine. Red Pine and Scots Pine are mostly resistant, so can be planted around other pine trees to mitigate any spread.

As with other types of fungus, spores are spread during wet weather during growing season by wind or rain. New infections begin in the summer to fall, and the disease has a number of symptoms. Reddish brown spots or bands appear on needles in the fall. Needle spots will eventually girdle the needle and what is beyond the band dies and turns brown. The base of the needles remains green. Tiny black fungal fruiting bodies show up in the dead areas of needles and release spores the following year. Older needles closer to the trunk are more vulnerable than young ones at the top. Infected needs sometimes remain attached for up to two years depending on their age when they were first infected.

Lab analysis is often necessary to distinguish Dothistroma Needle Blight from Brown Spot. But there is a great deal you can do to help control the spread. Always maintain good air circulation through and around trees Bottom branches can be removed to help air move through the canopy. Remove weeds under trees and spread a three to four inch deep layer of wood chip mulch to prevent further weed growth, leaving about a two inch space between the mulch and the trunk to allow for air movement. If your tree is on land where a sprinkler irrigation system is close by, you must make sure the water does not spray the needles. For trees with a history of this disease you can spread copper fungicides to protect new needles. Severe infection several years in a row will cause death.

Spruce Budworm

The (Eastern) Spruce Budworm is a moth native to North America that feeds mostly on Balsam Fir and White Spruce, and to a lesser extent on Red or Black Spruce. Where forests contain these trees, it is the most destructive on the continent. In an outbreak, tens of millions of trees can be completely defoliated over several years. An outbreak in Quebec in 2006 along the St. Lawrence River led to over 3,000 hectares being defoliated and by 2019, over 9.6 million hectares of forest had suffered.

Damage first appears in May. You will see destruction of buds, abnormal spreading of new twigs, defoliation of current-year shoots and, if an affected branch is disturbed, the presence of large numbers of larvae suspended from strands of silk. Defoliation begins at the top of a tree and typically becomes visible in late June.

The government has launched a research program called the Spruce Budworm Early Intervention Strategy to investigate a new pest management approach that would prevent Spruce Budworm outbreaks. The idea is to target hot spots or key areas when the population density is still low.

Forest Tent Caterpillar

The Forest Tent Caterpillar is native to North America and is the most widespread defoliator of hardwood trees on the continent. In northern Ontario, they target Trembling Aspen and other Poplars, as well as White Birch. In southern Ontario, the preferred hosts like Sugar Maple and Oak, but they can also be found eating the leaves of other hardwoods like Red Maple. Alberta, Ontario and Quebec have all seen large outbreaks.

The Forest Tent Caterpillar can cause serious damage through the widespread eating of leaves and shoots. It has numerous natural predators that include ants, birds and yellow jacket wasps. If an infestation is severe and sustained, it can stunt the growth of a tree and kill its branches. If a tree is weakened by other stresses such as drought or insect damage, or if defoliation occurs repeatedly, mortality can occur.

Full-grown caterpillars are 50 millimetres long, hairy and brown, with a blue stripe along each side and a row of keyhole-shaped white spots along the back. After feeding for six weeks, they spin yellowish cocoons in a sheltered place and then pupate inside. Outbreaks are an expected part of boreal forest ecology, considered natural like a forest fire, but they can be incredibly destructive. An outbreak in 2015 led to the loss of 4.8 million hectares of Canadian forest. These outbreaks only happen every 10 to 12 years and last anywhere from three to six years, depending on the location.

Bacterial sprays can help reduce the spread or protect a tree during outbreaks. Parasites such as the large flesh fly can also help. You really want a qualified arborist to inspect the site and make recommendations.

Beech Bark Disease (BBD)

Beech Bark Disease is caused by a combination of a beech scale insect (introduced from Europe) and a fungus. It arrived in Canada in the late 19th century and gradually spreading ever since, primarily through the Maritimes, Quebec and Ontario. It has done the most damage in eastern and southern Ontario, where it has existed the longest. It is most often found in mixed-hardwood forests with Sugar Maple and Hemlock present.

Because this disease is a combination of insect and fungus, the infestation is coordinated. First the insect burrows into the tree, feeding on the bark and creating cracks or cankers on the tree. Then the fungus enters those cracks, slowly rotting a tree and eventually, causing it to die. Approximately 50-85% of infected beech trees will die within ten years of infestation.

Symptoms to look for along the way include wilting foliage, yellowing or smaller-than-usual leaves, thin crowns, cankers, and small orange-red fruiting bodies. A tree will generally lack vigor and branches will be covered with waxy woolly secretions of the beech scale insect. It typically goes after large trees first, but it can also target forests with nutrient imbalances. Beech trees are an incredibly important part of Ontario’s ecosystem as their nuts offer nutrient and fat content for bears and deer, and their structures alone offer living accommodations for all kinds of animals.​

The disease is spread by wind and animals, and by fungal spores spread by rain and splash. There is no easy solution to preventing or halting this disease. If you have one on your property, you could try a power washer to remove any small insects. Don’t move firewood from the area they were cut as bugs hitch a free ride on them. If you have multiple beech trees in an area, retain any that show no signs of infection. Some beech trees have been shown to be resistant.

Brown Spruce Longhorn Beetle (BSLB)

The Brown Spruce Longhorn Beetle is native to central Europe, Russia and Japan. It attacks mostly White, Black, Red and Norway Spruce trees in eastern Canada. It also goes after stressed or dying spruce trees of all kinds, especially mature ones. It has the potential to threaten spruce forests across Canada experiencing drought or other types of damage, which is why the Canadian Food Inspection Agency has restricted its movement with regulations.

The adults are about 10-15mm long with a dark head and thorax and a brown abdomen. You will see L-shaped pupal chambers in the sapwood called galleries, which become visible when the wood is cut longitudinally. The galleries are usually packed with insect excrement and chewed wood fibres. White resin may be detected dripping along the hole of an infested tree. The crown may be thinning. Larvae feed on the inner bark, especially the lower section of the trunk and oval exit holes with tunnels can be found. Leaves or needles turn yellow, then brown.

A severely infested tree will die after a few years. If you suspect BSLB, best to contact a qualified arborist. There are some native predators and parasites that are enemies of this beetle, and you want to be very careful about moving firewood.

Birch Leafminer

Birch Leafminers are a type of sawfly native to Europe that were first introduced to Canadian forests between 1920 and 1960. They attack all species of Birch trees and have been found in pretty much all provinces except Nunavut. While outbreaks occur primarily in urban areas, these pests cause aesthetic damage and do not usually kill a tree unless other factors are present.

The damage to a tree appears in the spring or late summer, depending on the species – Birch Leafminer is only one type of Leafminer. Some merely cause the leaf’s edge to curl while other render foliage brown. Most species go after fully formed leaves, but one species attacks new shoots.  Adult Leafminers can be moths, beetles, or flies, and the tunnelling patterns of their larvae vary depending on the nature of the insect. For example, an Aspen Serpentine Leafminer creates winding or meandering tunnels whereas a Birch Leafminers create large blotches.

The clearest evidence of infestation is severe, translucent brown blotches on leaves and distortion of foliage. That browning may increase as damaged leaves dry out, or premature defoliation may occur. In certain cases, leaves may become thin, papery and translucent. When examined closely, black pellets can be discerned in tunnels, which are faecal deposits. If damage to the tree persists or it suffers from other sources of stress, it will be weakened and therefore more vulnerable to more serious problems like the Bronze Birch Borer.

If you suspect Birch Leafminers, contact an arborist. There are treatments available, but they should be handled by a professional. There are other natural enemies as well, such as parasitic wasps, that might help.

Bronze Birch Borer

The Bronze Birch Borer is a pest native to North America and of increasing importance in Ontario. It targets old and Birch trees under stress, boring underneath the bark and feeding off the sap conducting tissue. It is not to be taken lightly as it has killed thousands of trees in Canada.

Damage typically first appears near the upper part of the crown. The branches dry out just above the area affected and die a year later. Foliage will become discoloured, leaves drop prematurely, branch dieback appears on the crown as do winding galleries between the bark and the wood. Those galleries are often packed with digested sawdust-like borings, raised welts on the bark of the branches and the trunk, sap flows on the trunk near the larval tunnel entry holes and D-shaped holes on the trunk where adults emerge. As the years progress the damages moves down to the trunk until the tree dies.

If you suspect Bronze Birch Borer, we urge you to contact a qualified arborist immediately.

Mountain Pine Beetle (MPB)

The Mountain Pine Beetle is native to western North American forests and has reduced the growth of millions of trees while causing widespread mortality to commercial tree species. The most recent infestation led to the estimated mortality of hundreds of millions of trees, covering an area roughly five times the size of Vancouver Island. While the most extensive damage has been limited to the west coast, scientists predict it is only a matter of time before it reaches Ontario.

The MPB is a wood-boring insect and a small beetle that goes after Sugar pine, Western White Pine, Ponderosa Pine, Lodgepole Pine, Jack Pine, Limber Pine and Whitebark Pine. It does not attack Jeffrey Pine. When the beetle population is low, healthy trees can defend themselves against an attack by producing a toxic resin. But as more beetles come to that tree its defences are eventually overwhelmed. Unfortunately, when an epidemic occurs, the population of this beetle only declines when all the large pine trees in the area are weakened or dead.

Evidence of MPB includes red needles at the crown and sawdust collections at the base of a tree from larvae feeding. Larvae themselves are legless grubs with red-brown head and can be seen under the bark. The beetles transport a fungus to the tree as well that stains the sapwood blue.

Every government body on a provincial level has been running their own detection and control program that includes monitoring and burning of infested trees. The MPB has many natural predators that include woodpeckers, believe it or not, but their impact is not enough to prevent or control outbreaks.

Phytophthora Root Rot (PRR)

Phytophthora Root Rot is a soil-borne fungal disease that affects just about any species of tree, but especially Oak, Dogwood and many ornamental trees. There are numerous types of root rot that go by various names, including Cotton Root Rot, Mushroom Root Rot, Oak Root Rot and Armillaria Root Rot, but in Canada, Phytophthora Root Rot is the most common. Symptoms in general are very difficult to detect, in part because they are similar to insect infestation, but we will attempt to describe them here.

PRR goes after trees growing on poorly drained sites and proliferates in damp conditions. Over time it weakens a tree from oxygen-starved and decaying roots. If you suspect PRR, you want to inspect the base of the tree and excavate soil from around the trunk and root system, to examine tissues for crown and root rot symptoms. As you peel back the outer bark, look for the normally green cambium to appear orange or brown. Roots may be dark brown or orange or may slough off from the primary root. Wilting leaves can often be seen as they turn dull green, purple or yellow. The canopy might thin out or signs of slower than usual growth will appear. Major branches may perish. Dieback – the signs of stress in a tree – may become evident, and lead to a growing loss of vitality.

In fruit trees, Root Rot will likely cause a lack of fruit production on the lower branches. Roots will show signs of decay while the lower trunk and cambium layer of a tree will often have brown spots. As the disease spreads it is also possible white mushrooms or white fan-like plaques or strands will develop around the base of the tree trunk.

Some trees with Root Rot can survive for years, while others die quickly. PRR can survive in the soil for years where damp conditions prevail.  It spreads through irrigation water, runoff water or splashing rain so prevention measures are critical. There are several actions you can take to ensure your trees never suffer the fate of Root Rot. They include:

  • Always plan for proper water drainage when planting. Consider changing the soil composition to encourage draining. Don’t allow water to pool around the root.
  • Plant on mounds of soil & never go deeper than they were planted at the nursery.
  • When a tree dies, remove it and its root system immediately to avoid spread.

Cherry Leaf Spot Disease (CLS)

Cherry Leaf Spot (CLS) is a fungal disease known in colloquial terms as “yellow spot” or “shot hole” disease and it is considered serious here in Canada. Aggressive measures must be taken to prevent it or it can consume an entire orchard. Typically, you find leaf spot in the tops of trees, and the fungus releases spores that help the infection spread further through the canopy. Microscopic spores called Ascospores may be discharged during or after rainfall. As the disease progresses, small, round and red lesions begin to merge and turn brown, their centers sometimes falling out. This is what gives the leaf its “shot hole” appearance, though it is more common on sour trees than sweet ones. Severely infected trees may become defoliated by the middle of summer. Spores can be found on the underside of leaf lesions that look like a tiny mass at the center of that lesion; a white to pinkish color.

If CLS spreads unchecked, the tree will become more susceptible to water damage, which in turn will lead to a loss of fruit spurs, small fruit buds, shrinking fruit or fruit that ripens unevenly, and eventually, the death of the tree.  If a tree becomes infected early in spring the fruit will not even mature and appear pale, too soft to the touch and low in sugar.

Smart management is urgent. All fallen leaves should be destroyed. When all the leaves have been raked up and discarded, add a layer of straw mulch to the ground. Fungicides must also be applied from petal fall to the middle of summer. Begin doing so two weeks after blossoming when the leaves are completely open and repeat according to the manufacturer’s instructions throughout the entire growing season, including post-harvest. Look for fungicides with the active ingredient of myclobutanil or captan. And be careful about applying it too frequently as resistance can develop. The trick to avoid this is to alternate between myclobutanil and captan. We have also found that fungicides with the active ingredient copper may demonstrate some effectiveness against CLS.

Peach Scab Disease

Peach Scab Disease (PSD) is a fungus that affects all stone fruits but is most severe on peaches as they are more prone to disease and insect manifestations. It is also known as Black Spot or Freckles as this is how it appears on fruit and the scab is usually superficial; that said, the appearance is hideous. No apricot, peach, plum or nectarine varieties are resistant.

The first sign of infection is often small, round, green olive spots on the part of the fruit facing the sun or on the stem side. Gradually these lesions merge to form large brown blotches on apricots, or black ones on peaches, nectarines, and plums. Being on the alert for these early signs is important as it can take over an entire crop. Fruit with severe infections may crack open, stunted or misshapen. Fungi invade further through cracks and cause rot, among other things. Leaves will develop yellowish-green spots on the underside and eventually dry up and drop prematurely. The fungus can also infect the twigs. Look for slightly raised blotches, about ¼ inch in diameter. The twigs die back in rare cases.

PSD can persist through the seasons if it is not managed. Large numbers of spores are produced from lesions in the winter and can survive until moist conditions take over and wind and rain help it to spread. The spores in the lesions on fruit do their part as well, re-infecting twigs and leaves. Twigs, being the larger source of infection, really should be removed and destroyed in early spring, along with any unwanted fruit near your tree. This is critical. Pruning annually will ensure air circulation and pruners should inspect carefully for any lesions on branches or fungus on fruit. If more than 20 pieces show troubling signs, disease management must come in to play.

Fungicides approved for fruit trees can also be considered. They should be applied every ten days from the time petals fall to about 40 days before harvest. Spraying is important to reduce the possibility of infections later in the season. It is important to remember that brown spots on peaches may be unsightly, but they generally do not affect the quality of the fruit unless the infestation is severe.

Powdery Mildew

Powdery Mildew is a common disease that can affect pretty much any species of tree, but the most common are Maple, Basswood, Lilac, Crab Apple, Magnolia, Dogwood, Oak and Catalpa. It is caused by many species of fungi as well. It appears as a white to gray powdery substance on the surface of leaves and is easy to recognize. It looks a little like sprinkled baby powder or cobwebs. It is generally not fatal but it can disfigure a tree and limit its productivity. Like other fungal diseases, it proliferates in humidity, though once a tree is fully infected it can spread in any weather.

The disease tends to affect young leaves, water sprouts and green shoots the worst. Leave may turn yellow and fall prematurely during growing season, and in the fall, you might see tiny round orange to black balls form within white fungal mats. However, most trees are not seriously damaged. Fruit trees are the exception. The infection targets new buds, shoots and flowers by distorting new growth. On Apple. Apricot, Nectarine and Peach trees web-like scars develop on immature fruit and a rough corky spot appears at the point of infection. They must be monitored for deformed, puckered leaves.

The powdery substance is produced by millions of tiny fungal spores, all of which can be spread in air currents to cause new infections. For this reason, to avoid destroyed crops, prevention and treatment are important. Where deformed leaves are found, qualified pruners must be brought in immediately. Fungicides can help on remaining leaves and applications should be applied according to label instructions to protect trees throughout an entire season. Monitor your trees carefully for new shoots developing or signs of the mildew. Control damp conditions as much as possible. If you are attempting to prune the tree yourself, focus on increasing light penetration and air circulation throughout the canopy. Avoid fertilizing any trees suffering from this as fertilizers stimulate new growth and may hasten the spread of infection. Do not compost infected branches and leaves as the spores will infect other plants. And when planting, choose disease-resistant species as much as possible.

Butternut Canker

The Butternut Canker is a lethal fungus that targets Butternut trees of any age or size, as well as other species in the Walnut family. It is not known where it originated but it is believed to have come from Asia and was first noticed in Canada in the 1990s. Butternut trees are found in southern Ontario, southwest Quebec and New Brunswick and are now considered an endangered species in Ontario, listed under the Species at Risk Act. In the United States over 90% of their Butternut population has been wiped out, though the disease has been there much longer.

This is an aggressive fungus that kills most trees it infects, though some live longer than others. It is thought that some environments help increase a tree’s tolerance to the disease so that may be a factor. Dark, sunken elongated cankers develop under the bark and eventually surround the branches and main stem. They effectively strangle a tree, blocking the flow of water and nutrients. If a canker appears on a branch it will look like a white patch with a black center. On the trunk the fungus develops into deep grooves and gashes in the bark with black jelly-like material (containing spores) oozing from the cracks in the spring. In summer, fall and winter this black fluid dries up and leaves a sooty stain. The overall vigor of a tree declines. Small, abnormal branches grow from the trunk, induced by stress. Gradually it will kill the upper canopy and spread down to attack the trunk tissue. When the fungus has girdled the trunk, the tree dies. Fungal spores can spread through raindrops, traveling insects and birds and infected seeds. It is very hard to control.

There is no known treatment for trees infected with cankers, so education is critical. An arborist can identify symptoms. Never risk cutting a Butternut tree down yourself, always get it inspected first and get a qualified professional to do it safely.

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