In some cases, pest damage to a tree can be mistaken for normal seasonal color changes. Other times, a pest can cause serious tree disease.

Fungal diseases are often spotted as mushroom-like growths (called conks) on the bark or twigs of the infected plant. Stem pathologies are more severe, especially if the fungi get into the host’s vascular tissue. It is best to call Hummingbird Tree Care LLC experts for your tree care needs.

• Anthracnose

Anthracnose is a fungal disease that causes spots on leaves, fruit, and twigs. It is often found on shade trees like maples, oaks, and sycamores as well as shrubs including dogwoods and viburnums, stone fruits such as cherries and peaches, and vegetables such as tomatoes and cucumbers. Irregular, dark brown or black spots form on leaves which can lead to wilting and rot if the infection is severe. The spots also contain pink, gelatinous masses of spores. During warm, wet conditions anthracnose tends to spread rapidly.

The fungi that cause anthracnose overwinter within infected twigs or dead leaves on deciduous trees. In the spring, they produce spores that are spread through rainwater or sprinkler water to new leaves and twigs. Spores germinate on these tissues and can continue spreading until hot, dry conditions return in summer.

Leaves are the primary site of anthracnose but twigs and flowers can also be affected. Symptoms of anthracnose on leaves begin as yellow, then brown discolorations that coalesce into large dead areas of leaf tissue. Spots may be located across or between leaf veins and on the edges of leaves as well as on the undersides. Necrosis usually occurs across or near the margins of leaves and at leaf tips.

While there is no cure once an anthracnose infection develops, preventative care can help mitigate the problem. A professional arborist can provide fungicide treatments that reduce the amount of spores produced in spring and ensure that plants receive proper water and nutrients. In addition, a variety of tree species are available that are naturally resistant to anthracnose, and planting them can be an excellent way to minimize the risk.

• Verticillium

This fungus attacks and kills a wide variety of woody plants, from sunflowers and tomatoes to lilacs and maple trees. It enters a plant through root wounds and then plugs its water, nutrient, and sap pathways, leading to wilting and eventually death. Cool spring temperatures encourage disease activity, but symptoms may not appear until late in the season. Infected plants typically die the same year or decline slowly over many years. Symptoms are not specific but may include slowed growth, wilting, drooping branches and leaves, twig and branch dieback, undersized seed or cone production, and general decline. The fungus often causes severe damage to oaks, especially coastal live oaks.

The fungus is commonly spread by soil from infected plants, by wind, or by insects that carry it. It also survives in the spores produced on mushroom-like fruits called “conks” that grow on tree limbs, trunks, butts, and root flares. These conks can be mistaken for rot, so they should only be removed by a licensed pesticide applicator.

As its name implies, this fungal infection looks very similar to talcum powder. It appears as small spots on a leaf or branch and is usually accompanied by wilting and drooping. It is more common during periods of wet weather when the fungus thrives.

Most species of deciduous and evergreen trees are susceptible to this fungus. However, some hosts are more vulnerable than others: ash, box elder, birch, camphor, elm, hickory, Indian bean tree (Catalpa), Judas tree (Cercis), maple, stag’s horn sumach (Rhus), and smoke bush (Cotinus). The fungus is less common in spruce and hemlock. Never use wood chips from Verticillium-infected trees for mulch or potting media, as this can spread the fungus to new plants.

• Rhytisma Leaf Spot

If your maple leaves develop dark spots that resemble drops of tar by late summer, you have a common disease known as maple tar spot. It is caused by one of three closely related fungi in the genus Rhytisma. The symptoms vary slightly, but the dark tar-like spots are characteristic of all strains of fungus. The fungus grows from year to year in the form of fungal galls that survive the winter on fallen leaves and mature into fruiting structures in the spring, which are blown by wind to new foliage.

The galls that cause tar spots on maples begin as light to yellowish-green spots on the upper surface of leaf tissue. As the summer progresses, they grow and coalesce into tar-like areas that can be up to one inch in diameter on the foliar surface. Macroscopically, the black tar-like spots are splotchy with a ridged appearance. The galls also contain a sticky substance that looks like wet tar when seen from a distance.

Different strains of fungus produce tar spots on different species of maple trees. The most common maple tar spot occurs on non-native Norway maples planted extensively as street trees in Toronto. In this case, the fungus Rhytisma bacterium causes the dark tar spots seen on the city’s maples. The other two strains, Rhytisma americanum and R. puntatum cause smaller spots that are about pinhead size on our native silver, red, mountain, and bigleaf maples.

While tar spots on maples can be unsightly, they are not fatal to the trees and do not significantly reduce photosynthesis. The fungus does not typically spread from tree to tree or defoliate the trees. When the spores fall from the infected leaves with the weather changes in the autumn, they can become airborne and be carried by the wind to other susceptible maples.

• Fire Blight

Affected trees may be prone to scabs or wilts, and branches may break off easily. These diseases are also known as vascular pathogens and are typically caused by bacteria and fungi. They affect woody plants such as hickory, oak, sycamore, and maple, and are especially common in wet climates. Unlike fungal plant diseases, which are often fatal, some of these bacterial pathogens can be controlled with proper management techniques.

Typically, infection occurs when a tree is pollinated by honeybees that have visited infected blossoms or twigs. Once infection has occurred, it can spread from branch to branch through natural openings, such as wounds or insect damage. Infections are most serious during wet spring weather and can cause a tree to decline for a variable period or die.

Diagnosis is easy – the disease is identifiable by mushroom-like growths (called conks) covering the bark and discolored leaves. The fungus penetrates deeply into the tree, causing vascular lesions and a scab-like appearance.

Infections from these pathogens can be prevented by planting resistant varieties, and by reducing or eliminating insecticide use around the tree. Pruning of infected twigs and branches should be done early before the fungus starts to spread.

Several other vascular diseases can affect your trees. These include Phytophthora root and butt rot, which can be caused by a variety of factors such as soil conditions, poor internal tree drainage, low nitrogen levels, or the presence of certain species of nematodes. The fungus Phytophthora cinnamomi is associated with the death of shortleaf pines in the South and can be prevented by using resistant cultivars, adequate fertilization, and by controlling insects.

• Oak Wilt

Oak wilt is the most deadly disease that afflicts oaks. It is caused by the fungus Ceratocystis fagacearum and spreads through both aboveground and belowground mechanisms. Aboveground, the fungus is carried by sap beetles attracted to fresh wounds (from storm damage, pruning cuts, or other activities) on uninfected oaks, as well as through root-graft connections between trees of the same species. Oak wilt can also be spread by moving infected limbs or firewood, and by humans working near oaks.

Foliar symptoms include slowed growth, progressive and conspicuous leaf wilting, and rapid defoliation. Leaves may also develop water-soaking symptoms, where the middle of the leaf becomes discolored as it dries and wilts. Infected oaks will develop fungal mats in the soil beneath the bark, where spores are located. These mats may ooze a sticky liquid, and be spread by insects attracted to them. The fungus causes tree death within the year of infection in red oaks, and several years for white oaks.

The best method for diagnosis is through the submission of a tissue sample to a laboratory. Please note that dead limbs can not be used for this purpose and that the leaf should be separated from the stem for the sample to be valid.

The only effective treatment is through preventative management. Avoid wounding oaks, especially from February through June. Paint any fresh wounds on oaks with latex or commercial tree paint to prevent the arrival of sap beetles and the fungus they carry, and be sure to disinfect any tools used between trees. Buy and burn oak firewood only from local sources, and never move firewood between locations. Dig trenches at least 4 feet deep and 100 feet wide around the base of infected oaks to break up the underground spread of the disease.