13 Common Maple Tree Problems and Diseases

Maple Tree DIseases and Issues

When I worked as an editor of a gardening channel, people often asked me how to identify and address common maple tree pests, diseases, and problems. While maple trees can suffer from a number of general problems, verticillium wilt seems to be the most common and dangerous disease plaguing maple tree owners.

The fungi that cause verticillium wilt compromise the tree's vascular system and cause symptoms of wilting and yellowing leaves that are concentrated in one particular area. Failure to control verticillium wilt can kill your tree, so it is important to correctly identify the disease and quickly remedy the problem.

Whether you don't have any idea what anthracnose is or you don't know how to identify lichen, you'll find all the common issues below to help you understand what's plaguing your tree and how to treat the problem.

13 Common Maple Tree Diseases, Problems, and Pests

1. Maple Tree Tar Spot

• Identification: Black spots that range in size from a pin-prick to the size of a half-dollar (4 cm). Some reports say that the spots can get as big as two inches.
• Caused By: Fungi that tend to hide in leaf debris. Specific species include Rhytisma acerinum, R. americanum, and R. punctatum.
• Season: Late summer and autumn.
• Susceptible Species: Norway, silver, sycamore, and sugar, but almost all types of maple are affected by some form of tar spot or another.
• Treatment: It's mainly cosmetic. It shouldn't affect your trees in the long term. The fungus tends to hide in decomposing leaves. So the best way to protect against it is by cleaning up any dead leaves.
• Threat Level: Low (cosmetic)

The maple tree tar spot is fairly easy to identify. It's caused by a fungal pathogen in the genus Rhytisma. While this affects maple trees in general, it especially targets Norway, silver, and sugar varieties. Tar spot will not kill your trees, but it's unsightly and can cause them to drop their leaves before the fall season.

The spots first appear as small yellow spots in June. Then, they progress to the black spots on the leaves you see above. Their size ranges from one-eighth of an inch to an inch or more in diameter on the Norway Maple. The spots can also appear on the seeds (samaras).

The fungus winters over on fallen leaves. If the leaves are not raked up in the fall, the fungal spores will reappear in the spring and spread to nearby trees. Treating the trees is usually not effective because the spores can travel from a neighbor's tree onto yours.

If your tree has these spots, which then causes leaves to curl in mid-July, it may have a different disease called Anthracnose.

2. Anthracnose

• Identification: Anthracnose is a bit of a general term describing a wide range of symptoms. In general, though, these can be signs of the disease: leaves that curl around a dead-looking brown spot, tan or brown spots near the leaves' veins, cankers, dying young branches, and premature leaf loss.
• Caused By: Various fungi such as Aureobasidium apocryptum, Discula campestris, Colletotrichum gloeosporioides, and Discula umbrinella.
  
• Season: Late spring to early summer; especially prevalent during wet periods
• Susceptible Species: Young maple shoots of many varieties are especially susceptible to this.
• Treatment: Keep the tree dry if you can (by moving sprinklers elsewhere, for instance). Prune any affected-looking branches, and then rake them up and dispose of them properly. Rake any fallen leaves as well.
• Threat Level: Low because it's mostly cosmetic.

Anthracnose is commonly mistaken for tar spot. However, it inflicts much more extensive damage because it affects both the leaves and the branches. You'll find many more spots on the leaves that are typically smaller than the 1/8" tar spots.

This condition typically occurs when trees experience long periods of cold, wet weather. The affected areas may show small, dark spots and irregularly-shaped leaves with dead or brown areas. The leaves usually fall off in the early spring, followed by a second set of leaves which also die off. The branches can also develop cankers, which often strip them of their bark and kill them.

The disease continues when fungal spores over winter in dead leaves and infect trees during a prolonged wet spring. Once a tree is infected, the disease survives the winter in infected branches and then spread when the wind carries its spores to surrounding trees.

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Anthracnose can be controlled by removing dead leaves from the base of your trees in the fall. You can also apply fungicide, but you may need a commercial application depending on the size and number of trees that may be affected. You can call your local Cooperative Extension office to see what fungicides are legal in your state.

3. Sooty Mold

• Identification: As the name suggests, sooty mold looks quite a bit like soot. It typically presents as a black, powdery coating that can be brushed off.
• Caused By: Fungi, or plant- or insect-secretions.
  
• Season: N/A
• Susceptible Species: Trees in close proximity to those that honeydew-secreting bugs love, such as hickory or pecan. Also affects trees that honeydew-secreting plants love.
• Treatment: Since this is usually caused by insects such as aphids, whiteflies, and mealybugs, controlling them is the larger issue. You can control these populations by various non-chemical means including using neem oil.
• Threat Level: Low (doesn't do direct damage and is mostly cosmetic)

Sooty mold feeds on the sticky honeydew left by aphids and scale insects, which can sometimes be found on maple trees. You'll know it's this mold if it rubs off easily on your fingers when you touch it.

The good news is that sooty mold won't kill your tree and can be easily treated. Try using a gardening mix that works to protect plants. Make sure to follow the instructions carefully and apply it at the base of the tree to ensure the roots absorb the mixed ingredients. This method should also prevent reinfection for up to one year.

The University of Hawai'i has created an amazing PDF that explains the mold's lifecycle and is complete with many photos of the mold.

4. Lichen

• Identification: Lichen looks so many different ways. Its appearance is affected by a wide variety of circumstances, including altitude, temperature, photosynthetic component, and which other components make up the lichen (such as the fungus it's growing with).
• Caused By: Lichen is a composite organism made up of algae and/or cyanobacteria that creates filaments between shoots of fungi. They don't simply appear out of nowhere. Generally speaking, the lichen dries up, a piece breaks off, the wind carries it elsewhere, and then moisture revitalizes the broken-off piece at a later time.
  
• Season: N/A
• Susceptible Species: Slow-growing things, like the Japanese maple, tend to be more susceptible to this (it'll even grow on rocks!).
• Treatment: You can lift some lichens right off the tree without damaging it. You may want to wait until the tree's dormant period so that you avoid damaging any buds. Alternatively, you can also prune leaves or branches that are covered.
• Threat Level: Low (mostly cosmetic; doesn't damage tree), but it can be a sign that the tree is experiencing more traumatic issues such as root rot.

Lichen, pronounced "liken," isn't a plant. It's actually a combination of algae and fungus living symbiotically. It comes in vast swaths of colors and formations. There are an estimated 20,000 different varieties. Lichen isn't parasitic, meaning that it doesn't feed off of the thing that it's attached to—unlike mistletoe, for instance, which does feed off the host plant. Lichen gets everything it needs to thrive from the air rather than its host surface.

Lichen can be found on many maple varieties, but it's more commonly seen on mature trees. Fortunately, it's not harmful because it feeds off of the air rather than the trees. It doesn't seem to have any long-lasting effects on the places where it grows. It can make it harder for the tree to get the nutrients that it needs via photosynthesis, depending on how large the lichen is and how much of the tree it covers.

You shouldn't feel a pressing need to eliminate lichen because it's not harmful, but you can use copper fungicides if you don't want to see them on your maples. Make sure to follow the instructions carefully!

5. Verticillium Wilt

• Identification: The name describes a wide variety of wilts. However, symptoms tend to be localized to the base of the affected vegetation, to several limbs, to one side of the tree, or to the lower and/or outer parts of the plant. Symptoms can include wilting, yellow leaves, defoliation (meaning "leaf loss"), and stunted growth.
• Caused By: Fungi of the Verticillium genus, V. dahliae, V. albo-atrum, V. longisporum, V. nubilum, V. theobromae, and V. tricorpus. The fungi lives in the soil. It enters the tree via the roots.
  
• Season: Symptoms typically develop in July and August, particularly after a dry and hot summer.
• Susceptible Species: Maples
• Treatment: The disease spreads by laying dormant in the soil, by being ejected into the air when it falls from a tree, by bugs damaging the plant or tree (and thus giving the disease access to the plant or trees innards), and by root-to-root contact. To treat this disease, you'll need to fumigate the soil, plant resistant strains, or whatever it is that's affected, and in severe cases, remove the tree. The disease may resolve on its own, the tree may need additional care (such as watering or balancing the nutrients in the soil), or it may have to be removed. Removing the tree does not need to be your first option. You should be able to call an arborist to help determine if it is, in fact, verticillium.
• Threat Level: Moderate to severe; this disease does affect the health of the tree and can kill it.

One of the worst diseases that your tree can get is verticillium wilt. It affects the tree's vascular system, which usually kills the entire plant. The verticillium fungus is a soil-borne disease and can remain dormant in the soil years before it makes an appearance. The fungus enters through the tree's roots.

Two symptoms of verticillium wilt are yellowing leaves and wilting leaves. Sometimes, whole branches or the entire crown can wilt and die in a short period of time. You can also find a green or brownish-green color in the sapwood of affected trees.

Try to get a proper diagnosis to help you make the right decision for your tree. Call your local Cooperative Extension office to see if they can come out and take a core sample to test.

6. Root Rot

• Identification: There are several different types of root rot, including formes, ganoderma, phytophthora, and laetiporus. The symptoms of each type are pretty distinctive. Formes has shelf-like, mushroom-looking half-disc growths that protrude out of the trunk; this particular type of root rot is especially common in North America. Ganoderma forms similar mushrooms, but they are less wavy in appearance than the formes variety; this species seems to mainly appear in tropical climates. Phytophthora causes cankers that make it look like the tree is bleeding, and it doesn't form any mushroom-looking growths. Laetiporus does form mushrooms too. They are often bright yellow, orange, or some combination thereof. They look like a ruffly colonial-era cravat.
• Caused By: Most varieties are caused by water molds of the Phytophthora genus. In order to "activate," the molds need a sufficient amount of water. The spores are airborne and can also be carried by flying insects and soil-bound arthropods.
• Season: Particularly wet seasons.
• Susceptible Species: Everything is at risk; however, you can find specific types of root rot-resistant trees. For instance, if a particular type of root rot is common to your area, you can see if there's a tree resistant to that particular strain.
• Treatment: Infected specimens should likely be removed and destroyed immediately. Call your local arborist and consult their expertise both to ensure that the tree is infected and that the tree is properly disposed of.
• Threat Level: Severe (it's usually lethal, and there aren't treatments).

Phytophthora root rot is caused by a wet spring or leaving your maple tree in poorly drained soil. The main symptoms are yellow, relatively smaller-sized leaves and dark brown or black tree roots.

Unfortunately, trees with root rot usually can't be salvaged and need to be cut down to prevent injuring people or damaging property.

7. Sapstreak

• Identification: Because the fungus infects the tree from the inside, it can be difficult to notice symptoms right away. Initial symptoms include small leaves. The small leaves can become branch dieback in subsequent years. Branch dieback will most likely appear at the top, or crown, of the tree first. The trunk's wood will look tea-stained.
• Caused By: A fungus called Ceratocystis virescens
  
• Season: Late spring and early summer
• Susceptible Species: Sugar bushes, especially the sugar maple where logging activities are present
• Treatment: The main way to treat this is to prevent the tree's roots from getting damaged, as this is how the fungus usually enters the tree. Damage could mean anything from an insect infestation weakening the tree to a car driving over the roots and wounding them. Some trees go into remission and recover without any treatment for unknown reasons. Some trees go into remission and then exhibit symptoms all over again. You may have to remove a tree that's infected.
• Threat Level: Severe (the fungus damages the tree's innards, and generally containing the disease means removing the tree)

Sapstreak is a ground-living fungus that generally enters the tree's system via an injury near the roots or bottom portion of the tree. The fungus then inches upwards from the root system and infects the trunk of the tree. As time goes on, the fungus eventually affects the outward portions of the tree, such as the branches and leaves.

Sometimes sapstreak means a slow death for the tree, over a period of many years. Other times, a tree can succumb in as little as two or three years. In a report by the USDA tracking sapstreak in sugar maples, they note that "[s]ometimes disease progression . . . is arrested and recovery ensues, even in trees with more than 40 percent crown dieback." So this infection isn't necessarily a death sentence.

8. Phyllosticta Minima (Maple Leaf Spot)

• Identification: Not to be confused with tar spot, these spots appear on a maple's leaves, are tan to brown in the center, and are violently red to purple around the edges. The spots can also be small black pinpricks like a banana or mango that's going bad.
• Caused By: The fungus Phyllosticta minima
  
• Season: The fungus can overwinter in leaf debris and reinfect things in the spring.
• Susceptible Species: Many maples are susceptible to this, but Amur, Japanese, red, and silver maple seem especially so
• Treatment: The best treatment is an ounce of prevention. Be sure that the tree's canopy isn't overcrowded, that the tree isn't over or under-watered, that any fallen leaves are removed, that any infected-looking leaves are removed, and that it has proper nutrients. You can also plant resistant strains.
• Threat Level: Low (mostly cosmetic and doesn't seem to cause lasting structural damage)

Phyllosticta mimima is a relatively small threat to your tree. The damage done by this disease is mainly cosmetic. In severe cases, it can cause defoliation; otherwise, it's a minor stress on the tree's ability to photosynthesize. Having the right set of weather conditions (high humidity and frequent rain) ensures that this disease spreads. High winds also help it migrate. Some strains can cause cankers and blight.

9. Powdery Mildew

• Identification: A superficial, fine powder-like mildew will coat the leaves.
• Caused By: Erysiphales fungi in the order, especially Podosphaera xanthii (a.k.a. Sphaerotheca fuliginea).
  
• Season: Year-round, but seems to favor moderate temperatures and high humidity (such as that which you might find in a greenhouse)
• Susceptible Species: Norway maple and Japanese maple, but almost all species of maple are susceptible to one strain or another.
• Treatment: The mildew can be brushed off. You can also apply horticultural oils and neem to help prevent the mildew from spreading.
• Threat Level: Low (it's superficial).

Powdery mildew doesn't often cause lasting harm to the tree that it's on because it just sits on the top of the leaf (as opposed to getting inside the roots or eating holes in the leaves). The infection is often tree-specific, so a strain that affects a sugar maple likely won't affect a Japanese maple. You can let the mildew sit, and it may resolve itself. You can also resort to fungicides (both natural and chemical) to help fight it. Typically, those are only resorted to on commercial plants and bushes like roses and wheat.

10. Scorch

• Identification: Scorch presents itself as drying leaves. Sometimes scorch is as moderate as a lightly browned leaf edge. Other times, it can be as severe as brown, curling, dry leaves that fall off the tree. The leaves' veins may also show signs of browning.
• Caused By: Weather conditions such as low moisture, high temperatures, and dry wind.
  
• Season: Summer.
• Susceptible Species: All trees are susceptible to this, but Japanese, Norway, and sugar maple are especially sensitive.
• Treatment: Ensure that your tree is amply watered. You can also put mulch around the tree's base to help improve soil moisture retention. Additionally, you can prune any dead branches to help reduce the tree's stress.
• Threat Level: Low (if it's just scorch and not indicative of a greater problem).

Unlike many of the items in this article, scorch isn't caused by a bacteria or a fungus, which also means that it's not infectious. It's caused by unfavorably dry weather conditions. The leaves often show the first signs because they're one of the last tree parts to get water, thus they show under-watering signs first. That said, scorch can sometimes be an indication of a more severe underlying cause such as root rot, which affects the tree's ability to absorb water because the aptly named fungus damages the roots, or an insect infestation.

11. Maple Mosaic Virus

• Identification: White to yellow leaf discoloration that's often kaleidoscopic looking.
• Caused By: It's a viral infection caused by those of the genus Begomovirus.
  
• Season: It's believed to be spread by whitefly feeding, and they're most prevalent during warm weather.
• Susceptible Species: The flowering maple is especially prone to this.
• Treatment: There is no treatment for this, and some people even cultivate for this specific virus because consumers like the way it looks.
• Threat Level: Low (cosmetic).

This virus causes discolored leaves. The discoloration generally ranges from pale white to vibrant yellow. The virus is cosmetic and doesn't affect the tree's ability to flower, grow, and thrive. In some cases—especially with house plants—propagators actually select infected plants since consumers like the way that the infection looks.

12. Pear Thrips

• Identification: They're winged brown insects that are usually less than 2 cm big when fully grown. They damage the tree by scraping away bits of the leaf to feed, which can cause brown or yellow discoloration on the leaves, defoliation (in extreme cases), small or distorted leaves, or blister-like scars.
• Caused By: An insect called a pear thrips.
  
• Season: They usually emerge from the ground in April. You're likely to see adults in March to May.
• Susceptible Species: Sugar maples are preferred hosts.
• Treatment: There are no known treatment options.
• Threat Level: Low (they may damage the tree's foliage).

Pear thrips spend most of their life underground. The female lays her eggs on the leaves by burrowing into the leaf (this can result in brown scars on it). The eggs are usually lain near buds and blossoms, which the larvae then feed on until they're so heavy that they fall off the leaf. Once they fall to the ground, they overwinter there to reemerge in the spring months.

13. Galls

• Identification: There are many gall varieties. They can be green, pink, red, or black depending on which stage they're in. They often look like small wart-like protrusions.
• Caused By: Most of these varieties are caused by mites.
  
• Season: Because there are so many varieties, the seasons vary.
• Susceptible Species: Varies by mite.
• Treatment: Plant resistant varieties. Some people have said they've had success with insecticides and/or miticides, but scientific literature doesn't support this one way or another.
• Threat Level: Low (cosmetic damage).

Galls are abnormal structures formed when a plant's hormones mix with an insect's. This typically happens at a spot where the insect feeds on the tree. There are many different types; some of which are specific to certain trees, some are not. Some variants include:

• gouty vein gall (caused by Dasineura communis larvae, only affects sugar maples, appears on leaves' veins),
• maple bladder gall (caused by mites, looks like pimples, likes silver and red maples especially, usually appear in May),
• maple spindle gall (caused by eriophyid mites like V. aceriscrumena, looks like tall and skinny worms, common on sugar maples),
• and maple velvet gall (caused by mites; looks like a red velvet patches on leaves; primarily found on silver, Norway, and boxelder maples).

Sources

• The Connecticut Agriculture Department's "Common Diseases of Maple"
• Penn State's "Maple Diseases"
• Agriculture and Natural Resources, University of California's "Sooty Mold"
• University of Massachusetts, Amherst's "Sooty Mold"
• The Morton Arboretum's "Scorch"
• University of Nevada, Reno's "General Care of Maples: Managing Phyllosticta Leaf Spot Disease"
• University of Hawai'i's "Abutilon Mosaic"
• Cornell's "Sugar Maple and the Pear Thrips"
• University of Ohio's "Galls of Maple Trees in Ohio"

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: My maple tree is in a pot. I’m guessing, based on what I’ve read, that the roots are compacted. Can I trim the roots and repot it?

Answer: You may trim the roots, but only if it hasn't started to leaf out yet. It is best to trim the roots and limbs when the tree is in a dormant stage (early spring and late fall after the leaves have fallen off).

Eventually you should plant the tree in the ground. The taller it gets, the longer the roots are going to be. The roots are usually as long as the tree is tall, and often much longer.

Question: A big piece of bark fell off my large silver maple tree. It looks like there are eggs on the underside. What is this?

Answer: I would recommend calling your local Cooperative Extension for identification of the eggs you discovered on the tree bark. Bug varieties differ according to location, and I'm not an entomologist. It may be a borer beetle, but several different beetles affect maple trees. Once the eggs are correctly identified, you can decide the best way to treat your tree.

Question: The bark on my maple tree is peeling. Woodpeckers are active on the tree. Is the tree dying?