One of the most common maple tree diseases is known as maple wilt. The causal factors are Verticillium albo-atrum or Verticillium dahliae, which are fungi found in the soil. This is a common and serious problem that can even kill established trees. Maple wilt seems to be most common in Norway maples but is also found in silver, sugar, red, sycamore and Japanese maples.
- Description: A tree with maple wilt may have browning or scorched-looking leaves, and diseased branches will have small amounts of sick-looking leaves. Sometimes olive-colored streaks will be found in the sapwood of an affected tree. Cut the bark and look for these streaks, then take the bark to your county Extension Office for confirmation.
- How it spreads: The disease starts in the root system and spreads up through the sapwood into the upper branches of the tree, causing big limbs to start dying back.
- Prevention: A healthy, vigorous, well-established tree may be able to beat maple wilt, but most trees will die within a season or two of showing symptoms. Unfortunately, the best way to control the disease is to destroy infected trees to keep it from spreading. If that's not an option, or the tree is not seriously infected, pruning out affected branches may help the tree survive. Keep the tree well-watered during the time it is trying to heal.
Anthracnose (Colletotrichum gloeosporioides) refers to a group of diseases caused by fungi, and it can affect many shade trees. Similar fungi attack other trees such as sycamore, white oak, elm and dogwood trees. They cause a loss of leaves and are usually relatively harmless when the disease only strikes once.
- Description: This type of fungus is particularly common after unusually cool, wet winters and can affect bud formation, kill small twigs and leaves, or cause premature and repeated early loss of leaves. On maple trees, it causes brown or purplish-brown spots and stripes near the veins on the leaves, and the tree may lose its leaves prematurely. If the disease cycle repeats year after year, the tree may become stunted or deformed because it cannot keep its leaves long enough to grow.
- How it spreads: Anthracnose spreads by airborne fungus and is especially prevalent during a wet or rainy spring. In maple trees, it is spread in April or May in most gardening zones. Wind blows through the infected trees and spreads spores onto new maple trees. Wet springs provide the ideal conditions for anthracnose spores to take hold.
- Prevention: It's important to rake up all the fallen leaves each fall and compost them or burn them (if your area permits burning.) Fallen leaves provide the ideal breeding ground for anthracnose. Another option is to have an arborist spray a special fungicide containing a chemical called mancozeb on the trees. If the damage continues year after year, it could predispose the tree to other problems.
Another common maple tree leaf disease is tar spot, which can be caused by one of two different fungi: R. punctatum or Rhytisma acerinum.
- Description: Tar spot is an ugly but mostly harmless disease that strikes several maple species. As its name implies, tar spot disease looks like big black tar spots on the top of the leaves.
- How it spreads: Infection typically begins in early spring and continues into the early summer. The fungus is able to take hold when there are prolonged periods of wet weather that prevent the leaves from drying off. Leaf spots start out yellow and evolve into a dark, tar color.
- Prevention: Treatment is generally not recommended for tar spot because it is usually not a serious problem; however, raking up fallen leaves will keep tar spot at bay.
sapstreak (Ceratocystis coerulescens (C. virescens)) is a fungal disease that affects sugar maples. It is a fatal disease that discolors the wood, so salvage is not possible. This disease is mostly seen in parts of North Carolina, Michigan, Wisconsin and Vermont.
- Description: The disease causes foliage at the crown of the tree to become smaller, and bald spots often appear.
- How it spreads: Over time this dwarfing spreads and the tree ultimately dies. When the tree is cut down, a radiating pattern will be seen in the wood of the lower part of the tree.
- Prevention: The only way to get rid of sapstreak is to cut down the tree as soon as possible after noticing the problem. Sapstreak can spread with the help of insects through wounds on the trees, so removal of infected trees is important to keep other trees healthy, if you have multiple maples.
Like anthracnose, phyllosticta leaf spot (phyllosticta minima) is caused by a fungus.
- Description: phyllosticta causes raised tan or dark brown leaf spots. The spots may turn dry and brittle and crumble away, leaving holes in the maple leaves.
- How it spreads: As with anthracnose, the fungus that causes phyllosticta spends its winters hiding among the fallen leaves on the ground. It waits until the springtime, when damp conditions give it the opportunity to spread. Breezes carry the spores to new hosts.
- Prevention: Rake up fallen leaves each autumn and discard them properly to prevent fungal diseases such as phyllosticta.
Preventing Maple Tree Diseases
The best thing you can do for your trees to protect them from maple tree diseases is to take good care of them before they develop a disease. That means water regularly, fertilize annually, keep the area around the trees clean, prune when necessary and seek help immediately if you notice your tree looking ill or having problems.