Common Tree Fungus Identification and Treatment

Dutch Elm Disease

Fungal diseases are grouped into categories. The fatal Dutch elm disease falls into the "wilt" category. These fungi attack the vascular system. When its vascular system is weakened, your tree develops problems circulating water and nutrients.

The fungus behind it is spread by elm-bark beetles. Signs that an elm tree has it show up in the foliage. The leaves can curl, drop, wilt and turn yellow.

Growing resistant cultivars is the best option as treatment is difficult. 

Oak Wilt

The fatal oak wilt can attack any oak, but red oak (Quercus rubrum) is especially susceptible. This tree fungus is spread by sap beetles.

For identification, look for wilting, browning out of season (spring or early summer), and leaf drop.

Remove infected trees, especially if you are growing multiple oaks together, since the fungus spreads through intertwined root systems.

Verticillium Wilt

A third wilt disease is verticillium, which affects many trees and is potentially fatal. The fungi responsible are soil-borne. Trees with this fungus experience curling, drying up, and yellowing of leaves. Slow growth is another sign.

No fungicide kills this. Your only option is prevention. The best step to take is growing a tree not susceptible to it, like arborvitae (Thuja spp.) or birch (Betula spp.).

Powdery Mildew

Other fungal diseases fall into the foliar-disease category. Here, the fungal spores attack the leaves directly. If a plant has a powdery dusting on its foliage, then it is usually a sign of powdery mildew, the most common and easily recognizable of the foliar-disease tree fungus.

Perennials like tall garden phlox (Phlox paniculata) commonly get it, but trees get it too. It's not fatal, but, once a plant gets it, its appearance is spoiled until its leaves drop. To prevent it, space plants properly (promoting air circulation) and practice sound irrigation policies.

Shot Hole Fungus

Although another fungus in the foliar class, shot hole fungus goes beyond leaving blotches, and instead leaves holes. This leads to misidentification, as it looks like insects could have caused the damage.

Fortunately, reddish-brown spots precede the holes, so, if you're paying attention, you won't be fooled. By mid-summer, the leaves turn yellow and drop. 

One reason to take this fungus seriously is that it puts your tree under stress, and insects and other plant diseases can take advantage of the stress to infest the tree. Cherry trees (Prunus) are especially vulnerable. Helpfully, several fungicides can be used as treatment.

Sooty Mold

This foliar disease looks worse than it really is, although it does, indirectly, harm trees by depriving them of sunlight (thus stunting their growth).

It takes the form of a black film covering the leaves. Scale insects are ultimately responsible for the presence of sooty mold. Control these insects with neem oil, and you won't have trouble with this tree fungus.

Anthracnose

Anthracnose is another foliar disease, although its identifying tan or brown spots (sometimes surrounded by purplish rings) can also appear on a tree's berries. An infected tree suffers dieback eventually. Prior to that, you'll notice blotches and wilting on the leaves.

Like many fungi, it's especially active in spring. It's harbored in winter by leaves you allowed to stay on the ground from last year. Raking up those leaves and taking them elsewhere is the simple solution. Trees stressed for too long by this disease become susceptible to insect infestations.

Dogwood (Cornus spp.) is one of the popular trees that's highly susceptible to this fungus, which is fatal.

Cercospora Leaf Spot

Crepe myrtle (Lagerstroemia spp.) is one of the trees bothered by this type of foliar disease, characterized by brown circles surrounding tan blotches. As more spots appear, leaves become unable to carry out photosynthesis, and overall tree health suffers. Several fungicides are available for treatment.

Armillaria Root Rot

Root rot is another category of fungus diseases. Armillaria root rot is also known as "oak root fungus," but it affects far more than just oak trees. Various fungi are responsible for root rot, including Phytophthora and Armillaria. Root rot is fatal.

Although the presence of Armillaria is sometimes indicated by mushrooms, don't rely on mushrooms as an indicator. Look, instead, for browning or yellowing of leaves, wilting leaves, leaf drop, and stunted growth. 

Control root rot through prevention:

• When watering, direct irrigation at ground level and keep any spray off trunk and foliage
• Apply water at dripline, not up against trunk
• Avoid over-watering
• Ensure proper drainage

Or avoid the problem altogether by planting water-loving trees like pussy willow (Salix discolor).