Three Tree Diseases To Watch


Tree diseases continue to be a problem throughout the country causing harm and threatening otherwise healthy trees. The Davey Tree Expert Company’s Diagnostic Laboratory identified the top three most common diseases they diagnosed over the course of 2020.

These top three diseases affect the appearance of a variety of trees from conifers to deciduous, and, if combined with other factors, can be deadly for a tree. Keep a close eye on these diseases in your area because they can be anywhere, and they aren’t going away in 2021.

Tree diseases
(Credit: Adobe Stock)


Rhizosphaera Needle Cast. The number one most common disease of 2020 and the most problematic of the three diseases, Rhizosphaera Needle Cast is a fungal disease that can be found affecting conifer trees across North America. Spruce trees are the most susceptible to this disease, as well as fir, pine, and hemlock species.

Alone, Rhizosphaera Needle Cast will most likely not kill a tree after one season—but in combination with other elements, it can be a contributing factor that results in tree death or decline. There are simple visual symptoms one can look for that help determine if a tree has Rhizosphaera Needle Cast without a diagnosis from a lab. These include excessive needle drop, thinning of the canopy (especially from the bottom up), and a purple to dark brown color of the inner needles.

Tree diseases
Rhizosphaera Needle Cast

Fortunately, there is a treatment for Rhizosphaera Needle Cast. Since it’s a fungal disease, a fungicide application can be used as a preventive to reduce infection and damage to the tree. “We would use a fungicide application, and we would want to do this as a preventative,” says Kodi Riedel, supervisor of Davey’s Diagnostic Laboratory. “Let’s say last year I confirmed Rhizosphaera Needle Cast on a tree. I would want to treat actively this spring when the new foliage comes out. [In this way] it prevents new fungal infection. Then you would also have to do one more treatment in the growing season as well.”

On the downside, if Rhizosphaera Needle Cast is in the area, there is a strong chance surrounding trees will become infected. “The best thing to do is treat the trees preventatively and provide the tree with proper cultural care to reduce infection and slow the spread,” Riedel says.

Pestalotiopsis Tip Blight
Pestalotiopsis Tip Blight

Pestalotiopsis Tip Blight. The second most common disease of 2020, Pestalotiopsis Tip Blight is also a fungal disease that can be found nationwide and affects conifer trees, specifically Arborvitae. “Pestalotiopsis Tip Blight, like most other fungal diseases, thrives in areas that hold moisture either from excessive rain or irrigation in combination with little airflow,” Riedel says.

Pestalotiopsis Tip Blight alone will most likely not kill the tree in one season, but in combination with other elements, it can be a contributing factor that results in tree death or decline. This disease is recognizable by its appearance. The tips of the foliage will turn brown and yellow, as seen at right, and branches will die from the tip back.

Pestalotiopsis Tip Blight follows a similar treatment regimen as Rhizosphaera Needle Cast by using a fungicide application as a preventive. Fortunately, ensuring the tree is healthy and properly cared for will help protect it from Pestalotiopsis Tip Blight as well. Even if Pestalotiopsis Tip Blight is in an area, if a tree is healthy and has proper care, it will have a better chance of recovering from infection or avoiding infection in the first place.

Tree diseases
Anthracnose (Photos provided by The Davey Tree Expert Company’s Diagnostic laboratory.)

Anthracnose. And finally, the third most common disease of 2020 is Anthracnose, another fungal disease that can affect trees anywhere in North America. Anthracnose affects deciduous trees, such as maple, oak, sycamore, and dogwood species.

As with the previous two diseases discussed here, typically Anthracnose alone is not fatal to most tree species, but in combination with other stress factors affecting the tree, it could contribute to decline and possible death of the tree. However, dogwood trees can be severely affected by Anthracnose, so it’s important to keep a close eye on these trees specifically, especially if Anthracnose is in the area. “Anthracnose in dogwoods can be deadly because it can get down into the stems of the tree,” Riedel says. If Dogwood Anthracnose is in the area, there is a possibility that surrounding dogwood trees will become infected.

One way to help is with cultural methods. “There are some cultural methods that you can use: pruning out the severely affected tissue; pruning to allow more airflow throughout the canopy; and removing the affected debris from the ground as it falls,” says Riedel. (These cultural methods can be applied to all three diseases mentioned.) Anthracnose can be spotted by irregular browning around the veins and margins of leaves, as in the picture above.

Since Anthracnose is also a fungal disease, it will require a similar treatment method as Rhizosphaera Needle Cast and Pestalotiopsis Tip Blight. A fungicide application can help protect infected trees with a handful of applications over the course of a year, but usually these are unnecessary except for certain tree species or severe infections.

Jill GoldenGolden is a project manager in corporate communications for The Davey Tree Expert Company, an employee-owned corporation that provides tree, utility, lawn care, and environmental consulting services in the green industry throughout the United States and Canada.

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