Common Diseases That May Be Plaguing Your Dogwood Tree (And How To Prevent Them)

Dogwoods make a stunning addition to any garden. The compact trees are prized for their delicate white blossoms in the spring and bright red berries in the fall. One hidden downside to planting a dogwood tree in your yard is that, depending on the variety you get, they may be prone to a variety of diseases –- especially fungal ones like powdery mildew or leaf spot. The best way to avoid any of them is to opt for a more disease-resistant cultivar. Flowering dogwood (Cornus florida), for example, is native throughout North America, and cultivars like "Appalachian Joy" or "Appalachian Snow" are highly resistant to powdery mildew, one of the most common diseases seen on the tree.

Beyond choosing a resistant cultivar, selecting the best spot to plant a flowering dogwood is equally important. The right site can make your tree less vulnerable to developing a disease in the first place. That means planting your tree somewhere that gets morning sun so that the dew gathered overnight will evaporate off the leaves early to reduce the risk of disease.

Fortunately, even if you have one of the more disease-prone varieties and it's planted in a less-than-ideal spot, many common diseases either don't require treatment at all or are easy to manage when caught early enough. If it's too late to change the tree or its location, inspect it often for disease so infections don't spread out of control.

Many different fungi can cause dark spots to appear on the foliage of dogwood trees. Some of the most common culprits include Cercospora, Septoria, Pestalotia, and Phyllosticta. Regardless of which species is responsible for the spots you're seeing, leaf spot diseases are generally harmless. Treatment is rarely necessary, but when the leaves drop in fall, rake up the infected leaves and toss them to prevent the disease from returning next season.

Beyond raking leaves at the end of the season, it's better to just leave your dogwood be if it has leaf spots. While fungicides could help control the disease, you want to avoid using them except in extreme cases where the tree might die otherwise. There are a few reasons for this. First, sort of like antibiotics, fungi can develop a resistance to common fungicides. Using them more often than necessary or improperly can end up creating resistant strains in your garden that will be harder to treat if a more severe disease overwhelms your tree later on.

Second, fungicides are harmful to bees and other pollinators. Not only will the pollinators who visit your dogwood come in contact with the toxic spray, but they'll carry it back to their colony, exposing the entire hive. With that said, if leaf spot diseases have overwhelmed your tree multiple years in a row, triggering early leaf drop, you might want to spray a preventative fungicide early next spring, right before the tree comes out of dormancy.

Powdery mildew looks exactly like its name suggests. If you see leaves that look as if they've been dusted with a fine, white powder, this is likely the culprit. This fungal disease happens most often when trees are planted in shady areas. It's also common when trees lack air circulation, such as when planted too close to another plant, a fence, or another obstruction.

Fortunately, the disease is rarely deadly, but it can weaken or slow the growth of trees, so it's still a good idea to prevent it where you can and manage it where you can't. But fungicides probably aren't necessary here. To prevent powdery mildew from taking over, choose a site that allows for good air flow on all sides of the tree. If it's too late for that, prune annually to remove dead branches or thin out denser parts of the canopy. 

If you spot the disease early in the season, start by pruning affected leaves and branches. Then, rake the fallen leaves into a bag and throw them away so the fungus living on them doesn't spread further. If so much of your tree is covered in powdery mildew that you would have to prune more than a third of the branches, skip pruning because removing too much of the canopy can stress the tree. Instead, contact your local extension office to find out which fungicides are recommended for your area.

Another common dogwood disease is spot anthracnose. If you've had an especially wet spring, you might notice tan or reddish spots on the flowers. As the disease spreads, you'll see similar spots on the leaves, young shoots, and fruit. While spot anthracnose is not serious, a rarer, but similar-looking disease known as dogwood anthracnose can be severe. These two diseases cause similar visible symptoms, but are caused by different fungi. The latter can spread aggressively and potentially kill the tree.

To figure out which one you have, look for where the disease started and how quickly it's spreading. Dogwood anthracnose typically starts on the lower leaves and works its way up the tree. As the infected leaves die, they typically cling to the twig instead of dropping off. Meanwhile, with spot anthracnose, you might notice ragged holes in the middle of leaves where infected tissue has dropped out.

Since fungicides only prevent disease — not treat existing infections — it's too late to use them once symptoms appear. Instead, prune away what you can and dispose of the diseased leaves and branches so they can't reinfect the tree. If you have other dogwoods in the area, you might want to cover the infected tree to prevent the disease from spreading.

A fungus called Phytophthora cactorum causes crown canker (also called collar rot) when it enters a tree through a wound. Just as an open wound on your skin can become infected, so can cuts and scrapes on the trunk or branches of a tree. Early symptoms include smaller, paler leaves than usual. You might also notice your tree producing more flowers than usual.

If you see either of those symptoms, inspect the trunk of the tree. Often, the wounds are caused by lawnmowers or injuries sustained during the transplanting process. But it can also happen during the pruning process if you aren't careful. If you don't see anything around the trunk, check the branches, too. Look for discolored bark or grooves in the tree. These cankers might be oozing a dark fluid that dries into a powdery white residue.

For cankers that are small enough, you may be able to cut out the infected area to curb the disease's spread. If you do, apply a fungicide to the new wound right away to prevent reinfection. To prevent crown canker, avoid mowing around the base of the tree. When pruning, use sharp shears or bypass pruners that make a clean cut rather than crushing or ripping at the wood. These clean cuts heal faster, so they'll be less vulnerable to infection. But if you do live in a climate that's prone to fungal diseases, consider brushing or spraying a fungicide onto the cuts right after pruning.