5 Evergreen Tree Diseases to Look For in Late Winter

Although winter landscapes aren’t as colorful as those green spring and fiery fall displays, evergreens help add appeal to the winter garden. Large trees like pines and spruces tower overhead, and dwarf evergreens provide interesting shapes and textures to front-yard gardens.

While these species don’t lose their needles and leaves during the winter, this doesn’t mean they’re unaffected by the change in seasons. Cold temperatures, snow and sleet, and heavy winds can stress them and make them less able to fight infection. Therefore, pathogens often infect or spread during the colder months.

Checking your evergreen trees for diseases in late winter helps you catch any problems early on. After you notice and properly identify the disease, you can move on to treating or managing the issue.

Cedar Apple Rust

Although cedar apple rust rarely causes serious damage to the evergreens it infects, it can become problematic for apples, roses, and other plants in the Rosaceae family. That’s because this fungus requires both a member of the Cupressaceae family and the Rosaceae family to complete its life cycle. Therefore, monitoring your cedars and junipers for the fungus will help you keep other species healthy.

The fungus overwinters on juniper and red cedar species. It can either exist in the branches or in galls. Although you may not be able to spot an infection in the middle of winter, the fungi will begin releasing slimy, orange growths once the weather starts to warm. The growths eventually release spores that travel to and infect apples, serviceberries, hawthorns, and related plants.

Removing cankers and galls from your evergreens can help limit the spread of this fungus. Although these growths aren’t harmful to the cedars and junipers, removing them will help keep the other host plants healthy. 

Cytospora Canker Disease

This fungal disease most often affects Colorado blue spruces and Norway spruces, but numerous other evergreen species are susceptible. The fungus Leucostoma kunzei infects branches and eventually causes sunken cankers that often ooze with sticky resin. As the cankers spread, the infected branches die.

Since this fungus most often infects stressed trees, aim to keep your trees as healthy as possible. That means selecting a species that’s well-suited to your area (natives are often a good choice) and planting it in a proper location. Avoid planting in areas with wet or compacted soil, ensure there is plenty of sun, and water well during drought.

Although cytospora canker disease rarely kills trees, it can cause them to lose multiple branches and become disfigured. If you notice a branch is infected, you can prune them off during a dry period. This intentional pruning and proper tree care will help limit the disease’s spread.

Diplodia Tip Blight

This disease is caused by the fungus Diplodia pinea. It most frequently impacts pines with clusters of two or three needles—red, Ponderosa, and spruce pines are all susceptible species. While the fungus can infect other conifers, it generally isn’t a concern in firs, junipers, and other trees.

Diplodia tip blight most often infects old trees weakened by stressful events like drought, compacted soil, or snow damage. It affects the most recent year’s growth, causing the tips of branches to become yellow and then brown. Over time, entire branches can die back to the trunk.

Although tip dieback is easily spotted in spring, you can also spot the symptoms of this disease in late winter. Recognizing this disease in February or March will help you prepare for treatment.

The fruiting bodies begin releasing their spores in the spring, so your goal is to remove or kill the fungus before it has a chance to spread. The fungus overwinters in the dead needle tips and cones, so removing and disposing of them will help halt the disease. You can also spray an appropriate fungicide on your tree at bud break or consult with an arborist to provide treatment.

Leaf Spots

Although you may imagine conifers like spruces and pines when you think of evergreens, magnolias are also popular in much of the country. The trees’ large leaves are immune to many of the diseases that plague conifers, but they’re also susceptible to diseases like leaf spots.

Leaf spot is a general term for diseases that cause discolored spots on leaves. Magnolias are susceptible to leaf spots caused by fungi in multiple genera. These fungi cause yellow or brown spots of various sizes.

If you notice these spots on your magnolia in the early winter, consider your treatment options. The diseases are unlikely to cause serious damage to mature plants, so treatment is often unnecessary if you don’t mind looking at imperfect foliage. You can prevent the disease from spreading by collecting fallen foliage and limiting stress by proper watering.

Leaf spot is a more serious issue for smaller magnolias. Therefore, it’s worth consulting with a local, certified arborist about your fungicide treatment options.

Bonus: Winter Burn

Although it’s not a disease, winter burn is a phenomenon that often impacts evergreen trees in the late winter. This environmental condition occurs after trees become stressed by cold temperatures or snow damage. It’s especially likely to occur in newly planted trees or plants that are not well-suited to their growing zone.

While many people think extremely cold temperatures cause this damage, it can also result from warm, sunny winter days. These warm periods cause trees to lose water through their leaves. In most cases, plants can replenish this lost water. However, if the roots are frozen, the leaves dry out and later become brown.

This injury often doesn’t become apparent until temperatures start to warm in the late winter. You’ll spot brown leaf tips and possibly brown foliage closer to the interior of the tree.

Once the damage occurs, you cannot reverse it. Prune off the infected portions of the trees and mulch your trees to protect them from future damage.