Autumn is a beautiful time of the year for your clients’ trees to show off their fall foliage. However, weak trees can fall victim to various diseases during this period as well.
While some diseases cause mainly aesthetic issues to trees, others can pose a fatal threat.
Leaf Spot
Leaf spot diseases can be caused by fungal pathogens or bacteria. Leaf spot fungi favor cool, wet weather. They can weaken trees by disrupting photosynthesis, but most only impact a small area of the tree’s leaves.
Depending on the species of the tree impacted, leaf spots come in a variety of shapes, sizes and colors. Dead areas are typically black, brown, tan or reddish. Leaf spot is typically seen first on lower and inner branches where leaves are shaded and there is more humidity.
Mature leaf spots produce spores that spread the disease further throughout the tree. In years with very high humidity or frequent rain events, leaf spot can spread throughout a canopy, resulting in severe disease. If leaf spot kills too many leaves on a recurring basis, the tree’s health can gradually decline.
The spread of leaf spot can be mitigated by raking up and destroying diseased fallen leaves around the impacted tree. Pruning to improve air circulation can also help.
Powdery Mildew
Like its name would suggest, powdery mildew produces white, powdery patches on leaves and stems. This fungal disease prefers mild, humid temperatures. It can infect thousands of species, including maple, oak, hawthorn, dogwood, ash and birch. As the fungus spreads, leaves will turn yellow or brown and drop prematurely. Thankfully, this is another disease that does not significantly affect the health of the plant.
You can manage powdery mildew by not fertilizing infected trees and pruning to improve airflow. Fungicide treatments are effective at treating this disease, but they must be applied before the disease becomes severe.
Anthracnose
Oaks, maples and sycamores are some of the species most susceptible to anthracnose. It can infect both deciduous and evergreen tree species. For the most part, this is another cosmetic disease.
Symptoms include dead spots on leaves, premature defoliation and browning along leaf veins, depending on the tree species. However, repeated infections can lead to severe leaf blight and foliar dieback.
Like leaf spot, the best way to prevent the spread is to rake up infected leaves and twigs to prevent the fungus from re-infecting the tree the following year.
Verticillium Wilt
Verticillium wilt is a vascular disease that impacts woody ornamental trees like Japanese maples, redbuds, magnolias, and locust trees.
Symptoms include leaf scorch, leaf wilting, vascular staining and branch dieback. It can be confused with root and cankering diseases, so those causes have to be ruled out. The symptoms will be most visible during hot, dry periods.
Unfortunately, there are no effective fungicides for this disease. Trees can be pruned to remove infected limbs, delaying the spread of the disease, but most susceptible trees will decline and die within a few years following the first appearance of symptoms.
Needlecast Diseases
Needlecast is a catchall for a variety of diseases caused by a number of fungi. It is typically found in conifers, including spruce, Douglas fir, and Austrian pine trees.
An infected tree’s needles will turn yellow, then brown and eventually drop. They can commonly occur when plantings are dense and there is a lack of airflow. This disease infects conifers in the spring and becomes visible in the fall. Poor vigor will increase the severity of needlecast.
Because there are so many different diseases that cause needlecast, it’s important to send a sample to a plant clinic to identify which one it is. Fungal controls will vary depending on the culprit.
