Spring is Coming … Along with Foliar Diseases

After the cold, dark weather of winter, we are all looking forward to spring! Flowers will be blooming.  The birds will be singing. And while spring brings new life to trees, it also brings foliar diseases that attack their growing leaves. Foliar diseases impact the appearance of a tree’s leaves (resulting in spots, lesions or other visible defects) and hurt the tree’s ability to make food through photosynthesis.

Foliar diseases are found in most geographies and in a range of tree species. Here are some common examples.



Anthracnose is a disease that affects many types of trees, though some species are more susceptible including sycamore, oak (especially white), maple, ash and walnut. Symptoms typically appear anywhere from May to mid-June depending on the location and weather. These symptoms differ based on the species. For example, infected sycamore, white oaks, and maple develop large irregular brown to purplish lesions develop along leaf veins. Ash, black oak, and walnut, on the other hand, display more discrete circular or angular lesions on leaves.


Fireblight causes wilting and blackening of blossoms and leaves in many species such as crabapple, hawthorn, pyracantha, spirea and species in the rose family. Leaves will have a ‘scorched’ appearance and cankers usually develop on the stems. Fireblight activity occurs from May through early June depending on the location.

Apple scab

apple scab

Apple scab is a major disease of apple and crabapple trees. When early spring weather is warm and moist, and thus particularly conducive to disease development, hawthorn and mountain ash may also be affected. Smoky or smudgy spots on leaves is the first sign of a problem. As infection continues, the leaves and fruit become deformed and flowering is reduced.

Powdery Mildew

powdery mildew

Powdery mildew is an easily recognized disease characterized by white spots or patches on the foliage that look like powder. The disease impacts many species and develops most easily during warm, dry weather.

Leaf Spot Diseases

leaf spot

This is a common term applied to a number of different diseases that impact foliage with the chief symptom being spots. Entomosporium leaf spot is one example that attacks the rose family and certain fruit trees. Tubakia leaf spot, also known as Actinopelte leaf spot, is a late season fungal disease that commonly attacks oaks, particularly those in the red oak group.

Rust Diseases

rust disease

Another common term applied to a group of diseases, rusts result in freckle-like spots on the leaf surface – anywhere from a few dots to a speckled mass. The spots may have a reddish “rust” color, but can be other colors as well. Unusual growths called galls sometimes accompany these diseases. Cedar apple rust, pear trellis rust and hawthorn rust are a few examples.

Management of foliar diseases

Some insects and diseases can be effectively treated after symptoms become apparent. Not so with foliar diseases. For maximum efficacy, treatments should be applied preemptively. Controlling these diseases once symptoms are noticeable is notoriously difficult.  Treatments are far more effective when applied to prevent infection in the first place. While this does make application decisions challenging, it is a safe bet that if foliar disease symptoms were present in the past, they will likely return.

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