Leaf Spot Diseases: What You Need To Know

We talk a lot about how to spot tree risk in the form of tree location or position, but when it comes to spotting tree diseases, you need to take a closer look. Leaf spot diseases are one of the least talked about tree diseases, but they are just as important to look out for as EAB or Dutch Elm.


Leaf spot diseases interfere with photosynthesis – how plants create energy.  Just like if our energy creating processes (eating, drinking, breathing, etc.) are interrupted, a tree’s overall health can fail.

The most common forms of leaf spot disease in Minnesota are:

  • powdery mildew
  • anthracnose
  • apple scab

Fortunately, leaf spot diseases are easily spotted and treated.


While there are many forms of leaf spots, we are going to focus on helping you spot the ones you are most likely to notice in your own yard.


Powdery mildew looks exactly like it sounds – like a puffy cloud on your leaves.  Powdery mildew can be found on dozens of different species of trees, but, in general, each kind of powdery mildew is host-specific meaning it won’t transfer to other species of trees.

powdery mildew.jpeg

Photo via Wikipedia

The photo above is a great example of what to look for, but here are a few for tips for spotting powdery mildew:

  • Infected leaves may turn yellow or purple and fall off during the growing season.
  • In late summer and early fall, tiny round orange to black balls form within white fungal mats.
  • Cool temperatures and high humidity is ideal conditions for growth.
  • Plants in shaded areas are more likely to have powdery mildew. 


While wide spread, anthracnose is spotted on fewer trees than powdery mildew. It can be spotted on Ash, Birch, Black Walnut, Butternut, Buckeye, Elm, Hornbeam, Maple and Oak trees. Anthracnose results in a wide range of symptoms including leaf spots, blotches or distortion, defoliation, shoot blight and twig cankers.


Photo via Government of Maine

The photo above will help you identify anthracnose, but here are a few for tips for spotting it:

  • Brown shaped spots near the leaf veins.
  • Spring leaf drops.
  • Orange/brown blisters on young twigs.
  • Lower and inner branches are most susceptible to anthracnose.


Apple scab affects many ornamental trees and shrubs in Minnesota. The main symptoms of the disease are leaf and fruit spots.

apple scab.jpeg

Photo via University of MN

The photos above will help you identify apple scab, but here are a few for tips for spotting it:

  • On leaves, you will see velvety, green spots with feathery borders.
  • On fruit, there will be green spots that turn brown with time.
  • With age, the leaf spots grow together especially along leaf veins.
  • Severely affected leaves will fall mid-summer.


Misdiagnosis is common when it comes to leaf spot diseases, so call us. We will identify the leaf spots, diagnose if they are a minor or serious problem and offer you a treatment plan. Leaf spots can also be a sign of something more serious, so it is best to have an expert come look at your trees.