Question
A landscaping company planted 6 jaquemonti birch in my yard at the end of May. Two of them recently began developing darkish brown leaf spots in the center. They are getting plenty of water. On one of the leaves I found very tiny black insects. I did not find insects on any of the other leaves. What could the problem be and what should I do about it. I picked the infected leaves off that I could reach. I also collected the insects.
Answer(s)
From your description there may be two different things happening. The first possibility is that this particular type of tree is not heat tolerant and the leaves may be scorched because of our hot and dry weather conditions. This problem usually shows up in July and August with newly transplanted trees being the most susceptible because they have yet to develop extensive root systems. I have included a link below that has more information on leaf scorch.
The second possibility is that what you are seeing is a fungal disease. Fungal diseases usually start out as dark brown to black spots. These spots will spread and eventually grow together and in severe cases the leaves may die and fall off. When we have long periods of cool wet weather, like we did this spring, fungal diseases can be more problematic. If it is a fungal disease the good news is that now that we are having much hotter and dryer weather you should see a significant decrease in the spread of this problem. Below is a link with more information on a common fungal disease in our area. If it is fungal, some things you can do to help your tree out are to make sure to pick up and discard any of the leaves that fall off of this tree. This will help reduce the spread of the fungus. When watering the tree try to avoid getting water on the leaves. If the tree is watered by sprinklers make sure to run them early enough in the day so the leaves will be dry before nightfall. Below is a link with more information on watering trees. You can also apply a protective fungicide spray at bud break and to the newly emerging foliage next spring.
http://utahpests.usu.edu/plantdiseases/htm/ornamental/leafscorch
http://utahpests.usu.edu/PLANTDISEASES/htm/ornamental/anthracnose
http://extension.usu.edu/files/publications/publication/HG-523.pdf
If neither of these look like what you are seeing on your trees please bring a sample into our office at 2001 S. State Street #S1200 for help with diagnosis.
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