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Maple Leaf Spot

A wet spring contributes to leaf spot disease

By: Sandy Feather ©2011
Penn State Extension


Q. My lovely Japanese maple has white spots on the majority of the leaves. Although it looks OK otherwise, I am concerned about these spots. They appear to be drying up the leaves. Can you tell me what is causing this and how to prevent it in the future?
  
  A. The very wet weather we had this spring when plants were leafing out created very favorable conditions for a variety of fungal diseases to develop on all kinds of trees. Tender new growth is far more susceptible to infection than mature, hardened off foliage.

Going from your description, without seeing a sample, it sounds as though your Japanese maple is suffering from anthracnose.
  

Common Disease

This is a very common fungal disease that attacks maples, dogwoods, sycamores, oaks and many other species. To complicate matters, the disease has different causal organisms on different species of trees, and the expression of anthracnose can vary, depending on the causal organism.

Aureobasidium apocryptum is one of the fungi that causes anthracnose on maples. On Japanese maples, this anthracnose appears as discreet white spots. As the disease progresses – and the longer the leaves stay wet, the worse it will be – the spots generally run together and kill larger areas of the leaf, resulting in a scorched appearance. On occasion, this anthracnose can cause twig dieback, too, especially on very succulent shoots.

Although it does detract from your tree’s ornamental appeal, anthracnose on maples is not life threatening. Fungicides are best used to protect new growth as the tree is leafing out in spring. Nothing you can apply now will "cure" those spots. They will remain until the tree loses its leaves this fall.

Sanitation

The best practice is to clean up the fallen, infected leaves as thoroughly as possible this fall. Send them out with the trash rather than composting them. If the disease has resulted in dead twigs, prune them out in late winter and get rid of them, too. Treatment is not usually recommended since anthracnose does not cause significant damage to maples. If you cannot tolerate any damage, apply a fungicide containing mancozeb at bud break, and at seven to ten day intervals until wet weather stops and average daily temperatures are above 65 degrees.

Growing Site

It also helps if the tree is growing in full sun and is not crowded by other plants. This permits good air circulation and rapid drying of foliage after it rains. If this is not the case, perhaps you can prune the surrounding plants to permit better air movement around the maple. Not that it would have helped much this spring – the plants rarely had a chance to dry off in between storm systems.
  

 

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