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.
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.
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