What are those ugly black spots on the leaves of my tree and what can I do about it? That is an often asked question in a year such as this. The answer is giant tar spot and there is little to worry about and little to do about it.

Giant tar spot is caused by the fungus Rhytisima acerinum. The species name of this fungus (acerinum) gives a clue about which trees it infects. Acer is the genus for maples, and this fungus primarily affects maples and sycamores; but especially Norway maples. Giant tar spot and Norway maples are both from Europe. In late summer it becomes more apparent on the leaves until they fall in autumn. At that point there is something that can be done about it, which will be addressed later.

First, the fact that it is so widespread and apparent in Concord this year is at least partially due to the wet weather the town has experienced this summer and earlier in the spring. All fungi like moist conditions. If you looked closely in the spring, evidence of giant tar spot could be seen as lighter colored, yellow green blotches. I noticed a lot of these this spring in Concord on a large Norway maple that had to be removed because a crack had made the tree too risky to retain. This light green is referred to as chlorosis and those spots have less chlorophyll than the non-infected parts of the leaves. When the spots turn black in the summer, they retain a yellow border. These blotches can grow to an inch or more and sometimes coalesce into larger, irregular shapes. In extreme infestations giant tar spot can cause the leaves to drop earlier than normal.

Occasionally, in years with sustained wet conditions that are favorable, giant tar spot can be joined by other fungi that attack the leaves and petioles and cause the leaves to drop in July. Trees are great at storing energy, but if this is repeated in subsequent years it can eventually affect the overall health of the tree. If the leaves are still on at this point, in early August, the trees have already stored enough energy to maintain their vigor.   

So what can be done about giant tar spot? The fungus overwinters in the fallen leaves. When the weather warms up in the spring, spores are released which infect new leaves on the trees and the cycle begins all over again. One method of control is sanitation. Simply raking up the fallen leaves and disposing of them will eliminate the source of giant tar spot for the following growing season. Thoroughly composting the leaf debris can break the cycle.

It was once thought that the presence of giant tar spot meant a lack of air pollution. This was because giant tar spot was sometimes absent in urban areas. It is likely that the leaves were simply disposed of more consistently in cities. There was a time when it was customary in places to rake the leaves to the curb and burn them. That was both a source of air pollution (even if it smelled nice) and a way to kill the fungus. 

Andrew Hillman
Senior Urban Forestry Consultant
Davey Resource Group
August, 2017