Extension Educator, Local Food Systems and Small Farms
Extension Educator, Horticulture
May 24, 2012
Here we go again! We are into May and many area homeowners are reporting problems with their maple trees. Many times the problems begin on one side of the tree, and have the symptoms of brown or dropped leaves. What could be the problem? Well, anthracnose is back again. The fact that one side of the tree is affected is usually a function of wind currents (or the lack of them). The other side will probably show symptoms shortly.
What's anthracnose? It's a disease caused by a fungus. It's present most years, and affects many different plants. On trees, it only affects good quality shade trees. That includes maples, oaks, ash, and even sycamores. Silver maples are not usually affected by anthracnose, but are affected by other leaf spot fungi (that's plural for fungus).
Anthracnose can affect three different ways on shade trees. The first way is by affecting small twigs. In this type of infection, small twigs are actually killed by the fungus. The second way is affecting buds. In bud infections, the buds are killed. The third way is infecting leaves. The leaf stage is the most common, and shows up as dead areas along the tips and edges of leaves or as dead spots between the leaf veins. As these dead areas get larger, the leaves fall to the ground.
There is no cure for anthracnose. Once the fungus has infected the tree, we just have to ride it out. We might have prevented the disease by protecting new buds and leaves before the infection, but that isn't very practical on very large trees. This would take a fungicide application covering the entire tree every 10 days or so from when leaves first expand from the buds, until nighttime temperatures stay over 65 degrees. Usually the first three sprays are the most important to protect new leaves.
That leaves us with good news and bad news. First, the bad news is the infections are just starting and will get worse. There is nothing we can do but keep the trees in good growing condition (water when dry and add a little fertilizer). We can continue to have leaves infected for quite some time. The good news is that rarely is the disease a killer on established trees. Generally the worst that happens is the loss of the small twigs if that stage was infected, or having a bare tree in your yard for a few weeks. I say a few weeks because generally when leaf drop is severe, a new set of leaves comes out within 4-6 weeks. What usually happens is it takes more energy to shoot a second set of leaves, so that is why the water and fertilizer can be very important.
The fertilizer rate for trees is essentially the same as for lawns. Supply about one pound each of nitrogen, phosphorus, and potassium per 1000 square foot of drip area. This means a tree with about a 30 foot branch spread would require about eight pounds of 12-12-12 or 13-13-13 broadcast around the entire area with branches above it. Of course if you've fertilized the yard, you've already fertilized the tree.
Maple leaf bladder gall is also abundant this year. The gall is yellow to red when it starts out, then turns brown as the year wears on. The cause is a mite feeding on the leaf. The leaf then swells in response to the injury and toxins put into the leaf. The "bumps" are actually leaf tissue. I usually compare it to you getting a mosquito bite. There is no treatment, since the swellings are actually leaf tissue, and there is no damage other than the unsightliness.