 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.
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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 four to six 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 1 pound each of nitrogen, phosphorus and
potassium per 1,000 square feet of drip area. This means a tree with
about a 30-foot branch spread would require about 8 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 the toxins put into the leaf. The "bumps"
are actually leaf tissue. I usually compare it to a person getting a
mosquito bite. There is no treatment, since the swellings are
actually leaf tissue, and there is no damage other than the
unsightliness.
[By
JOHN FULTON,
University of Illinois Extension]
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