Seeing Spots (again)?
Recent Weather Favors Foliar Diseases on Trees
If you see leaves dropping from your maple trees and spots on the oak
leaves, or if the sycamore in the yard looks scraggly, blame it on a
repeat of the fungal attacks that hit New Hampshire trees last spring.
The excessively wet, cool weather during May set up the perfect conditions for a population explosion among a group of closely related fungi that cause diseases collectively known as anthracnose. Most common on maple, sycamore, ash, oak and dogwood, anthracnose may also affect linden (basswood), birch and hickory.
Symptoms of anthracnose
Leaf drop is common if prolonged cool
wet weather occurs while the new leaves are expanding—exactly the
conditions southern New Hampshire experienced during mid- to late May.
Other symptoms of anthracnose include dark, discolored areas along the veins or margins of the leaves, brown or black areas between the veins and leaf spots. Twig cankers and twig death are also common on sycamore and dogwood trees. Leaf drop and twig death often gives sycamore a scraggly appearance with tufts of leaves at the end of otherwise bare branches. Many trees that drop leaves in June will put out a second flush of scattered, large leaves.
Other leaf spot diseases prevalent
In addition to anthracnose, a wide range of other fungal leaf
spot diseases have appeared on lilacs, crabapples and many other trees
and shrubs this year. Because many of the same diseases were present last
year (thanks to similar weather last spring), dead twigs and fallen leaves
provided the source of infections for this year. Add in the wet weather
this year and viola! you basically have the perfect conditions
for a wide range of foliar diseases.
What to do
Although it may be alarming to see all the leaves on the ground
beneath your maple, or the spotted leaves on your crabapple, anthracnose
and leaf spots are generally considered a cosmetic or aesthetic problem
that rarely requires chemical control.
In fact, chemicals applied now for anthracnose and most leaf spots will do little to control the diseases. The symptoms now visible were caused by infections that occurred when the leaves were expanding, and these established infections can’t be “cured.” Rarely are fungicides warranted as protection from infection in the early spring.
In fact, spraying large trees is not only impractical and expensive, but also often unnecessary, since many healthy leaves remain on the tree. Fungicides are warranted only in severe cases where defoliation has occurred for three or more years. (One exception: anthracnose in flowering dogwood, which often requires fungicides for effective control.)
Management of anthracnose and leaf spots generally includes pruning infected twigs and cultural practices that improve tree vigor and prevent stress. These include fertilization after the leaves have fallen in the autumn or one month before the last frost in the spring will help maintain tree vigor. Provide supplemental water if rainfall is lacking for more than two or three weeks. Raking and removing leaves in the autumn helps control many of the leaf spots, but has minimal effect on anthracnose.
It’s important to remember that trees are resilient and leaf spots and anthracnose are just another part of the natural challenges Mother Nature exposes them to.
by Cheryl Smith, UNH Cooperative Extension Professor & Plant Health Specialist
