Tom Ingram: Ask A Master Gardener
Sunday, August 2, 2020
Q: I have an American Elm that started looking
like it had a problem and then died a few weeks later. What in the world
happened? JA
A: The culprit was likely Dutch elm disease. I was speaking
with Jen Olson of the OSU Plant Disease & Insect Diagnostic Lab recently
and she said she was seeing more Dutch elm disease this year than in recent
years, which is too bad because it’s one of the most destructive tree diseases
in North America.
Dutch elm disease was first discovered
in the Netherlands in the early 1900s, but it didn’t take long for it to make
its way to the U.S. It arrived around 1930 on beetles who were hitching a ride
on some logs headed our way to make furniture.
Quarantine helped control the disease
until 1941, but the nation then became more focused on fighting a war. Some
estimates suggest there were approximately 77 million elms in North America in
the early ’30s. By 1989, more than 75% of those trees were lost.
Dutch elm disease grows in the xylem of
the tree. The xylem is the tissue that helps bring water up from the roots
throughout the entire tree. You typically start to see evidence of Dutch elm
disease in the upper branches with leaves gradually browning, then yellowing
and eventually getting dry and brittle.
When adult elm bark beetles emerge from
under the bark of infected trees in the spring, they are covered with the Dutch
elm disease fungal spores. They look for tender young bark on healthy trees to
feed upon. Their feeding leaves wounds that become great places for the Dutch
elm disease to take hold.
If you suspect your elm may have Dutch
elm disease, you can cut off a branch about 1 inch in diameter from an actively
wilting section, peel back the bark and look for discoloration similar to that
in the photo (the dark strips). To be sure, you can bring a sample to your
local Extension Office, and we will send it off to OSU for positive diagnosis.
If you catch the disease early, the
infected areas can be pruned out, but you will need a minimum of 8 to 10 feet
of un-infected, streak-free wood below the infected areas.
Fungicides and insecticides are
available to help prevent Dutch elm disease; however, these chemical treatments
should be applied by a licensed arborist because a fungicide may need to be
injected into the tree. You can find a licensed arborist for our area at treesaregood.org.
There are a variety of elm species that
are resistant to Dutch elm disease, but none is immune. Resistant varieties
include Siberian elm, Chinese or lacebark elm, “Valley Forge,” “Princeton,”
“New Harmony” or “American Liberty’.”
Garden tips
- Now is the time to divide and
replant crowded hybrid iris (bearded iris). When planting, take care not
to plant the rhizomes too deeply. Cover them with an inch of soil or less.
Do not mulch iris.
- Water all plants deeply and
early in the morning. Most plants need approximately 1 to 2 inches of
water per week. Rather than watering daily, water less often and more
deeply.
- Some trees, such as sycamores
and river birches, lose large numbers of leaves in the heat of summer.
Trees do this to reduce water loss from their leaves. It is a coping
action by the tree; it is not dying.
