The Diagnosis
Dogwood Anthracnose
Dogwood Anthracnose is a disease caused by a fungus. Have you ever seen
mold on a piece of bread or on food left in the refrigerator too long? That
mold is another type of fungus.
There are two different type of anthracnose fungi on dogwoods. One type
is an undescribed species of a Discula fungus. The other, more deadly,
fungus is called Discula destructiva.
Left - Discula destructiva, a fungus believed to cause anthracnose.
Right - an undescribed species of Discula which also causes anthracnose.
Plants are able to survive because they contain chlorophyll
to make food. A fungus doesn't have chlorophyll so it can't produce its
own food. A fungus doesn't have a mouth like humans so it must have another
way of "eating."
Once the fungus is on the dogwood tree, it releases a substance that breaks
down the complex structure of dogwoods into something the fungus can absorb
and use for growth, energy, and reproduction. The fungus also releases toxins
that kill the leaves of the tree, can spread into the stem to infect new
limbs and can cause cankers.
How do you know if the dogwood has the killer fungus?
Researchers can remove a sample of the fungus from the tree. The photos
above show the difference between the fungi.
Another research tool is DNA amplification fingerprinting (DAF). DNA (deoxyribonucleic
acid) is the molecule containing the codes that pass characteristics from
one generation to the next. You have probably heard about the use of DNA
to identify both victims and criminals. The use of DNA to identify the type
of fungus on dogwood trees was developed at the University of Tennessee.
"DAF is like looking at the bar code on a coke can to borrow an analogy
from Dr. Peter Gresshoff. It tells you what that organism consists of,"
says Dr. Trigiano.
Bar code from a Pepsi can.
Bar code from a Diet Coke can.
Can you see the difference in the bar patterns? If you had a can of soft
drink with only the bar code and no label, you could tell what was in the
can by passing the bar code over a bar code reader.
This is the DNA of the fungus Discula destructiva. If you look
down these codes, they look pretty similar. That tells us that these examples
of fungus have many of the same genetic codes and probably came from the
same place.
The codes on this group of fungus are very different and show that these
fungi did not come from the same place.
Dr. Trigiano and the other researchers think "the fungus is an exotic
pathogen from outside the United States. For example, here in the United
States, if we took a random sample of DNA from a lot of different people,
we would see some of the differences in people because of where their ancestors
came from. If we went to an island where the same people had been living
for many generations, and took a random sample of DNA from those people,
their DNA would be very similar. That's why we think this organism originated
on both the East coast and the West coast."
Researchers are not sure how this fungus came into the United States.
"In Seattle, Washington, the researchers have evidence that the disease
spread both north and south in the range of the pacific dogwood. There was
also a central point in New York from which the disease spread both north
and south. Seattle and New York are both major port areas where a tremendous
amount of plant material is brought into the United States. It's very possible,
but there's no way to prove, that the fungus was brought into the country
on a diseased tree," explains Dr. Windham.
How bad is the disease?
In the 1980s, dogwood anthracnose swept through the Catoctin Mountain Park,
home to Camp David, in Maryland. By the time the disease had ran it's course,
very few dogwood trees were left in the park.
In 1990 at a nursery in middle Tennessee, nearly 344,000 trees in one field
were infested with the disease. The nursery isolated and condemned the field
before allowing the Tennessee Agricultural Experiment Station to use the
diseased trees in a fungicide trial.
