This was originally written in June 2019, when a certain soybean pest was first discovered. I am reposting this blog here.
Have you ever seen something that was actually something else? Or, rather, have you ever thought you saw something, but it actually was something else? Not necessarily as extreme as the whole Baby-Ruth-in-the-swimming-pool drama (what movie was that anyway?), but in life, we seem to come upon things, or people, or events, or places that seem to be what they’re not. Early in my science classes, I learned about the “false advertisers” of the caterpillars that have spots to pretend to be snakes, the viceroy butterflies who mimic the markings of the poisonous monarch butterflies, or the stick insects that look like twigs and leaves that even look like they sway in the wind!
I have learned that not everything is what it
seems. That is why, in agronomy and in
the field, I have to look twice to identify those imposters.
Probably the most common example of mistaken
identity in corn is misidentification of purple leaf sheath as physoderma brown
spot. Later in the growing season, as
the stalk widens, the leaf sheath tends to collect particles. When moisture and heat react with these
leftover dust, pollen, and “trash” particles, they discolor the leaf sheath,
leaving a benign purplish, blackish spot on the top where the leaf is
connected. Often, this is misidentified
as physoderma brown spot, a fungus which can develop into a stalk rot. Purple leaf sheath isn’t a disease—it’s just
a funny looking spot similar to a mole on your arm.
Sometimes, when a new disease is found in the area,
people begin to “see” it everywhere.
Recently, bacterial leaf streak was identified in corn. This is a bacteria that causes wavy leaf
lesions that are yellow in color when backlit.
It can look like other diseases that cause leaf lesions, especially grey
leaf spot. Unfortunately, grey leaf spot
is a fungus and controlled by fungicides, which will not control bacterial leaf
streak. Sometimes corn leaf blotch miner
feeding can look like bacterial leaf streak because they feed on the juicy
inside of the leaf and leave the outside film.
Other times, sun scald or wind damage can look like leaf lesions from a
disease.
Another disease that’s new to our area is tar
spot. This is one that looks similar to
our common or southern rust, but there are slight differences. A rust is called that because it “rubs off”
when you scrape at the lesions. Tar spot
does not. The spots are usually dark in
color, and not the rusty color that common and southern rust start out at,
either.
And, last, but certainly not least, was the one that
baffled me last year. I was near Pender
in a soybean field. This grower had
quite a bit of flooding earlier in the year, and had lost some of his field
from that damage. The growing season
turned into a hot one. This brought
quite a bit of disease to the corn, which didn’t surprise me. I expected some disease to show up in his
soybeans as well. That is why, when the
edges of two of his other fields which were close together showed symptoms of
what seemed to be phytopthora, it didn’t alarm me. The patches of droopy, dying plants were just
one more thing for this poor guy to have to treat. However, when I went to check on the soil and
root system, the roots weren’t brown and rotting like the rest of the plant! This isn’t classic phytopthora at all! My puzzlement was alleviated this winter at a
conference when I discovered the clever little soybean gall midge, a new
species discovered in the area affecting soybeans. The larvae of this little striped fly burrows
into the soybean stem, killing the plant and causing symptoms that look like
phytophthora without the root rot.
Huh. One more imposter to put on
our list and remind us why we always need to keep our eyes peeled and our minds
sharp!
Monday, June 3, 2019
Julie S. Paschold