My name is Dr. Terry Spurlock,
I'm an Extension and Research Plant Pathologist
with the University of Arkansas System
Division of Agriculture.
And today, we're at Lon Mann Cotton Research Center
in Marianna, Arkansas,
talking about corn disease I.D. and management.
So, I happen to be standing in the middle
of a corn research field.
These are in plots.
This is a population study
done by Dr. Jason Kelley's lab,
so I appreciate Jason lending me some space to do this
disease I.D. and management talk today.
Corn diseases in Arkansas are always a hot topic,
especially for us extension specialists receiving calls
during the year about: Do I spray?
When do I spray?
How much disease should I have before I spray?
And typically, that's a conversation that we like to have,
but we'd also love to have the conversation beforehand
about planning for disease management,
especially foliar disease management,
before a seed ever goes in the ground.
So, some important fundamental things to understand
about plant disease management, in general,
are that to have disease,
we have to have a susceptible host.
Meaning a susceptible host plant,
the pathogen has to be present.
So, one of the diseases that we talk a lot about
in the state of Arkansas on corn is southern corn rust
caused by a fungus called Puccinia polysora.
Now, that fungus is an obligate parasite.
And so, when we say obligate parasite,
what we mean is it must exist on living plant tissue,
and it doesn't overwinter here in the state of Arkansas.
So it has to be blown in.
And so, there are some management considerations
with respect to this particular disease.
Also, the last and most important aspect of disease
management is the environment.
And the environment has to be conducive
for disease development.
This is the completed disease triangle.
So by the time southern corn rust happens to blow in
from the south, Louisiana, Texas, where it typically does
overwinter, where it's a warmer climate,
maybe there's earlier planted corn and the spores can
infect, and then produce more spores that produce
what we would refer to as inoculum,
that blows north to our area.
The environment has to be conducive
for disease development.
Now, we actually have some southern rust here,
along with some other diseases in this field that I'm in.
So, the leaf I'm holding in my hand is southern rust,
and southern rust has sort of a
bright orangey-red appearance.
This leaf actually came from just below the ear leaf.
This time of year, southern rust is quite present in a
cornfield that's finishing up.
What's diagnostic about southern rust
and different than common rust,
besides, what you would see under a microscope, is
that southern rust, one has a brighter orange color
than common rust,
which is darker maroon or burgundy. And southern rust
most often is only on one side of the leaf.
Where on the backside of the leaf,
we don't see any pustules and spores,
but on the on the front side, we would see pustules
bearing spores to be disseminated.
We recently had a grad student finish a project that was
funded by the Arkansas Corn and Grain Sorghum
Promotion Board, where the grad student —
a master's student named Justin Bailey,
actually mapped the spatial distribution of southern rust
in some fields in Arkansas.
And what we found was that Southern rust tended to
build preferentially in areas that had a denser
corn canopy.
As an example, the image on the left represents the
distribution of severity of southern rust
in a field at about R4
The image at right is a normalized difference
vegetation index calculated from a multispectral sensor
mounted on an airplane and captured
about a month earlier.
These distributions agree suggesting southern rust
followed a spatial pattern of canopy density.
And so we're not exactly sure why,
but there's this idea that it's somewhat protected and
that possibly if you sought out these areas
to scout corn,
this might be preferential for your scouting and also
might make you more efficient in the scouting process.
So to sum up, what we've talked about today is there's
numerous corn foliar diseases in corn that we see every
year in Arkansas.
But, typically the most important foliar disease
that we see is southern rust.
And we want to make sure of a few things.
One, that we plan for scouting for southern rust
during the year.
And we know when we should scout for southern rust
and when it's going to be important to control southern
rust with a fungicide if need be.
That's R3 or earlier.
Second, we also want to understand what southern rust
looks like relative to a similar looking disease
called common rust and not confuse the two so that
we're not spraying common rust
thinking it's southern rust.
A list of fungicides labeled for southern rust
can be found in the MP154 publication.
along with an efficacy chart indicating which products
might be best to control that disease and others.
And lastly, we always want to understand that the
extension specialists are here to help you.
So give us a call should you have questions about
plant disease identification.
We also have a disease diagnostic clinic in Fayetteville
and samples can be sent there.
But also questions about how much southern rust
am I seeing and when do we need to apply a fungicide
or if we need to apply a fungicide
to control that southern rust?
Thank you.
