Hi I'm Damon Smith, Extension field crops
pathologist for the University of
Wisconsin-Madison. Today we're out here
in a field of dent corn and we're
looking at some pretty significant
disease here. We're noticing quite an
epidemic of spot tar. Tar spot is a new
disease of field corn here in the state
of Wisconsin. Tar spot is pretty new to
actually the entire US, being first found
in Indiana in 2015. Since that first
find in Indiana, we have found tar spot
the last several seasons in 2016 and
2017 and Wisconsin. However it hasn't
been a very significant disease, usually
just causing some blemishes on the
leaves and having little impact on yield.
In 2018 however that situation has
changed.
We now have pretty significant disease
as you can see here with this particular
field and we're studying the impacts and
trying to understand why. Tar
spot is caused by a fungus called
Phyllachora maydis. In Latin America
it's been known for some time down there,
however it usually occurs in combination
with another fungus called M. maydis.
In the U.S. we don't know
if we have M. maydis. We
are actively trying to understand if it
is here given the severity of symptoms
we see. The symptoms of tar spot are
actually quite interesting. You can see
as the disease first starts on the green
foliage here you get this sort of tar
splattered appearance as the disease
progresses of entrance it then converts
into more of an eye spot type symptom
that you can see here where we get some
necrosis around that that tar spot
lesion now if you look at the literature
from Central and South America we know
that in order to have that significant I
spot lesion or have actual necrosis we
have to have that second fungus
monograph Ella made us present we don't
think again that that particular fungus
is here in the US but it is interesting
that we're noticing this type of damage
on these particular leaves as symptoms
progress past the the tar spot
appearance on the leaves and into that
high spot symptom eventually we get to a
situation like this where we have
complete death of the plants and early
senescence of the plants this is a
result we think mostly of the damage
by a tar spot maybe in association with
other fungi however in the process of
trying to work that out and understand
the biology around this particular
disease now as we move forward into the
harvest time it's important to scout
fields look at the severity at our spot
because we're noticing that as these
fields prematurely dry we're also
noticing loss and stock integrity there
for lodging and yield reductions from
lodging will also be a problem by
confirming fields hit hard by thar spot
you can harvest those fields first and
try to get on top of it before
significant lodging takes place in those
particular fields as you chop silage
it's important to keep testing for
mycotoxins we are also noticing more ear
rot as these plants are stressed by thar
spot more iraq may lead to higher levels
of mycotoxin such as vomitoxin the only
way to know what levels of vomitoxin you
actually have there or other mycotoxins
is to simply test
there are many labs here in in the US
that can test for these micro toxins and
we've included a table in our a 3646
publication table 2-16 which gives a
list of certified labs that can test for
mycotoxins in terms of the actual
effects on yield and quality we're not
exactly sure what will actually happen
here however it is safe to assume that
will likely at least see losses and
tests wait however as farmers have
started to cut corn here in southwestern
Wisconsin
we're actually noticing pretty
significant yield reductions and also
off test weights and again this is a
result of the fact that these corn
plants are basically senescent too early
and can't finish filling out those ears
we're also looking at fungicide
applications and their possible efficacy
against our spot we know very little
about fungicides and at this time we
can't make any clear management
recommendations harbor's you move
forward and prepare for the 2019 field
season it's important to visit those
hybrid trials make some plans to look
for hybrids with resistance as I think
we will be dealing with thar spot in the
2019 field season we do believe that the
Thar spot fungus can overwinter here in
the Upper Midwest so being prepared for
2019 will be extra
important for more information about our
spot or other corn diseases please visit
badger crop dot-com or contact your
local Extension agent
you