January 2020 - 24

SWEET CORN
Midwest growers fight bacterial leaf streak
By Dean Peterson
VGN Correspondent
Tar spot captures the most attention
as a new foliar disease in corn in the
Midwest, but bacterial leaf streak may be
as big of a concern to sweet corn growers.
Bacterial leaf streak can infect plants early,
has alternate hosts, can't be controlled
with fungicides - and sweet corn is more
susceptible.
" In Minnesota, it's a bigger problem
in sweet corn than in field corn, " said
Dean Malvick, professor in the University
of Minnesota's Department of Plant
Pathology. " We have seen high levels of
damage in sweet corn. " Minnesota grows
more than 100,000 acres of sweet corn with
two-thirds of it going to processing.
Bacterial leaf streak was first positively
identified in the U.S. in Nebraska in 2016
and the worst damage there has been in
popcorn. The disease became progressively
worse in 2017 and 2018 and spread into
Oklahoma, Texas, Kansas, Colorado, Iowa,
Illinois, South Dakota and Minnesota.
But 2019 was a little different. " We didn't
see much this year in Minnesota, " Malvick
said. " It seems that the weather patterns
that favor it weren't here this year. "
Part of the problem in some states is
bacterial leaf streak is easily misidentified
as gray leaf spot.
crop residue. Rotation is a control, but
resistant varieties may eventually be the
long-term solution.
There are varietal differences in
susceptibility to bacterial leaf streak
and multiple sweet corn varieties are
being evaluated at the University of
Minnesota's Southern Research and
Outreach Center at Waseca.
All of the varieties tested in 2019
were inoculated with the disease at the
V4 stage. This produced disease levels
high enough to cause significant yield
losses in some varieties.
" We're trying to determine how to
Bacterial leaf streak in sweet corn appears as yellow, tan or brown lesions on the leaves.
The lesions can be small flecks or up to several inches long and have jagged or wavy
margins. Photo: Dean Peterson
Both diseases appear as yellow, tan or
brown lesions on the leaves. The lesions
can be small flecks or up to several inches
long. However, the lesion margins of
bacterial leaf speck are jagged or wavy
while the margins of gray leaf spot lesions
are usually smooth.
Lab tests are the only way to positively
identify the foliar symptoms as bacterial
leaf streak. The disease isn't controlled by
the fungicides sprayed for gray leaf spot.
Bacterial leaf streak can infect sweet
corn as early as the V4 stage. With an early
start, there's greater potential for damage.
Bacterial leaf streak has several alternate
hosts. The list includes green foxtail,
shattercane, yellow nutsedge, certain
varieties of orchardgrass, oats, and timothy,
certain varieties of warm-season, native
grasses and a few others. This makes good
weed control in rotation crops essential.
Bacterial leaf streak of corn has been
reported in South Africa and more recently
in Argentina. The source of the outbreaks
in Nebraska and across the Midwest is
unknown. " We don't know where it came
from or how it got here, " Malvick said.
The disease survives on infested
inoculate plants the most effectively in
the field, " Malvick said. " We inoculated
at V4. Sweet corn is clearly susceptible
at that stage. " The research is also trying
to assess bacterial leaf streak's effect on
yield and quality.
However, the disease has a high
degree of variability. Sweet corn varieties
inoculated with the disease at the
University of Minnesota's Rosemount
Research and Outreach Center showed
no infection. Rosemount is about 60 miles
from Waseca. With such variability, the
exact conditions that favor the disease are
hard to determine.
" There are a lot of questions, " Malvick
said. " There's a lot we don't know yet about
bacterial leaf streak of corn. " VGN
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