March 2020 - 31

ONIONS
microbe biology. " The development
of fungicide resistance means
growers are faced with a rapidly
reducing set of fungicides with
which to rotate, leading to increased
selection pressure for resistance in
the fungicides remaining. "
The increase of fungicide
resistance is a major reason why SLB
has recently become the dominant
foliar pathogen in onions. The
disease first appeared in New York
in 1985, but only caused significant
damage during the 1990, '91 and '92
growing seasons, before re-emerging
in 2011.
That's when study co-author Christy
Hoepting, an Extension vegetable
specialist at Cornell Cooperative
Extension in Orleans County, started
receiving calls from concerned
growers about premature leaf death
and smaller onion bulbs. She realized
the symptoms were similar to SLB,
so she sent some field samples to
Pethybridge and Frank Hay, senior
extension associate in the School of
Integrative Plant Science.
" Our first step was to look at
the pathogens and determine what
the main diseases were, " said Hay,
the study's lead author. " We were
surprised to see stemphylium leaf
Onions affected by Stemphylium leaf blight. Photo: Sarah Pethybridge/Cornell University
blight disease because it had been
seen as a minor disease of onion
compared to others. "
In the laboratory, Pethybridge
and Hay grew samples of the fungus
on artificial media and exposed
them to varying concentrations
of each fungicide. By evaluating
how different fungicides inhibited
the pathogen's growth, they could
determine which fungicides were
most effective against SLB.
Fungicides are organized into
groups depending on which cellular
process they attack within the fungus.
Laboratory and field trials demonstrated
that stemphylium had developed
geographically widespread resistance to
at least two groups of fungicides, and
that other fungicide groups were at risk
of developing resistance.
Growers quickly adopted
guidelines to reduce the onset of
fungicide resistance, including
limiting the number of applications
of any particular fungicide group
each season, and rotating between
fungicides groups.
" Managing stemphylium with
fungicides is like whitewater rafting:
The effective fungicides keep falling out
of the boat as we negotiate the rapids, "
Hoepting said. " Our work will prevent
crop losses in a high-pressure year
because we will ensure that growers will
be using fungicides that work instead of
those that SLB is resistant to. "
The researchers plan to develop
molecular and genetic tools that will
help detect fungicide resistance faster,
and they hope to identify a disease
prediction model to accurately guide
growers' fungicide use.
" Christy and her colleagues have
been instrumental in helping us
determine what concentrations are
still effective, " Mortellaro said, " and
to help us strategize our fungicide
use so we don't lose these materials
as time goes on. "
Collaborators included
postdoctoral researcher Sandeep
Sharma and graduate students David
Strickland and Karen Luong, all
from Pethybridge's lab.
This research was supported by
funding from the USDA National
Institute of Food and Agriculture, a
USDA Specialty Crop Block Grant
and the Federal Capacity Fund
Initiative. VGN
Expect
More Than
Great Seed
VGN | MARCH 2020 | 29
March 2020

Table of Contents for the Digital Edition of March 2020
