October 2020 - 10

MSUE VEGETABLE NOTES
Tool detects cucurbit pathogen in air samples
By Mary Hausbeck
Michigan State University
Five years ago, a team of Extension
educators and researchers, growers,
Pickle Packer International and other key
stakeholders came together to develop
much needed solutions to downy mildew
diseases affecting cucumber, pumpkin,
melon, squash, basil and spinach.
This effort was led by Michigan State
University and included researchers from
North Carolina, Florida, California, New
York, New Jersey and others. By working
together, we've developed and tested an
advanced tool to accurately and quickly
detect cucurbit downy mildew in field air
samples. Downy mildew of cucurbits has
been a focus of my lab and we've combined
laboratory and field aspects to monitor the
arrival of downy mildew into cucumber
production regions as one way to know
when the crop needs fungicide protection.
Even a minor delay in spraying fungicides
can result in foliar blighting (Fig. 1) that
cannot be easily stopped, especially during
wet and rainy weather.
Prior to 2004, the major cucumber
cultivars had genetic resistance to downy
mildew which protected the crop and
fungicides were not needed. In 2004, the
downy mildew pathogen re-emerged
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Figure 1. Cucumber DM leaf: Downy
mildew disease causes rapid blighting
and leaf death of cucumbers.
Photos: Michigan State University
The cucurbit downy mildew pathogen
doesn't overwinter in regions that
experience a hard frost, and it wasn't
certain that it would become a yearly
problem for Michigan's farmers. It soon
became clear that cucurbit downy mildew
was going to pose a threat each year. For
the last 15 years, it has come into Michigan
fields via air currents at different times
during the growing season. Some years,
the pathogen comes in relatively early for
northern growers (June) and other years it
occurs later (August).
Historically, the cucumber crop has
not needed to be protected with downy
mildew fungicides. Knowing when the
in the U.S. and was able to infect all
cucumber cultivars. Researchers and
growers have scrambled to stay ahead of
this destructive pathogen ever since.
downy mildew pathogen has " arrived "
in the state could be very useful so
that sprays can be applied when they
are needed based on field air samples.
Timing fungicides based on the field air
samples could save growers money in
years when the pathogen arrives late in
the growing season.
Prudent use of fungicides is also
important to delay resistance to
fungicides from developing in the
pathogen. When the downy mildew
pathogen arrived, it was already armed
with resistance to several important
fungicides including mefenoxam
(Ridomil Gold is an example) and the
strobilurin fungicides (Quadris and
Cabrio are examples).
Without cucumber cultivars with
superior genetic resistance to this " new "
downy mildew pathogen, fungicides
have been an important tool that farmers
have had to protect the crop. Downy
mildew reproduces at very high levels
and is known to develop the ability to
overcome fungicides.
It was not too surprising that after
a few years of intensive fungicide use,
additional fungicides were added to
the " no longer working against downy
mildew " list and included cymoxanil,
cymoxanil + famoxadone (available as
Curzate and Tanos, respectively) and
fluopicolide (available as Presidio).
Today, some of the downy mildew
Equipment that makes dollars & 'sense'
fungicides show variable results across
major cucumber growing regions in the
U.S. which means that recommendations
from some regions of the country may
not be appropriate for all production
regions. With our team of researchers,
we coordinate the fungicide testing
across different production regions to
ensure that the growers have the best
information for their farms. Further,
fungicide programs needed to protect
cucumbers from downy mildew may
be different than those for squash
and pumpkins. To make the situation
even trickier, Michigan research has
highlighted variable fungicide results year
to year for the propamocarb fungicide
(available as Previcur Flex).
For years, this fungicide was a mainstay
of our control recommendations but then
it became clear from our field studies
that it was no longer working well for
Michigan growers so it was replaced by
other downy mildew fungicides. It was
surprising when propamocarb (available
as Previcur Flex) proved to be effective
again during the last couple of years.
Now, the 2020 field research indicates
that Previcur Flex is not providing good
control against downy mildew.
Tackling cucurbit downy mildew has
not been easy! We've used spore trapping
devices (Fig. 2) in Michigan's pickling
cucumber fields for more than a decade.
It seemed clear that for Michigan growers,
detecting the pathogen's arrival in field air
samples coupled with effective fungicides
was needed for our pickling cucumber
industry which ranks No. 1 in the U.S.
Burkard spore traps are considered the
" Cadillac " of spore traps and can sample
the air for 7 days.
Reading the tapes from the spore traps is
labor intensive and requires identifying the
downy mildew spores based on their size
and shape using a microscope (Fig. 3) and
a small army of students. Improving this
particular system with new molecular tools
was possible with the efforts of Chris Smart
(Cornell University), Lina Quesada (North
Carolina State University) and Frank
Martin (Agricultural Research Service,
California). This team provided us with the
new molecular biology tools that we tested
in our spore trapping system established in
several Michigan counties.
Figure 2. Spore trapping: The Burkard spore
trap samples the air and is used to detect
downy mildew spores in cucumber fields.
In 2018 and 2019, the spore trap
results were obtained using molecular
tools and were compared to the results
using light microscopy. In 2020, we
moved wholly to these molecular tools
to process the spore trap results. The
samples were processed within one day
after the spore trap tapes were returned
to the lab and required fewer students
than using light microscopy. The results
were posted on the lab's webpage
immediately to inform the growers.
For each field location, airborne downy
mildew sporangia were detected in the
field prior to the disease symptoms.
Figure 3. Cucumber DM magnified: The
spores of the downy mildew arrive to
Michigan production fields via air currents.
An important benefit of this system
is increased accuracy. Cucurbit downy
mildew sporangia look exactly like the
hop downy mildew sporangia as far as
size and shape. The molecular tools " look "
at the genetics of the two downy mildews
and can tell them apart. This is a key
factor as the hops acreage continues to
increase in Michigan and that particular
downy mildew successfully overwinters in
the plants rhizomes.
This project was supported, in part,
by funds from the National Institute of
Food and Agriculture, USDA, under
award number 2015-51181-24285,
the Agricultural Research Fund of
Pickle Packers International Inc. and
the Michigan State University Project
GREEEN GR15-020. VGN
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October 2020

Table of Contents for the Digital Edition of October 2020
