March 2022 - 21

CUCUMBERS
Advances made against cucumber downy mildew
By Dean Peterson
VGN Correspondent
Downy mildew is the disease to beat
in pickling cucumbers, and there are
three questions to ask:
When do you begin fungicide
applications?
What fungicide should be used?
Once a fungicide is sprayed, how
many days between sprays?
Cucumber downy mildew doesn't
overwinter in Michigan or other areas
that experience a killing frost. Knowing
when the sporangia arrive by air
currents from the southern states or
from northern cucumber production
greenhouses is critical to know when
Michigan growers should begin their
fungicide programs.
" Downy mildew is driven by weather
and weather conditions, " said Mary
Hausbeck, Michigan State University
(MSU) vegetable pathology professor
and Extension specialist. Hausbeck
talked about downy mildew during the
pickling cucumber session at the 2021
Great Lakes Fruit, Vegetable & Farm
Market EXPO held recently in Grand
Rapids, Michigan.
There have been some new advances.
MSU had 13 stations in its spore
trapping network in 2021. The stations
were located in the cucumber production
areas of the state and the results posted
on MSU's downy mildew, spore trap
webpage.
The spores of the downy mildew
pathogen that infects hops is also
being monitored. Hop downy mildew
is closely related to cucumber downy
mildew, and the spores of these
pathogens appear identical under a
microscope.
Hop downy mildew overwinters in
dormant buds or the crown of the hop
plant and the spores of this pathogen
can be in the air early in the spring.
Since the spores of hop downy mildew
and cucumber downy mildew cannot be
distinguished from each other visually,
it's possible that an early detection of
hop downy mildew spores would be
mistaken for the arrival of cucumber
downy mildew spores and trigger an
earlier than needed fungicide program.
Molecular tools are now used
exclusively in Michigan to conclusively
identify cucumber downy mildew
spores from hop downy mildew
spores collected from the spore traps
throughout the season. Using these
tools allows the detection of even a low
number of spores in the air.
" We are able to detect cucumber
downy mildew spores before disease
symptoms are detected in the field, "
Hausbeck said.
MSU researchers use Burkard
volumetric spore traps to monitor
cucumber downy mildew spores in the
air. The machine is outstanding, but is
not always the easiest to use.
" The goal is to move toward a
grower-based tool, " Hausbeck said.
Forecasting models - such as
TOMCAST for use on tomato (early
blight), but adapted for use on
asparagus (purple spot) and carrot (leaf
blight), Blitecast for potato and tomato
(late blight), and DMCAST for grape
(downy mildew) - have potential for
use with cucumber downy mildew.
" Should spray intervals be five, seven
or 10 days? " Hausbeck asked. " These
programs may be able to help determine
that. "
A robust fungicide program that
alternates among different fungicides
is required to control cucumber downy
mildew. Each year, MSU conducts
extensive field trials to assess which
fungicides are effective and should
be used by the state's growers. The
cucumber downy mildew pathogen is
highly adaptable and has developed
resistance to many fungicides over the
last 15 years.
MSU researchers have conducted
product trials and rated the amount of
foliar infection.
" At the end of the season, Orondis
Opti had near perfect control, "
Hausbeck said. " This trial used a highly
susceptible cucumber cultivar and was
conducted for an extended time period
under high disease pressure, so the
results are especially impressive. "
Previcur Flex, Ranman and Omega
were also highly effective in MSU's
2021 trial. Zampro and Elumin also
performed well in Michigan's 2021
field trial, but offered more moderate
control.
A downy mildew-specific fungicide
must be tank mixed with a fungicide
that has multi-site activity to delay the
development of pathogen resistance.
These products include Gavel, Zing!,
Bravo WeatherStik and Koverall.
The preharvest interval (PHI) is an
issue with some fungicides that are
effective against cucumber downy
mildew. An example is Omega, which
has a PHI of 30 days.
A seven-day spray interval can be
standard, but there can be instances
when a shorter or longer spray interval
would be appropriate.
" There have been times when a
seven-day interval has not been
adequate because the weather was
highly favorable for disease, " Hausbeck
said, " but how do you know when a
five-day interval is needed to maintain
control? When weather is less favorable
for the pathogen, a 10-day interval may
be sufficient. "
MSU recently conducted a field trial
to determine whether some of the
forecasting programs in use for other
crops may be useful for cucumber
downy mildew. The programs tested
included TOMCAST, Blitecast and
DMCAST. Each of these programs
analyzes various weather conditions to
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determine whether the risk of disease is
low, medium or high, and if a fungicide
spray is needed. The MSU research field
tests sought to find out if any of these
programs could help to determine the
number of days between sprays for
cucumber downy mildew.
The models were evaluated under
different disease severities and
risk levels, and were compared to
fungicide sprays applied according
to a seven-day and 10-day calendar
schedule. The models' performances
were acceptable in protecting
cucumbers from downy mildew,
although Blitecast and DMCAST did
better than TOM-CAST, but more
work on the models needs to be done.
" It makes sense to use them, "
Hausbeck said, " if they can tell us how
often to spray and can save growers
money. "
In the meantime, the plan is clear.
" Is the weather favorable for the
pathogen? Then you need to spray, "
Hausbeck said. VGN
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March 2022

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