June 2019 - 14

MSUE VEGETABLE NOTES
Growers target downy mildew in cucumbers
Figure 1: Yellow/brown angular downy mildew symptoms on left, upper side of a
cucumber leaf, and, right, dark spore masses of the pathogen on the underside of a
cucumber leaf.
By Mary Hausbeck
Michigan State University
Downy mildew pathogens are highly
destructive and threaten several high
value crops. The cucurbit downy
mildew pathogen infects cucumbers,
squash, pumpkin, cantaloupe,
watermelon and other cucurbits.
Cucurbit downy mildew is different
from the downy mildews that affect
spinach, basil, broccoli, sunflower and
other ornamentals. Downy mildew
became a priority for Michigan's
pickling cucumber industry after the
pathogen marched across the state in
August 2005, leaving blighted fields
(Figure 1) in its wake. Since that
time, the Michigan State University
Vegetable Pathology Lab has focused
on early detection and control
strategies for cucumber producers.
The microorganism responsible
for downy mildew is the fungallike
pathogen Pseudoperonospora
cubensis. Since this pathogen only
infects and colonizes living plants,
it cannot survive in any location
that experiences a frost that kills its
cucumber host. The downy mildew
pathogen can survive a Michigan
winter on infected cucumbers or
other cucurbit crops growing in
heated greenhouses. Production
regions in the southern U.S. or
Mexico that grow cucumbers or
other cucurbits year-round and do
not experience a hard frost are also
safe harbors for the downy mildew
pathogen. The production of large
quantities of spores which gives a
" dirty " appearance on the underside
of infected leaves helps to identify
the disease in the field (Figure 1).
It is these same spores that can
be carried by air currents to new
plantings, making this problem one
that affects all cucumber producers
in a growing region.
For the best results, fungicides must
be applied to susceptible crops before
disease develops in the field. Spore
trapping is used in Michigan to detect
when the downy mildew spores arrive
via air into the state's key production
areas. The spore counts are posted
for each monitored site along with
field scouting reports (www.veggies.
msu.edu/downy-mildew-news, www.
downymildew.msu.edu). Growers
can decide when to initiate fungicide
sprays based on whether downy
Figure 2: Results of products applied for control of downy mildew on cucumber Vlaspik.
mildew disease has been detected
on cucumbers in the state and/or if
airborne spores have been detected
via our trapping network. Our lab
works closely with growers and scouts
and serves to verify downy mildew
suspect samples to ensure accurate
information is posted.
Fight the blight
Downy mildew pathogens are
known to quickly overcome fungicide
chemistries. When cucurbit downy
mildew first arrived in the state in
2005, the pathogen was already
resistant to several important
fungicides. Some additional fungicides
that had been effective in recent
years are no longer working well
in Michigan. If downy mildew
is not controlled early, yield and
quality suffer. Keeping the spray
recommendations current has been
challenging because the cucurbit
downy mildew pathogen continues to
change and overcome some of the most
effective fungicides. Each year, my lab
runs extensive season-long field trials
that form the basis for the next season's
recommendations for Michigan's
cucumber growers.
Our field research identifies the
effective fungicides that can be
included in a control program. This is
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June 2019

Table of Contents for the Digital Edition of June 2019
