December 2020 - 30

MSUE VEGETABLE NOTES
Fungicides help onions against stemphylium leaf blight
Fungicide Programs*
By Doug Higgins and Mary Hausbeck
Michigan State University
1
Onions are tough plants, struggling
to hold on to their green leaves for
as long as possible striving to reach
maturity. Those that do not make it die
trying - their foliage dies with their
boots on.
Spots and blights on the leaves can
get in the way of good onion growth.
In Michigan and other Eastern
production regions, producers wage
a yearly battle to protect onions
from various foliar diseases. Purple
blotch used to be a common disease
of onion leaves in the Northeast
U.S. but stemphylium leaf blight has
now become the most formidable
opponent for many onion producers.
Stemphylium leaf blight is caused by
a fungal pathogen of the same name
(Stemphylium vesicarium).
Early symptoms of stemphylium
leaf blight can be difficult to diagnose.
Tip burn may be associated with
herbicide damage but can also be a first
symptom of disease. The brown spots
of stemphylium leaf blight are small
at first but increase in size, extending
down the leaf (Figure 1). These
elongated lesions can appear similar
to symptoms of bacterial stalk and leaf
blight. An accurate diagnosis can be
obtained by sending plant samples with
symptoms to a university diagnostic
lab. In healthy onions the oldest onion
leaves die first. However, premature
leaf death resulting in a mid-season
loss of green leaf tissue is typical of
stemphylium leaf blight. Without
healthy foliage, bulb size is reduced
and so is total yield.
No
Fungicide
Bravo WS,
-alt- Miravis Prime,
-alt- Bravo WS,
-alt- Miravis Prime,
-alt- Bravo WS,
-alt- Miravis Prime,
-alt- Bravo WS,
-alt- Bravo WS,
-alt- Bravo WS.
*All fungicides were applied at labeled rates.
Luna Tranquility,
-alt- Tilt + Bravo WS,
-alt- Omega,
-alt- Luna Tranquility,
-alt- Tilt + Bravo WS,
-alt- Miravis Prime,
-alt- Omega,
-alt- Aprovia Top,
-alt- Miravis Prime.
Bravo WS,
-alt- Bravo WS,
-alt- Bravo WS,
-alt- Bravo WS,
-alt- Luna Tranquility,
-alt- Bravo WS,
-alt- Luna Tranquility,
-alt- Bravo WS,
-alt- Luna Tranquility.
Bravo WS,
-alt- Bravo WS,
-alt- Bravo WS,
-alt- Bravo WS,
-alt- Miravis Prime,
-alt- Bravo WS,
-alt- Miravis Prime,
-alt- Bravo WS,
-alt- Miravis Prime.
2
3
4
5
Figure 2. The percentage (%) of green tissue on Aug. 24, below and yield, above, in onions treated with fungicide programs (2 -5) and no
fungicide treatment (1). See table for fungicide programs.
leaf blight pathogen is resistant to
fungicides classified as strobilurins
such as azoxystrobin, picoxystrobin,
pyraclostrobin and trifloxystrobin.
On the other hand, Omega SC, Luna
Tranquility SC, Miravis Prime SC, Tilt SL
and Luna Experience SC effectively limit
stemphylium leaf blight. Producers have
asked how best to use these fungicides to
maximize their effectiveness and provide
the best return.
To answer this question, we
Figure 1. Brown elongated lesions of
stemphylium leaf blight and the spores
of the fungal pathogen that causes the
disease.
Many of the fungicides that were
historically used to manage purple
blotch do not adequately protect
against stemphylium leaf blight. Field
research from Michigan State University
has shown that the stemphylium
30 | VegetableGrowersNews.com
conducted a replicated field study
with a grower cooperator to evaluate
the early and late timing of Miravis
Prime SC applications in a rotation
program with Bravo WeatherStick SC.
We used the pungent yellow Bradley
onion grown in muck soil. Fungicide
treatments (See table) were applied
as a foliar spray to 20-foot long plots
using a carbon dioxide backpack
sprayer. Nine fungicide applications
were applied at seven-day intervals.
The percentage of green tissue in
the plot was estimated frequently
throughout the season. Onions were
harvested by hand, cured, topped,
graded and weighed.
All plots treated with fungicide
had more green tissue at the end
of the growing season and yielded
substantially more than the plots
that were not treated with fungicide
(Figure 2). The fungicide treatment
that included applications of
Miravis Prime SC at the start of the
fungicide program alternated with
Bravo WeatherStik SC, resulted in
a significantly higher percentage of
green tissue and an increase in total
yield than when Bravo WeatherStik
SC was applied early and Mirvais
Prime SC applied later.
Fungicide programs to protect the
onion's foliage from stemphylium leaf
blight should include highly effective
products at the front end of the season
(see treatment program two and three
as examples) to prevent the pathogen
from becoming too well established
when the plants are young. Protecting
the onion leaves early in the season
helps to ensure that the plants have
a fighting chance to reach maturity
and achieve their full yield potential.
When the protectant fungicide Bravo
was applied early in the season with
the more effective products added in
later (see treatment programs four and
Protecting the onion
leaves early in the
season helps to ensure
that the plants have a
fighting chance to reach
maturity and achieve
their full yield potential.
five as examples) the leaves were not
protected as well and the yield suffered
as a result. Based on these results, it
appears that producers should not
" save the good stuff " but should
consider using the most effective
fungicides right at the beginning of the
season to ensure the best return.
This research was supported by a
Michigan Specialty Crop Block Grant,
awarded to the Michigan Onion
Committee.
Doug Higgins is post-doctoral
research associate in Mary Hausbeck's
vegetable pathology program at
Michigan State University. VGN

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December 2020

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