Managing
Field Corn Infected with Common Rust
Erick
J. Larson
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Introduction
Common
corn rust (Puccinia sorghi Schw.) can be
found in most temperate areas of the world where
corn (Zea mays L.) is grown. Disease
development depends upon the presence of the
pathogen, favorable temperature, and humidity. In
temperate regions, the uredospore stage of the
fungus is considered too fragile to
overwinter.
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Thus,
the uredospores needed to initiate rust epidemics
are believed to be windblown into the continental
U.S. from tropical and subtropical areas.
Germination of spores of the common rust fungus may
occur over a wide range of relatively cool
temperatures (approximately 54°F to 82°F) and
requires nearly 100% humidity for several hours
(Headrick and Pataky 1987). These conditions
frequently occur during the early part of the
growing season in the Midsouth. Thus, monitoring
uredospore populations of common rust on corn is
considered more useful than using weather-based
forecasts to determine the need for fungicidal
control.
Common
rust rarely causes significant yield reduction of
dent corn grown in temperate regions. Yield
reductions are more likely in tropical areas and
where corn planting is staggered throughout the
growing season. In Hawaii, common rust reduced
grain yield of 10 double-cross hybrids an average
of 35%, compared with the paired resistant hybrids
(Kim and Brewbaker 1976). Fungicide applications
have been used to reduce disease severity and
increase sweet corn yields when common rust was
severe (Dillard and Seem 1990).
Corn
grain development is very sensitive to stress
timing. Corn is extremely susceptible to different
kinds of environmental stress, including water
deficit (Grant et. al 1989), light deficit (Kiniry
and Ritchie 1985) and defoliation (Shapiro et al.
1986) from silking to approximately 2 weeks after
silking. These stresses reduce grain yield by
limiting photosynthesis. During pollination, corn
grain development is extremely dependent upon
current photosynthate production, even when
accumulated carbohydrates are plentiful (Schussler
and Westgate 1991). The sink capacity of the ear is
limited, compared with stalks, during this
transition from vegetative to reproductive growth
(Edmeades and Daynard 1979; Setter and Meller
1984).
Although
common rust epidemics are rare, the amount and
timing of common rust development on Mississippi
field corn during the 1997 growing season warranted
evaluation of fungicidal control. The primary
objective of this study was to determine whether
fungicide application would control common rust
development in field corn and improve grain
yields.
Materials
and Methods
Four
grower field sites were selected to evaluate common
rust control using fungicides. Pioneer 3223
(Pioneer Hi-Bred International, Inc., 1997), a corn
hybrid susceptible to common rust, was grown at
sites 1, 2, and 3. Pioneer 3394, a moderately
resistant hybrid, was grown at site 4. A fungicide
treatment of either propiconazole (4 ounces of Tilt
per acre) or mancozeb (1.5 pounds of Dithane DF per
acre) was applied aerially from either an Air
Tractor 402 Turbine or 502 Turbine airplane
traveling at 130-140 mph. The spray adjuvant Latron
CS-7 was added to the Dithane DF treatment at 0.25%
by volume to improve initial spray deposit,
fungicide redistribution, and weatherability. The
fungicide treatments were applied at 25-32 pounds
per square inch in a spray volume of 5 gallons per
acre at an altitude of 4-8 feet above the crop
canopy. The airplane was equipped with CP nozzles,
manufactured by CP Products, Co., Inc., angled
straight back with the deflector set at 45 degrees.
The treatments were applied in adjoining blocks. A
sequential treatment of Dithane DF was applied as
an additional treatment 1 week after the initial
application.
Visual
estimates of rust severity were performed on corn
ear leaves at weekly intervals after fungicide
application. These ratings were based upon
percentage of the total leaf area infected with
uredinia (pustules) using the Peterson scale
(1948). Rust severity at different time intervals
represented a cumulative estimate of uredinia.
Twenty ear leaves randomly selected from plants
were evaluated for each treatment. Data were
analyzed as a series of completely randomized
designs combined over sampling dates and
sites.
Grain
yield was estimated by hand harvesting ears from
0.001-acre plots within each block and shelling
them in a manual single-ear sheller. Grain yields
were calculated from grain weights, and moisture
was adjusted to 15.5% moisture. Three replications
were evaluated for each treatment. Data were
analyzed as a series of completely randomized
designs combined over sites.
Results
Common
rust was first detected in the 1997 growing season
during the first week of June. The state average
daily temperature during this week was 71°F, which
is 5°F below normal. Symptomatic corn was at the
V9-V12 growth stages as defined by Ritchie et al.
(1996). Cool, humid environmental conditions
promoted rapid disease spread and development over
the next several weeks. The average daily
temperature did not reach the 82°F upper
developmental threshold for common rust until the
first week of July. Common rust development was
minimal after this time.
Fungicide
application controlled rust development, compared
with untreated checks. Rust severity on ear leaves
1 week after fungicide treatment was significantly
reduced at each site, except site 2, where no data
were collected at this interval (Figures 1-4). The
reduction in disease severity by fungicide
treatment averaged 41%, compared with the untreated
plots. This level of control is slightly less than
the 50% to 60% levels reported by Pataky and
Eastburn (1993) and Raid (1994). Control
differences were no longer evident between treated
and untreated plots 2 weeks after treatment at site
1 and 4 weeks after treatment at sites 2 and 4.
Rust severity was only measured 4 weeks after
treatment at site 2 (Figure 2). Although the use of
two applications of Dithane was more effective than
one application at lowering rust severity, the
treatments were not significantly
different.
Fungicide
efficacy appears to be associated with fungicide
application timing relative to crop development
stage. Fungicide efficacy lasted longer when the
corn was treated at an earlier vegetative growth
stage. Differences in rust severity between treated
and control plots were no longer evident after
pollination, except at site 3. This finding could
have two possible causes: hybrids becoming more
resistant to common rust infection after
pollination as reported by Headrick and Pataky
(1987); and/or average daily temperatures exceeding
the upper developmental threshold for common rust.
The hybrid grown at site 4 is also moderately
resistant to common rust.
Yield
response resulting from fungicide treatment was
dependent upon factors at each site. Fungicide
application significantly improved grain yield at
site 3 (Table
1).
There was no significant difference between
treatments at the other locations, although the
yield was slightly higher with fungicide
application. The cultural factors for sites 2 and 3
were similar, except for timing of the fungicide
application. The fungicide application at site 2
did not occur until the corn reached V18 stage. At
site 3, the fungicide was applied at the V12 growth
stage. Therefore, early fungicide treatment timing
increased the likelihood of yield improvement
associated with control of common rust. Sites 3 and
4 were grown under similar culture, except that the
hybrid grown at site 3 was susceptible to common
rust. Thus, the lack of yield response at site 4
may be due to the moderate disease resistance of
this hybrid. The lack of yield response at site 1
may be related to the yield level. Location 1 was
not irrigated and produced lower grain yields
compared with the other locations. Sites 2, 3, and
4 were all irrigated and produced grain yield
levels exceeding 150 bushels per acre. This is
considered a high yield level for any region of the
world. Thus, the magnitude of yield response to
fungicide treatment of common rust was reduced in a
lower-yielding environment, probably due to other
limiting factors.
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Table
1. Corn grain yield response to fungicide
treatments. 1
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Location
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Untreated
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Treated
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Treated
twice
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bu/A
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bu/A
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bu/A
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Site
1
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130a
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134a
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--
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Site
2
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199a
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208a
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--
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Site
3
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156b
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189a
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183a
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Site
4
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220a
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218a
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232a
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1Values
assigned the same letter are not significantly
different at the 0.05 level of
probability.
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Conclusions
The
timing of common rust infection in relation to corn
growth stage can critically influence the amount of
potential grain yield reduction and fungicidal
effectiveness. Common rust development on corn was
reduced by fungicide application before tasseling.
Fungicidal control of common rust increased yield
of a susceptible hybrid grown in a high-yielding
environment, compared with an untreated control.
This improved grain yield is especially significant
since the high-yielding environment is also
conducive to common rust development. Corn grown in
a lower-yield potential (dry land) environment may
be less likely to produce a yield
response.
Yield
response was attributed to depletion of
photosynthate during pollination. Corn grain
development is extremely dependent upon current
photosythate production during and shortly after
pollination. This dependence decreases as grain
approaches maturity.
This
research supports the use of a fungicide when an
action threshold of 1% to 2% disease severity
(about six uredinia per leaf) is reached on
susceptible field corn hybrids before tasseling, as
proposed on sweet corn by Pataky and Headrick
(1988) and Dillard and Seem (1990). This action
threshold should be relevant for the ear leaf and
higher leaves (upper six to eight leaves), since
they produce the majority of photosynthetic energy
required for grain development.
Yield
reduction resulting from common rust infection
after pollination is less likely for several
reasons. Corn grain yield is less sensitive to
stress as kernel development approaches maturity.
Pre-tassel treatment differences in disease
severity were no longer evident after tassel
emergence at two of three sites. This finding could
have resulted from hybrids becoming more resistant
after tassel emergence, as reported by Headrick and
Pataky (1987), or from hot weather arresting the
disease development. Temperatures normally exceed
the upper developmental threshold for common rust
during late June to early July. Thus, fungicide
applications after anthesis will likely not be
warranted except for susceptible hybrids heavily
exposed to common rust in a environment highly
conducive to disease development.
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List
of Figures
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Figure
1. This figure shows common rust control
after Tilt fungicide application at site 1
(Lowndes County, MS). Rust severity is
rated as the percent of ear leaf area
covered by disease uredinia. The treatment
was applied to corn at the V15 growth
stage. Values assigned the same letter are
not significantly different at the 0.05
level of probability.
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Figure
2. This figure shows common rust control
after Tilt fungicide application at site 2
(Sunflower County, MS). Rust severity is
rated as the percent of ear leaf area
covered by disease uredinia. The treatment
was applied to corn at the V18 growth
stage. Values assigned the same letter are
not significantly different at the 0.05
level of probability.
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Figure
3. This figure shows common rust control
after Dithane application at site 3
(Sunflower County, MS). Rust severity is
rated as the percent of ear leaf area
covered by disease uredinia. Initial
treatment was applied to V12 growth stage
corn, and a sequential treatment was
applied after 1 week (Treated 2X). Values
assigned the same letter are not
significantly different at the 0.05 level
of probability.
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Figure
4. This figure. shows common rust control
after Dithane application at site 4
(Holmes County, MS). Rust severity is
rated as the percent of ear leaf area
covered by disease uredinia. Initial
treatment was applied to V12 growth stage
corn, and a sequential treatment was
applied after 1 week (Treated 2X). Values
assigned the same letter are not
significantly different at the 0.05 level
of probability.
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Acknowledgments
Thanks
to Novartis Crop Protection, Inc. (Dale Brown,
Nancy Crane, and Scott Hendrix) and Rohm and Haas
Company (Larry Walton, Jonny Spivey, and Nathan
Buehring) for product and technical assistance with
this project. Thanks to Charlie Pilkington, Burke
Fisher, Lee Simmons, and Bill Thomas for allowing
this project to be conducted in their
fields.
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