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March 20 - April 10, 2000
C.O.R.N. 2000-7
In This Issue:
A) The
Results are in on ALS-Resistant Common Ragweed Populations
B) Resistant
Varieties to Manage Sclerotinia Stem Rot
C) So
You’ve Got Soybean Cyst, Now What?!
D)
Predicting
Stewart’s Bacterial Leaf Blight on Corn
E) Wheat
Growth and Development Update
F) Wanting
More Information on GMOs, OSU Web Site Up and Running
At
this point in time testing has been completed on 63 common ragweed populations
from around the state for ALS-resistance. Resistance populations have been
identified from this sampling. Additional common ragweed populations as well as
giant ragweed are to be tested. Progress
reports as well as a comprehensive summary on all ragweed populations will be
published here after testing is complete.
The
procedure used in testing for ALS-resistance is a 2X rate (0.6 oz/A) of
FirstRate. FirstRate was chosen because it is the most effective ALS-herbicide
for ragweed. Because we are using
FirstRate, there is a possibility that the common ragweed resistant to Classic
or Raptor would not be identified, but our research in 1999 showed that common
ragweed is cross-resistant to all ALS herbicides.
The 63 populations tested were collected from 15 counties. Populations were grouped into four categories based upon their response to FirstRate.
Group 1 had populations that were statistically similar to a known resistant population, meaning these populations contain mostly resistant plants.
Group 2 are populations that were statistically different from the known resistant or known susceptible populations tested, meaning these populations have a mixture of susceptible, resistant, and injured plants within a population (the populations were comprised of about 50% resistant/injured plants and 50% susceptible).
Group 3 populations were statistically similar to the known susceptible population, but contained one and sometimes two plants that were controlled between 5 and 55%. These populations may have resistant plants in them, but without further research we are not confident that these are resistant populations.
Group 4 populations had all plants controlled by treatment.
Of the 63 populations, 40% were in the highly resistant category Group 1. When combining groups 1 and 2, 67% of the populations tested were confirmed to be ALS-resistant, with a higher percentage of the populations being resistant in northwestern Ohio. Eighty-one percent of the populations tested are either confirmed ALS-resistant or may contain resistant plants in the population (Groups 1,2 and 3).
The
following counties have confirmed ALS-resistant common ragweed populations based
upon this research: Auglaize, Darke, Defiance, Fulton, Hancock, Marion, Mercer,
Morrow, Paulding, Putnam and Van Wert. Sandusky
and Union counties have one population in each county that were placed into the
may contain ALS-resistant category (Group 3). Darke,
Paulding and Putnam Counties had 7 to 10 fields in each county that were
confirmed ALS-resistant.
The
results from many of the studies of the North Central Soybean Research Program
are posted at a new web site: http://www.plantpath.wisc.edu/NCSRPwhitemold. The
most economical and best means to manage Sclerotinia Stem rot or White Mold is
by planting varieties that have some level of resistance. All varieties are susceptible, the difference is how much damage occurs
from the infection. Some varieties
will have very high levels of disease like Williams82 while others have lower
disease levels under the same disease pressure. The web site includes a number of articles and links to websites that
discuss Sclerotinia stem rot as well as the final recommendations from the White
mold working group.
Fifty-five percent of the 3,678 soil samples had soybean cyst nematodes. Thanks to the Ohio Soybean Council and the North Central Soybean Research Program for sponsoring the soil sampling program and the SCN campaign to raise awareness. We can now begin to manage these populations to limit yield losses.
The next question is how many eggs or cysts do you need before you start losing beans? Fortunately, our SCN team has been running field trials to find that very answer. The results from a trial in 1999 can be found on the IPM web site: http://www.ag.ohio-state.edu/~ipm/scn/scn.htm
From the study, susceptible beans out yielded resistant beans by as much
as 5 to 10 bu/A when SCN populations were below 200 eggs/200 cc of soil.
However, when SCN populations were between approximately 2000 to 6-8,000
eggs/200 cc of soil, the resistant varieties had a 5 to 10 bu/A advantage over
susceptible varieties. Above 10,000 the resistant varieties had a greater than
10 bu/a advantage.
Remember
that SCN populations are quite variable across a field, and your average
eggs/200cc of soil counts are dependent upon the pattern in which you took soil
samples were collected. SCN occurs in fields in pockets or hot spots
Figure 1.
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Figure 1. |
Generally, if we could maintain our three-crop rotation, we would not see dramatic increases of SCN over time like we do now in continuous soybean systems (Figure 2). Figure 1 gives an example of some real data and estimated data from Ohio production fields under different cropping systems.
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Figure 2. |
Our
next biggest challenge will be to maintain our sources or resistance. Within every SCN field there are individuals that have an appetite (can
reproduce) on the different sources of resistance. When we utilize those sources of resistance exclusively such as the
PI88788, we increase nematode populations that reproduce well on that resistance
source. Figures 3,
4, and 5
illustrate what can happen to populations in a field where susceptible or
PI88788 cyst resistant beans are planted. In
order to manage SCN effectively over the long-term it is essential to rotate
crops AND rotate sources of resistance to avoid any yield losses due to SCN.
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Figure 3. |
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Figure 4. |
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Figure 5. |
Stewart’s
bacterial leaf blight was at abnormally high levels throughout the state in
1997, 1998 and again in 1999 causing extensive leaf damage and premature death
of plants in some fields. Mild
winter conditions favor the over wintering flea beetle adults that carry the
bacterium. In 1998 and 1999,
numerous flea beetles were present when young corn seedlings emerged. Flea beetles transmit the bacterium when feeding on young plants. Over the past two years we saw the rare occurrence of the systemic wilt
phase of the disease in field corn. Generally,
hybrid corn has sufficient resistance to restrict the wilt phase. Some fields had up to 30% dead plants within the first month after
planting due to Stewart’s wilt.
Corn flea beetle adults become active in the spring when soil temperatures reach 65 F. Adults are most active on sunny, warm windless days. They hide in cracks in the soil during windy, cool or cloudy periods. After feeding and mating, adult females lay eggs at the base of the corn plants. Larvae feed on corn roots and are full grown in about two weeks. There are at least two generations per year in Ohio. The beetle over winters as an adult in the soil near corn fields. It prefers bluegrass sod, but may be found in fence rows, roadsides, and woods (OSU Extension Fact Sheet CV-1000-94, not available on the web). See also Stewart's Bacterial Leaf Blight on Corn, Soybean, Wheat, and Alfalfa Field Guide, Bulletin 827 and Corn Diseases on Field Crop Disease Management, Bulletin 631-98.
The
corn flea beetle harbors the Stewart’s wilt bacterium during the winter and
initiates the disease in the spring when corn plants emerge. Any non-infested beetle that feeds on an infected plant can acquire the
bacterium and spread it to other plants. Continued
feeding by beetle's spreads the disease to other leaves during the remainder of
the growing season. As the season
progresses, an increasing percentage of the beetle population carries the
bacterium. Leaf blighting can be
severe on susceptible hybrids. The only effective control is to grow hybrids
with a high degree of resistance. Most seed companies have resistant hybrids
available. Insecticides can be used to control corn flea beetle populations, but
has not been shown to reduce the disease on susceptible corn lines.
Predicting
Flea Beetle Survival and Stewart’s Wilt Severity: The occurrence of
Stewart’s bacterial leaf blight and wilt is totally dependent on the level of
flea beetle survival over winter. For
many years, winter temperatures have been used to predict the severity of
Stewart’s disease because higher populations of the corn flea beetle survive
during mild winters than during cold winters. A ‘flea beetle index’ was calculated as the sum of the average
temperatures (in degrees Fahrenheit) of December, January and February.
Several
factors can contribute to the variability in accuracy of predicting epidemics of
this disease. High populations of
beetles may survive cold winters where they are protected by continuous snow
cover or with high levels of corn residues, such as in no-till or reduced
tillage fields exist. Also, a high
number of beetles carrying the bacterium can occur following an epidemic season.
Table
1. Monthly average temperatures and Flea Beetle Index for predicting severity of
Stewart’s Bacterial Leaf Blight for different location in Ohio, 1999-2000.
| Location | December | January | February | Sum | Predicted Severity |
| Hoytville | 32.1 | 23.3 | 33.6 | 89.0 | Negligible |
| Wooster | 33.3 | 26.4 | 35.2 | 94.9 | Moderate |
| Delaware | 33.0 | 25.7 | 36.0 | 94.7 | Moderate |
| S. Charleston | 33.0 | 25.4 | 35.6 | 94.0 | Moderate |
| Piketon | 35.9 | 30.0 | 40.6 | 106.5 | Severe |
| Jackson | 35.1 | 29.5 | 39.8 | 104.4 | Severe |
*Index
represents the sum of the average temperature for December, January and February
where: <90 = negligible disease, 90-95 = moderate, 95-100 = moderate to
severe and > 100 = severe disease.
The
weather data for various locations in Ohio indicate that December and February
average temperatures were above normal and January temperatures were normal or
below. This temperature data
predicts that flea beetle survival over winter is low in northwest Ohio,
moderate in central Ohio and high in southern Ohio.
In January, there was persistent snow cover throughout the state,
especially during the coldest periods. The
snow cover moderated the temperatures at the soil surface and may have protected
adult flea beetles. If this is the
case then the Flea Beetle Index may underestimate the survival of flea beetles
in areas with persistent snow cover in January.
It will probably be wise to select hybrids with higher levels of resistance to Stewart’s bacterial leaf blight to plant this season. With severe levels of disease over the past few years and the high probability that a larger than normal proportion of adult flee beetles will be carrying the bacterium. This will be especially true for corn growers in southern Ohio. Contact your seed dealer for information concerning the level of resistance to Stewart’s bacterial leaf blight in available hybrids.
The relatively warm weather in February essentially jump-started wheat growth this year. In most years wheat does not begin to grow until mid to late March, but during the past three years many fields showed renewed growth by early March. Favorable weather for growth last fall provided an abundance of tillers on plants and over winter survival of wheat has not been a problem again this year. Most wheat producers have already top-dressed their wheat. Even plants in the most delayed fields are producing new roots to begin the resumption of growth this spring.
There has been concern that the wheat is too far advanced for this time of year and that the wheat may be prone to cold temperature injury. We generally do not worry about cold temperature injury until plants advance to Feekes’ growth stage 6, which is when you can find the first node on individual, tillers (first node visible). Cold temperatures (mid-teens) can cause damage to wheat at this stage. Cold temperatures for a few short hours will not damage the wheat, but extended periods of cold temperatures at this growth stage can cause damage to the growing point.
We
have examined wheat from several locations in the state.
A sample from Ross County in southern Ohio was in Feekes’ growth stage
5 which is the “leaf sheath strongly erect” stage. At this stage the plants appear to be very upright, but when you strip
down the leaf sheaths to look for the stem, you cannot find a node.
At this growth stage the growing point is still at the soil surface or
only a fraction of an inch above it. This
is still a very safe growth stage for cold injury sensitivity. Samples from other fields in Fayette Co., Shelby Co. and Clark Co. were
in growth stage 4 where the leaf sheaths were beginning to lengthen.
amples from more northern counties were advancing from growth stage 3 to
growth stage 4 and should have not problem handling cold temperatures.
According
to the calendar date, the wheat is a little more advanced than what would be
expected for this time of year, perhaps a week earlier than last year for fields
in southern Ohio. If the weather
returns to 60-70 degree range for an extended period of time the wheat will
advance quite rapidly. Be
sure to check the tillers in all wheat fields to check the growth stage of the
wheat before applying certain herbicides to avoid injury to the crop.
The
Ohio State University College of Food, Agricultural, and Environmental Sciences
has formed a committee to address concerns and issues raised by GMO's
(genetically modified organisms). Faculty serving on the committee represent a
diverse group of disciplines and interests, including animal sciences,
biotechnology, crop production, economics, environment, food safety and social
sciences. The committee has
established a web site http://ohioline.ag.ohio-state.edu/gmo/
that contains a
range of information on GMO's as well as links to other web sites that provide
information on this topic or related issues. In addition a list of commonly
asked questions (Q&A) concerning GMO's has been prepared and included on the
web site.
Readers can subscribe electronically to this newsletter by sending an e-mail message
to: corn-out-on@postoffice.ag.ohio-state.edu.
A successful subscription message will receive by an automatic reply from the listserv.
Contact your local Ohio State University Extension Office or e-mail
Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
Contributors to C.O.R.N. this week include: State Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Production), Jay Johnson (Fertility), Jim Beuerlein (Soybeans & Small Grain), Mark Loux (Weed Science), Jeff Stachler (Weed Science); District Specialists: Curtis Young (IPM), Ed Lentz (Agronomy) Jim Jasinski (IPM); Extension Agents: Mike Estadt (Pickaway), Andy Kleinschmidt (Van Wert), Roger Bender (Shelby), Dave Jones (Allen), John Barker (Knox), Steve Bartels (Butler), Larry Lotz (Fayette), Ray Wells (Ross), Clark Hutson (Seneca), Barry Ward (Marion), Dennis Baker (Darke), John Hixson (Union), Bruce Clevenger (Defiance), Gary Wilson (Hancock), Greg La Barge (Fulton), Howard Siegrist (Licking) and Steve Prochaska (Crawford).
Editor: Greg LaBarge Web Editor: Nathan Watermeier
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