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June 12-18, 2000
C.O.R.N. 2000-17
In This Issue:
A.
Head Scab at Low Levels; Glume Blotch Increasing
B. Poor Soybean
Stands and Postemergence Weed Control
C. OSU Weed
Science Field Day
D. Summary on ALS-Resistant
Ragweed
E. Slug Injury
Continues
F. Signs of First
Brood Corn Borer Beginning to Appear
G. Timely
Detection and Control of Armyworm
H. Treat Those
Beans and Change Those Genes for Soybean Replanting!
I. Identifying
Vegetative Growth Stages in Corn
J. 2000 Southern
Ohio Crop Protection Workshops
It
appears that most of the states wheat crop has escaped head scab. Most fields
have very low incidence of scab 1 or 2 heads per 500), but a few fields have as
much as 5 to 10% heads affected. This indicates that scab will be an
insignificant problem for growers and the milling and baking industry this year.
Although the wheat crop received rain during the vulnerable flowering
period for infection, the temperatures during this time were too low (below 60
F) for infection to take place. Regardless, wheat producers should scout their
fields for head scab now to determine incidence of the disease. Look for heads
with bleached spikelets that were killed by the scab fungus. It is typical to
see a few spikelets to sections of the heads killed by the scab fungus.
Incidence can be determined by counting the number of affected heads per 100
heads randomly sampled in the field.
The
low and variable soybean stands many farmers are seeing are likely to cause
problems with postemergence weed control programs. The soybean stand provides most of the weed control within
several weeks after a typical postemergence herbicide application.
When there is an inadequate number of plants or plant spacing becomes
variable, mid- to late-season suppression of weeds by the soybeans will be
reduced. This is likely to result
in the need for a second postemergence application in some fields.
Giant foxtail, burcucumber, giant ragweed, and black nightshade are
likely weed targets for the second postemergence application in these
situations. Some things to
remember in this situation:
-
weeds emerging in mid-season are less competitive with the soybeans compared to
early-emerging weeds, but these weeds can still produce seed and interfere with
harvest.
-
a second postemergence application may be needed only in areas of fields with
poor soybean stands - the rest of the field may have adequate stand and weed
control
-
do not delay the first postemergence application in order to "catch"
the flushes of later-emerging weeds. This
can result in yield reduction due to weed interference and difficulty in
controlling large weeds. Plan for a
second postemergence application instead.
-
weed escapes from total preplant/preemergence programs may require a
postemergence treatment where soybean stands are poor (good soybean stands will
often suppress these weeds adequately)
-
when making a second postemergence application, or applying in late June or
July, check labels for recrop restrictions and restrictions on the number of
times a postemergence herbicide can be applied. For example, the full rate of Flexstar can only be applied
once per season.
-
we strongly suggest that you switch herbicide mode of action for a second
postemergence application or a postemergence application following preplant/preemergence
treatments. This will reduce the
rate of development of of herbicide-resistant weed populations.
The OSU Weed Science field day will be held on Wednesday, July 12 at the OARDC Western Branch near South Charleston. This tour of our research plots is free and mostly self-directed. The tour starts at 9 AM and typically ends around noon. Lunch is not provided, but there are restaurants in nearby Springfield. Western Branch is about 5 miles south of I-70 on State Route 41.
We
have tested a total of 60 giant ragweed populations, two times (usually a total
of 20 plants/population), looking for resistance using FirstRate at a 2X rate
(0.6 oz/A) in the greenhouse. Seeds
were collected from fields last fall where control with an ALS-herbicide of
Raptor, Synchrony or FirstRate that gave poor to adequate control or we had no
history and observed poor control. The
giant ragweed's response to FirstRate in the greenhouse fell into two broad
groups, one that was statistically different from the known susceptible
population in at least one time of testing, usually both times, (referred to as
resistant) and the other group was not statistically different from the known
susceptible population. Twenty-two
percent of the samples were in the resistant group, leaving 78 percent in the
other group. We further segregated
the not statistically different group into three sub-groups, one, known as
limited or intermediate resistance, that had 1 plant with a level of control
between 10 and 55 % and 1 to 6 plants with a level of control between 56 to 80
%, another group, known as possibly resistant, that had 1 or 2 plants with a
level of control between 65 to 80 % and the last group susceptible to ALS-herbicides.
Thirteen percent of the total populations were in the intermediate
resistant group, 13 percent in the possibly resistant group and 52 % in the
susceptible group. Therefore 35 %
of the populations were in the resistant or intermediate resistance groups! The resistant and intermediate resistant populations are in
the following counties: Auglaize, Clinton, Crawford, Delaware, Greene, Hardin,
Hancock, Pickaway, Union, and Wyandot.
We
have tested an additional 23 common ragweed populations for a total of 86
populations using FirstRate at a 2X rate as above. Fifty-six percent of these populations were resistant with 64
% of the total 86 populations registering in as ALS-resistant.
Three Ohio counties and one Indiana county were added to the already 11
county list. These new counties in
Ohio are Crawford, Greene, and Wood and Wayne in Indiana.
ALS-resistant ragweeds are usually identified in the field by observing dead plants next to slightly to severely injured plants that are next to perfectly healthy plants all in the same field. If this is observed and the field has had an ALS-herbicide applied in 4 of the last 5 years then the field is probably resistant.
To control ALS-resistant giant ragweed use a glyphosate product on Roundup Ready soybeans, Basagran plus Flexstar, or Basagran plus Cobra. Two applications in fields with heavy pressure of giant ragweed may be needed. For common ragweed the previous products can be used but Flexstar can probably be used by itself. Be sure that you are not applying the same mode of action two times in the same growing season. This is one reason why we have the ALS-resistant ragweed.Slug injury is still continuing in Ohio. Slugs have got bigger and continue to feed. Although temperatures were very warm in Ohio this past weekend, the slugs are doing fine. The benefit from these warm temperatures will be that the crops will be growing quicker and hopefully getting ahead of the slugs. However, if the corn or soybeans are still small, slugs are still capable of causing serious damage and thus, these fields should be watched. There are probably many fields in Ohio where a treatment will still help the situation. There are many questions concerning replanting soybeans due to slug feeding. If the field had not been treated for slugs, you need to remember that the slugs are still there and are capable of causing repeated damage. Thus, treatment might be warranted. If you decide not to treat, you have to hope that the crop germinates quickly and outgrows any feeding. There are numerous soybean fields that were being heavily damaged earlier and had been treated, but still suffered heavy stand loss. These losses probably had occurred before the treatment had been applied. These fields should have fewer slugs, and thus, should not need further treatment. However, growers should examine the soybeans that have come out and check the new growth. If the newer growth on these plants appear relatively clean of continued feeding, the chances are good that the slug population was reduced and treatment might not be necessary. If however, the newer growth is being feed on, treatment should be considered.
Adults
moths of the spring flight of European corn borer (ECB) may be observed at this
time in corn and wheat fields. Foliar
feeding symptoms such as shot holes and window pane feeding may be observed in
corn fields that almost knee high (i.e. V6 or V7 stage of development).
Egg masses my be found on the underside of corn foliage if one searches
long and hard. However, to date, I
have not found any early stages of 1st brood ECB larvae feeding in whorl stage
corn despite the fact that I was sampling corn in five counties across north
central Ohio during the past week.
In
whorl stage corn that is almost knee high, signs of early larvae activity may be
found if one inspects a significant number of plants.
However, the central whorl is generally lacking any signs of larvae
activity and inspection of the inner whorls draws a blank.
Presumably, the heavy rains have drowned any ECB larvae hatching during
the past week or two.
Most of the corn observed while driving about central Ohio remains in early V3 or V4 stages of development and is unlikely to attract or support development of 1st brood ECB. Corn fields that were planted early and currently approaching mid-whorl stage should be inspected for possible economic infestations of 1st brood ECB. If whorl injury (shot holes and window-pane feeding) appear abundant, then about 20 plants should be inspected at 5 locations in the field to determine the proportion of stand exhibiting whorl injury. In the inspection process, a number of whorls should be pulled and opened to determine presence or absence of ECB larvae. When larvae are found, the average number of larvae per plant may be estimated based on the proportion of stand exhibiting whorl injury and the proportion of injured plants actually having larvae present. If the number of larvae found exceeds an average of one or more per plant and the larvae have not yet begun to burrow into the stalks, the infestation may warrant a rescue treatment.
During the past week, we inspected a field in Allen County that had been planted as no-till corn in a rye cover crop and subsequently required treatment for armyworm. The corn was still in an early stage of development, a significant proportion of the stand exhibited feeding injury on the lower leafs, and a few plants had been chewed down to stubble prior to treatment. The application of a timely rescue treatment controlled the infestation, new growth exhibited no new injury, and a significant loss in stand had been prevented. Although there have been minimal reports of armyworm activity this season, this case provides a reminder that corn planted no-till into rye cover crops or old hay fields are vulnerable to armyworm infestation and should be closely monitored.
Phytophthora
sojae has been positively identified in many fields in the state so far this
spring. Saturated soil conditions
favor both Phytophthora and Pythium. Every
time the soil becomes saturated these water molds will produce swimming spores
and attack the roots. Phytophthora
will continue to attack plants throughout the growing season.
Symptoms of Phytophthora will become easier to diagnose as this is the
only soybean pathogen which will colonize the plant from the roots and move up
the stem.
Did
you replant? Did you treat that
bean seed? Late planted beans
should be treated with the seed treatments specific for Phytophthora, both
Allegiance and Apron XL will be effective.
Use the highest labeled rates for the most effective control.
Did
you replant? Did you change your
Rps gene combination? Phytophthora
populations have continued to increase in the state which can cause disease on
many of the currently used Rps genes (Rps-1k, Rps-1c, Rps-1a and Rps-3a).
If you had to replant a field which originally had one of these Rps
genes. Change your gene.
Switch to a variety with a different gene or to a variety with a
combination of genes. One caution,
Rps-1a is not effective in Ohio. Phytophthora
sojae, so far, has been like an elephant in that once it can attack and cause
disease on a soybean with an Rps gene, it will continue to do so.
What
else can you do? Choose cultivars
with the best partial resistance or field resistance score.
Remember to read the fine print at the bottom of the table.
Different companies use the 1 to 10 scale differently, in the Ohio
Performance Trial, varieties with high levels of partial resistance score 3.5 to
4.5. Also, use a seed treatment if
your are depending on partial resistance to help get a good stand established.
With partial resistance some disease does develop.
Seed treatments help the
soybean plant get established so then it can limit Phytophthora infections.
In making an assessment of yield losses in corn due to defoliation and other types of plant injury, it is necessary to establish the stage of plant growth at the time when damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.
Several
systems are currently used to stage vegetative corn growth. The "leaf
collar" system is probably the method most widely used by Extension
agronomists in the Corn Belt. With this method, each leaf stage is defined
according to the uppermost leaf whose leaf collar is visible. The first part of
the collar that is visible is the back, which appears as a discolored line
between the leaf blade and the leaf sheath. The oval shaped first leaf is a
reference point for counting upward to the top visible leaf collar. This oval
shaped leaf is counted as the #1 leaf when staging. Normally a plant at the four
leaf stage will have parts of the 5th and 6th leaves visible, but only four
leaves with distinct collars. A field is defined as being at a given growth
stage when at least 50% of the plants show collars.
Another
widely used staging method is the "hail adjustor's horizontal leaf
method" developed by the National Crop Insurance Service. Rather than using
the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40
to 50% exposed and whose tip points below the horizontal. Typically a given
"horizontal leaf" growth stage based on the hail adjustor's method
will be 1 to 2 leaf stages greater than the collar method. One problem with this
method is that it is often difficult to identify the uppermost horizontal leaves
in fields that have recently experienced severe leaf damage. Hail adjustors get
around this problem because they usually assess hail damage 5 to 10 days after
the storm, by which time 1 or more leaves have emerged from the whorl.
Certain
postemergence herbicide labels specify that the product can be applied up to a
certain leaf collar stage or plant height whichever comes first. You can often
have some fairly short plants that reach the restrictive leaf collar stage
specified by the product label, especially when early season growing conditions
cause corn to grow slowly from the plant height standpoint. Therefore, since the
restricted collar stage may come sooner than the restricted plant height, go by
the leaf collar stage not plant height.
At
about the six-leaf stage of the collar method (V6) or 8-leaf stage of the hail
adjustor's method, increasing stalk and nodal growth combine to tear the
smallest lower leaves from the plant. Degeneration and eventual loss of the
leaves results. Hail damage, insect feeding, and fertilizer/herbicide burning
promote this process.
To
determine the leaf stage after this loss of lower leaves, split the lower stalk
lengthwise and inspect for internode elongation. You can identify leaf location
since leaf #5 is usually attached to the top of the first noticeably elongated
internode. (The first node above the first elongated stalk internode is
generally the fifth leaf node. This internode is about 0.4 inches in length.)
Leaf growth stage can then be determined by counting from the fifth leaf to the
uppermost collared leaf, or the uppermost leaf that is 40-50% exposed from the
whorl (depending on which growth staging system is being used).
Remember that prior to the six-leaf stage (collar method), the growing point is below or at the soil surface. As long as the growing point remains healthy and intact, severe defoliation during early vegetative stages seldom translates into significant yield losses.
June
27 and 28, 2000 - Washington Court House, Ohio.
Crop Producers and agribusiness personnel are encouraged to attend the
2000 Southern Ohio Crop Protection Workshops to be held at the Fayette County
Farm near Washington C.H., Ohio. The
Fayette Co. Farm is located 2 miles north of Washington C.H. on St. Rt. 38.
This
year's workshops will be offered on Tuesday, June 27th and Wednesday, June 28th
with the same program offered each day.
These field days will give participants a "hands on"
opportunity to observe and learn the identification and economical treatment and
management of crop production problems. The
Fayette County Farm offers a land laboratory atmosphere where producers can roll
up their sleeves and improve field crop troubleshooting skills.
Numerous demonstration plots are established yearly to use in teaching
diagnostics and new agronomic technology.
The
topics and speakers for this year's workshop include:
1.
Plant Nutrition and Diagnostics - Ed
Lentz, NW District Agronomy Specialist
2. Corn Growth and Development - Peter
Thomison, OSU Extension Corn Specialist
3. Site Specific Technology - Nathan
Watermeier, OSU Extension Technology Specialist
4. Beneficial Crop Rotations and Tillage Practices
- Peter Bierman, OSU Soil and Water Specialist
5. ALS Resistant Weeds and Weed Management - Jeff
Stachler, OSU Weed Control Specialist
6. Crop Disease Diagnostics and Soybean Cyst Nematode Management
- Pat Lipps, OSU Plant Pathologist
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: Peter Thomison (Corn
Production), Pat Lipps (Plant Pathology), Anne Dorrance (Plant Pathology), Erick
DeWolf (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed
Science), Hal Willson (Entomology) and Ron Hammond (Entomology)
Editor: David A. Jones Web Editor: Nathan Watermeier
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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