http://www.ag.ohio-state.edu/~corn/ |
|
May 15-21, 2000
C.O.R.N. 2000-13
In This Issue:
A)
The Lack of Rainfall and It's Effects on
Preplant/Preemergence
B) Crops Are
Emerging Without a Burndown Herbicide in No-tillage Fields - What Can Be Done?
C) What's the
Impact of Early Season Leaf Damage in Corn?
D) Wheat Beginning
to Flower, Powdery Mildew Still Spreading
E) Will Head Scab
Be Important this Year?
F) Barley Yellow
Dwarf in Wheat
G) Early Season
Soybean Disease
H) Black Cutworms
Feeding on Central Ohio Corn
I) Alfalfa Weevil
Activity Declining Rapidly in Central Ohio
J) Bean Leaf
Beetle Activity on Soybeans
K) Slugs in Corn
L) Research
Results Available On-line
Much of the preplant and preemergence
herbicides applied within the past several weeks may not have received enough
rainfall for effective activity. We generally consider a single rainfall event
of 0.5 to 1 inch to be sufficient to move herbicide far enough into the soil to
affect germinating weeds. Single rainfall events of 0.2 to 0.3 inches over a two
week period totaling less than 1.0 inch of rain is usually not enough to provide
effective weed control. When the upper few inches of the soil is dry, as it has
been the last two to three weeks in some areas, more rainfall will be required,
compared to moist or wet soil. As the amount of surface residue increases, so
does the amount of rainfall necessary to move herbicide through residue into the
soil. The amount required varies with herbicide and soil conditions. Products
containing atrazine, flufenacet (Axiom, Domain), chlorimuron (Canopy, Canopy
XL), pendimethalin (Prowl, Squadron, Steel), imazaquin (Scepter, Squadron,
Steel), flumetsulam (Python, Bicep Magnum TR, Hornet) cloransulam (FirstRate) or
imazethapyr (Pursuit Plus, Steel) tend to require more rainfall for activity
than the "average" herbicide, while products containing isoxaflutole
(Balance, Epic), clomazone (Command), acetochlor (Harness, Surpass, Topnotch),
or dimethenamid (Frontier) may provide weed control with less rainfall than
other herbicides.
While loss of herbicide may occur from
various processes when it remains on the surface for long periods, we generally
expect that there will eventually be enough rain to move herbicide into the soil
and provide weed control. However, weeds may begin to emerge before this rain
occurs. While there can be enough uptake of herbicide from the soil to kill
emerged weeds that are very small following significant rain, this type of
activity is inconsistent. It is generally wise to treat these weeds as escapes
and act accordingly. In no-till fields where preplant herbicide was applied, but
weed escapes are evident at the time of planting and before the crop emerges,
application of Roundup Ultra, Touchdown, or Gramoxone Extra may be called for.
Rates of these products required to kill small weeds should be low, keeping
costs to a minimum. In tilled fields, early scouting following weed and crop
emergence may detect weed escapes at an early growth stage, allowing low rates
of postemergence herbicides to be used. Results of OSU research on reduced rates
has generally shown that weeds up to 1 inch tall can be controlled with 25% of
the labeled rate, and weeds up to 2 inches tall can be controlled with 50% of
the labeled rate.
The rotary hoe is another tool to
"buy some time" when rainfall within 7 to 10 days after planting has
been insufficient to "activate" preemergence herbicides. The rotary
hoe provides little incorporation of herbicide, but can effectively eliminate
those weeds that are starting to emerge. Some tips on the effective use of a
rotary hoe:
Ideal time for operation is after weeds
germinate but before the shoot emerges (white stage). This may be as early
as 3 to 7 days after planting, depending upon date of planting. Weeds
germinate and emerge more rapidly at later planting dates.
A second rotary hoeing 5 to 7 days
after the first will improve control, and may be necessary if rainfall
continues to be lacking.
Once weeds can be seen, they are
probably past the stage of maximum rotary hoe effectiveness, especially
large-seeded weeds such as velvetleaf and giant ragweed.
The rotary hoe is most effective when
the soil surface is smooth, dry, and firm, or where a crust is present. Rain
shortly before or after rotary hoeing can greatly reduce effectiveness.
For best results, operate the hoe with
crop rows at a minimum speed of 6 mph.
Take precautions to reduce stand loss.
Crop injury is more likely when the seed is not planted deep enough. Avoid
covering the crop with soil as it emerges. Corn can be hoed up to a height
of 4 to 5 inches. Avoid hoeing corn planted in furrows in loose soil from
the spike to one-leaf stage to prevent covering plants.
Soybeans should not be hoed between the
crook stage (just prior to emergence), until approximately 3 days after
emergence. Hoeing soybeans during emergence results in 5 to 10% stand loss.
If necessary, rotary hoe a test strip
and evaluate crop damage before proceeding over the entire field.
The first thing to consider for these
fields is to spray as soon as possible so that weeds do not get any larger.
Secondly scout the fields to determine which weed species are present so the
proper herbicide is chosen for the weed problem. Most of these fields will still
need an additional application later in the season unless residual herbicides
are applied and timely rainfall occurs shortly after application.
There is less concern with these fields
planted to corn as compared to soybeans. For corn apply any soil-applied
atrazine premix except Axiom AT and add 0.5 pint/A of Banvel or 2,4-D. If the
grasses are less than 1 inch tall be sure the amount of atrazine is at least 1.6
pounds per acre and add a crop oil concentrate and nitrogen source. Do not use a
liquid fertilizer as the carrier unless the label allows for this application.
If the grasses are between 1 to 2 inches in height add Accent at a half rate and
if over 2 inches add Accent at a full rate. Basis Gold plus Banvel (0.5 pt/A)
can be effective in this situation but grasses need to be 3 inches or less in
height. If Clearfield, Liberty-Link or Roundup Ready corn was planted more
effective control can be expected as long as these fields are sprayed
immediately as well as adding Banvel to the Lightning and atrazine to the
Liberty.
For soybean fields in this scenario
weed control will be difficult unless Roundup Ready soybeans were planted. Use
no less than 1.0 qt/A of Roundup Ultra on Roundup Ready soybeans with 1.5 qt/A
providing more effective control.
If STS soybeans were planted use
Synchrony STS at full rate and add a grass herbicide. It may be necessary
depending upon weed species and height to add additional Classic for a total of
3/4 oz/A as well as Cobra at 8-10 oz/A or Flexstar at 1.0 to 1.3 pt/A.
For regular soybeans common
lambsquarters, Canada thistle, horseweed (marestail) and other perennial
broadleaf weeds will be difficult to control. For common lambsquarters be sure
to use Pinnacle at 1/4 oz/A or Raptor at 5.0 oz/A and be ready to accept the
injury from Pinnacle on small soybeans. The injury will cause little yield loss
as compared to lack of weed control. For Canada thistle apply Basagran at 1.0
qt/A when thistles are 8 inches to bud stage. An additional application of
Basagran may be necessary for more complete control. For horseweed use FirstRate
at 0.3 oz/A with additives such as Methylated Seed Oil (MSO) at 1.2 %v/v and a
nitrogen fertilizer. If the field has a population of ALS-resistant horseweed
then it will not be controlled by the FirstRate and there are no other options
available.
For other perennial broadleaf weeds
tank-mixing Classic or FirstRate with Cobra (10 oz/A) or Flexstar (1.3 pt/A) is
your best option. For all other weed problems a combination of Flexstar at 1.3
(north of I-70) to 1.6 pt/A (south of I-70) and Raptor at 5.0 oz/A or FirstRate
at 0.3 oz/A should provide good control of broadleaf weeds. The Flexstar will
probably antagonize the Raptor for grass control which can be sprayed with a
post-grass herbicide after regrowth and before it reaches 6-8 inches in height.
The Flexstar plus FirstRate combination will need a post-grass herbicide
tank-mixed with it to control grasses and Pinnacle for common lambsquarters.
Another option is Raptor at 5.0 oz/A plus FirstRate at 0.3 oz/A. This should
control most weed species.
When assessing early season leaf injury
caused by frost, 28% fertilizer burn, insect feeding, etc., don't make a
decision on extent of damage and stand loss too quickly. Destruction of leaf
area in small corn plants is seldom as serious as appearance might suggest.
Young corn seedlings can be severely damaged above ground and still survive with
little or no effect on yield. The key to predicting survivability is
understanding where the growing point is inside the plant with respect to the
stage of plant development. The condition of the growing point is a good
indicator of plant health.
The growing point of a corn plant is
located deep inside the seedling where all the remaining leaves originate, as
does the tassel. You can observe the growing point by digging up a seedling and
splitting the stem from top to bottom. The growing point will be located at the
top of the pyramid-shaped whitish stalk tissue near the base of the seedling. In
corn seedlings that have just emerged, the growing point is usually located
about 3/8 inch below the soil surface, just above the crown area. The growing
point remains below ground for about 3 weeks or until five to six leaves have
fully emerged, after which its position elevates as the stalk begins its rapid
elongation phase.
While the growing point is below
ground, the corn plant is relatively safe from above-ground damage to the leaves
and stem. Severe damage from late frosts, wind blasting, hailstones, anhydrous
burn, insect feeding above ground, etc., will usually not kill corn plants
younger than the 6-leaf stage as long as the growing point is not damaged. On
the other hand, while the growing point is below the ground, the plant is more
sensitive to below-ground insect feeding, flooding or saturated soils.
To determine the viability of young
corn plants damaged by frost or some other defoliation event, the simplest
advice is to wait for several days after the date on which injury occurred for
signs of regrowth. Generally 2 to 4 days of 70 degree F or warmer temperatures
are sufficient to stimulate new leaf growth on an affected plant. If
temperatures have been cooler than normal, regrowth may not be readily evident.
In that event, you can determine plant viability by splitting the stem and
looking at the growing point. If the plant is "healthy" the growing
point will be firm and white or cream colored. A darkening or softening of the
growing point usually precedes plant death.
One of our recent field studies
demonstrates that stand losses due to early season leaf damage can be increased
by unfavorable growing conditions following a defoliation event. In 1998 and
1999, we conducted experiments comparing the effects of defoliation on different
types of corn. The defoliation treatments were intended to simulate leaf
destruction caused by various conditions such as frost, hail, insect feeding,
etc. One of the defoliation treatments involved 100% leaf removal at the V4-5
(4-5 leaf collar) stage of development, with all leaf tissue above and including
the first rounded leaf removed. In 1999, 100% defoliation had little or no
effect on stand at 4 test sites; however, in 1998, 100% defoliation at V4-5
caused major reductions in stand at 2 of 5 test sites (unrelated to corn type).
Stand losses were attributed to seedling blights caused by wet, cool conditions
following defoliation. The 2 sites with major stand losses in 1998 experienced
significant rainfall ( 3 inches of rain or more) and below average temperatures
during the week following 100% V4-5 defoliation. At sites where conditions were
drier, or warm and dry, stand loss following the defoliation treatment was
generally minimal.
Wheat growth rate was rapid last week
due to very warm temperatures, but recent cooler weather over the weekend has
slowed development to more normal rates. Most wheat fields in southern Ohio are
in heading (growth stage 10.3) to flowering (growth stage 10.5.1) growth stage.
The crop is in beginning to head (growth stage 10.1) in northen Ohio. The ten
day forecast is for moderate temperatures (lows in mid 50s and highs in low 70s)
and high humidity with normal precipitation.
Over the past two weeks our predictions
for POWDERY MILDEW have been right on target. At Wooster, powdery mildew is not
attacking the flag leaves. The weather predicted for the next week will continue
to favor the spread of powdery mildew. Any fields that had powdery mildew
earlier in the season should be scouted again right now. If powdery mildew is
beginning to show up on the leaf below the flag leaf and the variety is rated as
susceptible, a fungicide application could be made to control further yield
loss. Make the decision to spray based on your estimates of overall yield
potential of the field and the current price of wheat.
STAGONOSPORA LEAF BLOTCH is beginning
to show up in some fields. The rains and high temperatures last week spread more
spores. The disease is now detected on the F-2 leaf (third leaf down the stem
where the flag leaf is leaf number 1) at Wooster on plants that are just
beginning to head. The cooler temperatures expected over the next 10 days will
slow disease development, but rain will help splash spores up the plant to cause
new infections. Scout fields now to see if Stagonospora leaf blotch is on the
F-1 leaf (leaf below the flag leaf). A fungicide spray may be needed if the
variety is susceptible to Stagonospora nodorum.
Fungicides are labeled for applications up until the wheat is in mid-flower (growth stage 10.5.1). Tilt (Novartis Ag Products) has label restrictions that prevent later applications. The wheat crop is rapidly approaching flowering and the time when growth stage will limit the use of fungicides. It is important to visit fields now to scout for diseases before it is too late to spray.
The wheat crop is now entering the stage of development that is critical to HEAD SCAB development. Head scab is caused by Gibberella zeae, a fungus that survives from year to year in crop residues, especially corn residues on the soil surface. This fungus produces spores that are carried by wind currents to the heads of wheat. We have been monitoring the amount of spores in the air around wheat fields at Wooster over the past week. We have found low levels of spores in the air. We have also examined corn residues in the field and have found the fruiting bodies of the Fusarium fungus with maturing spores. It appears the fungus is ready to infect the crop. The occurrence of head scab is also dependent on an extended period (24-72 hours) of precipitation at the time of flowering to initiate the infection process. The weather that occurs this week and next week will be critical to infection and disease development. So far this month we have seen only single days of precipitation or scattered thundershowers. Unless we get several days of precipitation over the next week or so, we will not see head scab as a major problem. Our best advise is to watch your wheat fields to determine when the anthers are visible on the heads. This is the flowering period. If rain occurs for more than a 24 hour period during this flowering period, scab could be important. You will be able to detect the first symptoms of infection (bleached spikelets on heads) within 7-10 days after infection. For more information, visit the Fact Sheet on Head Blight or Scab of Small Grains.
Barley yellow dwarf is becoming more easy to see in fields of early planted wheat as the plants begin to head. The most notable symptom is the yellowing and reddening of the tips of the flag leaves on affected plants. Although you will see an abundance of aphids on plants now, their occurrence will not tend to make the disease more severe from this time forward. The most severe symptoms occur on plants infected in early fall. The warm temperatures that extended into late fall must have favored the movement of aphids soon after planting last year. We are seeing a few plants and groups of plants with barley yellow dwarf in some fields planted on or soon after the Hessian Fly Safe Date. The Hessian Fly Safe date is based on an average over many years. This date is used as a predictor for Hessian fly and because it is based on temperature accumulations, it also works for other insect related problems as well. However, we must remember that weather is variable and these dates are to be used as estimates of an 'average' year. We should expect some disease in seasons like last year due with extended warm autumn conditions. The good news is that in fields planted on or after the Hessian fly safe date, few plants are affected and yield losses will be negligible. For more information, visit the Fact Sheet on Barley Yellow Dwarf of Wheat, Oats, and Barley.
Pythium and Phytophthora sojae are two
water molds that can attack seeds and seedlings when the soils are saturated.
Pythium tends to be more active when the soil temperatures are cool and P. sojae
when the soil temperatures are at or above 60F. Symptoms on the plants for both
of these organisms are brown to light tan discoloration, sunken, and soft tissue
of the roots and hypocotyl. Pythium seed and seedling diseases can be managed
with a seed treatment (Allegiance or Apron XL) as well as increase soil
drainage. There are no soybean varieties that are resistant to Pythium.
Phytophthora is best managed with resistance, in combination with seed
treatments (at the highest labeled rates) and improving soil drainage.
Typically a resistance gene (Rps gene)
will provide good protection of soybean seedlings to Phytophthora. But over
time, populations of Phytophthora shift, in that they are able to cause disease
on the Rps genes used in that field. When this happens, producers need to change
to varieties with different Rps genes or look for varieties with combinations of
Rps genes or rely on varieties with partial resistance. If a producer is faced
with a replant situation, do not put the same variety back in the same field.
Also do not plant varieties with the same Rps gene. If the variety that was
wiped out has Rps-1a, switch to varieties with Rps-1c, Rps-1k, or Rps-3a. If the
variety that was wiped out has Rps-1k or Rps-3a, switch to varieties with gene
combinations. In both cases choose varieties with high levels of partial
resistance. READ the fine print on the tables, scores for partial resistance
range from 1 to 10. For some companies 1 indicates high levels of partial
resistance and for other companies 10 indicates high levels of partial
resistance. For the Ohio performance trials, scores of 3.5 to 4.0 indicates the
highest levels of partial resistance and should yield even under Phytophthora
pressure.
As more and more fields in Ohio develop
P. sojae populations which can cause disease on varieties with the different Rps
genes, it becomes critical that producers keep track of which Rps genes they
have used.
Rhizoctonia can also be a problem. Both
seeds and seedlings can be attacked. Symptoms of Rhizoctonia are depressed areas
that are brick red to dark brown in color on the roots and/or stem at the soil
line. Plants that do emerge will have an off-yellow color. There are a number of
seed treatment compounds which are effective for managing Rhizoctonia.
Black cutworm larvae ranging in size
from 3rd instar (ca. 1/2 inch long) to early 6th instar (more than an inch in
length) were found feeding on field corn in the VE and V1 stages of development
at the Western Branch station in Clark County last week. At the time of the
observations, cutting
was predominantly above ground and cut
plants would likely recover if a timely rescue treatment is applied. In
contrast, below ground feeding is generally characterized by wilting plants that
have been cut below the growing point. Visible cutworm injury to corn tends to
shift from above ground feeding to below ground feeding as corn plant
development progresses.
Where visible cutworm injury is
observed on 4% or more of a stand, application of a timely rescue treatment is
warranted, will arrest cutworm feeding, and prevent additional stand losses. If
a significant cutworm infestation is detected too late, below ground injury will
intensify and stand loss will increase to a point that application of a rescue
treatment may achieve marginal results. Thus, early detection of cutworm
infestations and timely application of rescue treatments are the keys to
achieving effective stand protection where preventive treatments have not been
applied.
For more information, visit the Fact Sheet on Cutworms, Stalk Borers and Webworms. Also visit the following images: Black Cutworm Below Ground Damage on Corn Plant, Black Cutworm Larva in Base of Corn Plant, Black Cutworm above Ground Damage on Corn Plant.
Sampling of alfalfa treatment trials at
the Western Branch station during the past week demonstrated a significant
decline in the number of weevil larvae. In untreated plots, alfalfa weevil
larvae were predominantly in the final 4th instar stage of development and pupae
could be found on the tips of most untreated plants. A high proportion of the
pupae cases contained pupae of parasitic wasps which indicates a vigorous return
of beneficial parasite activity and regulation of weevil activity in the future.
Soybeans have been emerging across the
landscape and bean leaf beetle feeding can be readily observed in the fields. In
some cases, the first soybeans emerging will exhibit significant feeding injury,
but the level of feeding injury will likely decline as the ratio of foliage to
beetles increases.
Slugs have began feeding on newly emerged corn in Wayne County. While the feeding is slight so far, the slugs are still small and will get much larger. This feeding might become more severe over the next few weeks. Growers and scouts should keep close watch on fields with a history of slug damage. This damage might also start being seen in soybean fields.
The 1999 Agronomic Crops Team On-Farm Research Reports are available on the web as Microsoft Word (*.doc) files. They can be found at: www.ag.ohio-state.edu/~corn/library/restrial/
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), Mark
Loux (Weed Science), Jeff Stachler (Weed Science), Hal Willson (Entomology), Ron
Hammond (Entomology) and Randall Reeder (Ag Engineering); District Specialists: Ed Lentz
(Agronomy); Extension Agents: Steve Bartels (Butler), Barry Ward (Champaign),
Steve Prochaska (Crawford), Dennis Baker (Darke), Bruce Clevenger (Defiance),
Greg LaBarge (Fulton), John Barker (Knox), Glen Arnold (Putnam), Ray Wells
(Ross), Clark Hutson (Seneca) and Roger Bender (Shelby).
Editor: Clark Hutson 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.
TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868
| C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |