|
|
|
In This Issue:
A) Late Postemergence Herbicide Application Precautions
B) Herbicide Label Updates
C) Erratic or Thin Soybean Stands
D) Impact of Drought and Heat Stress on Corn Yield
Potential
E) Spider Mites on Soybeans
F) 1999 First Year Corn Rootworm Survey
G) Potato Leafhopper Activity Increases
H) The 1999 Southern Ohio Crop Diagnostic Field Day
Now that we have gotten some much needed rain, the weeds will grow very rapidly, unless it gets too cool. Actively growing weeds will be easier to control than last week, unless the weeds were severely stressed last week. If weeds are actively growing and are small then use the least injurious adjuvants with the herbicides. However, if the weeds are large and were previously drought stressed, then keep the MSO's in with ALS herbicides to get effective weed control, even though crop injury may occur. Soybeans injured this week from herbicide injury should not reduce yield. However, soybean injury at the end of June through August can cause yield reductions.
Permit has received a supplemental label for postemergence applications to popcorn and sweet corn for control of yellow nutsedge and certain broadleaf weeds. The label states that it should not be used unless the seed company, processor or State Agricultural Extension Service has tested Permit on the particular hybrid/variety and specifically approves and recommends the use. Do not use Permit on 'Jubilee' sweet corn. The first application of Permit can be applied broadcast from spike to layby stage of growth. The label allows Permit to be applied in a single application at 0.67 ounces per acre and if a second is needed that it must be applied with drop nozzles. The label says do not apply Permit to sweet corn or popcorn that is under stressful conditions such as drought or too wet as well as others.
Plant population has very little effect on soybean grain yield. When rows are 7.5 inches apart there in virtually no yield difference for uniformly distributed populations of 75,000 to 150,000 plants per acre. Seeding rate studies have shown that plant populations as low as 50,000 plants per acre can produce yields that are 95% that of the more normal stands of 100,000 to 150,000 plants per acre.
Soybean grain yield is effected more by the distribution of plants than the number of plants. The ultimate goal is to have a complete leaf canopy by the end of June. The yield lost due to delayed canopy formation (late July) is not large, and the most serious effect of thin stands is the loss of competition with weeds.
When areas of fields have less than 50,000 plants per acre, it is acceptable to thicken the stand in those areas by inter-seeding an additional 100,000 seeds per acre. Always save as many existing plants as possible when inter-seeding. It is usually unwise to till and replant the entire field.
Although drought and high temperatures have resulted in some plant death, most stressed corn is "holding on". In many stressed fields, there is soil moisture available but it appears beyond reach of most corn roots. Current weather conditions have inhibited root development. Plants with root systems restricted to the upper four to five inches of the soil profile have been stunted and show drought stress during afternoon hours.
Development of the nodal root system (or permanent root system) in corn occurs during early vegetative development and, by the sixth collared leaf stage, nodal roots have usually replaced the smaller seed (seminal) root system as the primary source of water and nutrients to the plant. The hot, dry weather during the past two weeks has resulted in soil temperatures exceeding 90 degrees F and under such conditions growth of nodal roots ceases. Purple corn, a problem related to restricted root development (or root damage), was described in C.O.R.N. 99-11 (for week of June 1-6, 1999).
Given the severe stunting, wilting, and off-color appearance of drought stressed corn plants, a commonly asked question will be whether stressed plants still have the potential to produce normal yields, i.e., will stressed plants recover when favorable growing conditions return?
Past research indicates that drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking (about the 17th collared leaf stage). Moisture stress during these later vegetative growth stages (i.e. the three to four week period before silking) can result in grain yield loss of 3% per day. But yield reductions as high as 8% per day can occur as the result of severe moisture stress during the reproductive stages, tasselling and silking. Since most stressed corn is not far beyond the ninth collared leaf stage, the impact of early drought on yield may be limited if favorable growing conditions return shortly.
Severe moisture stress has caused stand loss in some fields and this will have a direct effect on yield. Where dry weather has resulted in corn stands with uneven germination, emergence, and development, yield loss may range from 5 to 20% depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight. If drought stress has enhanced plant injury from herbicides, fertilizers and insects, then some permanent reduction in yield potential is possible. Limited soil moisture has contributed to problems involving root injury from pop-up starter fertilizer and anhydrous ammonia which in some cases have caused significant stand loss.
Although there will be yield losses in certain areas of the state due to prolonged drought and heat stress, corn which has been less severely stressed may be in a better position to withstand drought later this summer. Moderate moisture stress during early corn growth may be advantageous because it promotes deeper early season root growth which can prove beneficial later if moisture supplies are deficient. In 1994 we experienced a similar period of hot, dry weather early in the season but rains in late June alleviated the stress and corn crop yields were among the highest on record.
 |
Spider mite damage to soybean leaves |
Field Symptoms: Spider mites feed on the underside of the foliage with sucking mouth parts and may be very destructive when abundant. Under hot and dry field conditions, spider mites thrive on plants that are under stress. Soybean foliage infested with spider mites initially exhibit a speckled appearance. As plants become heavily infested, foliage turns yellow, then bronze, and finally the leaves drop off the plants as the effect of heavy feeding leads to dehydration and death of plant.
In the 1988 spider mite outbreak on soybeans, we developed a scheme for evaluating infested fields base on observable symptoms and conditions as follows:
1. Mites barely detected at field perimeter or other dry locations. Multiple plants need to be inspected before mites are found. Assessment: Clearly non-economic.
2. Presence of mites easy to detect at field perimeter and dry spots. Difficult to detect within field. Foliage is still green but stippled feeding injury with few mites under lower sides of leaves detectable, but not on every plant. Assessment: Clearly non-economic, but warrants monitoring.
3. Entire field (all plants) exhibits some sign of infestation with speckling and some discoloration of lower leaves. Foliage exhibiting various levels of stippled feeding injury on relatively healthy foliage. Field perimeter and dry spots exhibit severely damaged plants. Assessment: Rescue treatment warranted at this point, especially if immature stages are in abundance or heavy egg laying present.
4. Infestation widespread with discolored and wilting foliage easily detected throughout field. All plants heavily infested when examined closely. Severe damage evident. Assessment: Effective rescue treatment will save field.
5. Total field discoloration and drying down of foliage. Significant foliage and stand loss evident. Assessment: Field may be beyond point of recovery if rescue treatment applied. However, new growth may resume if treated.
In making an assessment of a spider mite infested field, it is important that one recognize the early signs of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when foliage is still green. In addition, it is essential that one uses a good hand lens to view relative abundance of mites in egg, nymph and adult stages. Significant infestations observed during the past week were on no-tillage soybeans. However, it is important to note that one field may exhibit a severe spider mite infestation while fields nearby may exhibit minimal or no spider mite activity.
Rain over the weekend followed by cooler temperatures may reduce spider mite infestations or their effects. However, vigilance is still recommended since populations of spider mites could rapidly increase if dry, hot weather returns.
For additional information, consult the Ohio IPM web site: http://www.ag.ohio-state.edu/~ohioline/icm-fact/fc-24.html
This year, we will again be monitoring WCR activity in soybeans with
the Pherocon AM traps. We would like to have a minimum of four soybean
sites monitored per county to obtain an index of WCR activity in soybeans.
Monitoring a field sites requires deployment of at least four traps in
a field which must be serviced on a biweekly schedule from mid July (7/15
or 7/16) through late August (8/26 or 8/27). In Extension, we are providing
traps to monitor at least four sites in counties requesting to be participants
in the survey. Growers, consultants or dealers interested in participating
in the survey should contact their local OSU Extension Ag Agent.
 |
Pherocon AM trap in soybeans. |
 |
Western corn rootworm on soybeans leaf. |
 |
Potato leafhopper damage to alfalfa. |
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
(Plant Pathology), Anne Dorrance (Plant Pathology), Jeff Stachler (Weed
Science), James Beuerlein (Soybeans & Small Grains), Peter Thomison
(Corn Production) and Hal Willson (Entomology); DISTRICT SPECIALIST: Ed
Lentz (Agronomy, Northwest); EXTENSION AGENTS: Dave Jones (Allen),
Steve Prochaska (Crawford), Larry Lotz (Fayette), Greg LaBarge (Fulton),
Gary Wilson (Hancock), John Barker (Knox), Howard Siegrist (Licking), Clark
Hutson (Seneca) and Woody Joslin (Shelby)
Editor: Clark Hutson Web Editor:
Clark Hutson
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
|