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For Week of June 15-21, 1998
C.O.R.N. 98-14
In This Issue:
A) Saturated Soils = Phytophthora Root Rot in Soybeans
B) First Brood Corn Borer Activity - "Are There Any Out There?"
C) Rootworm Larvae Detected in Continuous Corn
D) Big Weeds in Small Soybeans
E) Stewart's Bacterial Wilt Causing Stunted, Tillering, Dying Corn Plants
F) Early Season Hail and Wind Damage to Corn
G) Potential for Double Cropping in 1998
H) The 1998 OSU Weed Science Field Day
Heavy rains at the end of last week and this weekend have provided ideal conditions for development of Phytophthora seedling blight and Phytophthora stem rot. The amount of damage caused by this fungus will depend on the age of the plants when they are infected as well as the type of resistance present in the soybeans. Young seedlings without any resistance will die as result of Phytophthora under saturated soil conditions. Soybeans which are in the unifoliate stage or older that do not have resistance may develop Phytophthora stem rot later in the season. Soybeans which are in the unifoliate stage or older and have high levels of partial resistance (tolerance score 1-3.5 in Ohio Soybean Performance Trials) may develop some Phytophthora root rot, but the effects on yields should be minimal.
Soybeans which are in the unifoliate stage or older with single Rps genes will be very resistant to Phytophthora unless there are races of Phytophthora in those fields that overcome that specific Rps gene. If producers are finding soybeans with Rps genes dying from Phytophthora they should switch to a different Rps gene. For example, if the soybean variety has the Rps1a gene (see the Ohio Soybean Performance Trials - score of 1 for Resist. No.) choose a variety with Rps3a (score of 6) or Rps1k (score of 10) the next time soybeans are planted.
Soybean varieties with a combination of single Rps genes and good levels of partial resistance or field tolerance and are treated with Allegiance or Apron XL seed treatment provide the best protection under conditions which are highly favorable for Phytophthora.
First instar larvae of 1st brood European corn borer (ECB) were observed in Clark County Wednesday, June 10th. Although most field corn is currently in the pre-whorl stages of development, ECB larvae could be detected in early planted field corn that was in the early whorl stage. Symptoms of early feeding on the foliage were minimal. However, 1st instar larvae could be found in plants exhibiting very small window pane feeding injury in the central whorl.
Given the rain storms that have passed through the state during the past week, mortality of early ECB larvae in infested corn should be significant. Since development of most corn fields is behind schedule and unlikely to be infested with 1st brood corn borer, the incidence of significant 1st brood corn borer injury this year may be minimal. However, corn stands in the early whorl stage should be scouted at the present time.
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European corn borer larva. |
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European corn borer "windowpane" leaf feeding. |
Early rootworm larvae could be readily detected in untreated continuous corn plantings during the past week. Early larvae were detected last Wednesday in our continuous plots, where corn stands ranged from V3 to V5 stages of development. Since rootworm hatch in early and corn development is delayed this year, the rootworm injury could be severe in fields having significant rootworm activity. However, if early lodging does not result from feeding injury on the root systems, the root systems may regenerate after rootworm feeding is concluded. The impact of rootworm injury on corn yield is dependent on a combination of: (1) rootworm abundance and (2) weather conditions that could either: (a) enhance the effects of rootworm feeding resulting in maximal damage or (b) favor root system growth, reducing the impact of rootworm feeding. Given the mix of weather conditions we have been having this year, the impact of rootworm on corn this year could be either severe or minimal.
Additional reading information is available through the Field Crops Pest Management Circular #16, Corn Rootworms, Ohio Pest Management and Survey Program.
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Corn stalk "goosenecking" resulting from corn rootworm root damage. |
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Corn rootworm larvae. |
Last week we wrote about weed control in dry weather and this week, we get to discuss how to control large weeds when excessive rain prevents timely post-emergence application. As we have mentioned in previous weeks, the weeds have generally been growing faster than the soybeans, and the rain we have received and continue to receive is further stimulating weed growth and emergence of new weeds. The common question seems to be: how will I control large weeds when it finally dries out enough to spray? By large, we mean outside of the normal spray range, or more than 6" tall.
Producers who have weeds taller than 6" and have planted a significant acreage to Roundup Ready soybeans should consider themselves fortunate. Roundup is more capable of killing larger weeds than other post-emergence herbicides, while not injuring the soybeans. The Roundup label calls for 24 oz/acre up to 6" weeds, 32 oz/acre up to 12" weeds, and 48 oz/acre for weeds taller than 12". Many fields are likely to have giant ragweed taller than 12", with other weeds still less than 12" tall. The strategy in these fields may depend upon soybean size. Where soybeans have significant size (>3 trifoliates ?), one application of Roundup should be sufficient. Be sure to select the Roundup rate based on weed size. Where soybeans are still small but weeds are large, it is possible that later-emerging weeds could cause problems. In this situation, consider applying 32 oz/acre of Roundup now, and following with an additional 16 to 24 oz/acre later as necessary. This has been an excellent program in areas where giant ragweed populations are dense and for control of late-emerging grasses.
In soybean fields without the Roundup Ready trait where weeds are large, apply non-Roundup herbicides as soon as conditions permit. Increase herbicide and adjuvant rates as necessary (and per the label). Additional herbicides can be added to the spray mix to improve control (again per the label), but this will also increase soybean injury. With the soil moisture high and high temperatures forecast for the end of the week, we anticipate that post-emergence herbicides will be extremely active on weeds and crops through the end of this week into next week. Therefore, think twice before making herbicide mixes too "hot". OSU research has generally shown that small soybeans sprayed in early July or later have less time to regrow following herbicide injury, and this can result in yield loss.
With regard to the time of weed removal and yield loss, OSU research shows that removal of weeds before they exceed about 6" in height will prevent yield loss across a range of conditions. However, in some experiments, weeds could be removed at a height of 12" with no yield loss. There is obviously quite a lot of variability in the effect of weeds on the crop across years, soil type, tillage, etc, but we do know that waiting until weeds are 9" to 12" tall before removal will reduce yield in some fields. So, try to control the weeds before they are 9" tall, even in Roundup Ready soybeans.
There will be a rush to spray Roundup and other herbicides on soybeans as soon as conditions permit, but be aware of the conditions that promote herbicide drift. Nearby fields of corn, non-Roundup Ready soybeans, vegetables, as well as homeowners' gardens can be affected by drift of most herbicides. Consider increasing spray volume and switching to drift-reducing nozzles to reduce drift. The large weeds present in many fields may require a spray volume of 15 to 20 gpa for adequate coverage, even with systemic herbicides. Increasing spray volume will improve spray coverage more than increasing spray pressure, assuming the pressure is already at least 40 to 50 psi. Increasing pressure beyond this point will generally not improve control, but will increase spray drift.
We have received many samples of stunted corn plants that have produced several tillers and a dead main stem. The main stem of some young corn plants appears wilted while the tillers look fairly healthy. These plants are affected by Stewart's wilt, a bacterial disease that is transmitted by the corn flea beetle. Beetle feeding and transmission of the bacterium must have occurred as soon as the young plants emerged from the soil to cause the wilt phase of the disease. Most young plants also have long yellow stripes on leaves within the whorl, this is the leaf blight phase. Not all plants are showing the yellow stripes, but are systemically infected causing the main stem and growing point to die. These plants will not recover. Other, taller plants are just showing the yellow stripes with no stunting. You can expect these plants to show more advanced symptoms as the season progresses. The wilt problem appears to be quite widespread in the state, but we have not had enough samples to determine the extent of the problem. We rarely see systemic infection from Stewart's wilt in field corn, it is more common in sweet and pop corn. Growers should expect to see continued spread of the leaf blight phase of the disease throughout the summer. Spread will occur through continued feeding of beetles on corn plants. The amount of yield loss from the leaf blight phase will depend on the susceptibility of the hybrid and the extent of leaf damage. Severe blighting of the upper leaves soon after tasseling will result in yield losses.
Severe thunderstorms accompanied by hail and strong winds have the potential to cause considerable damage to corn. The extent of the damage is dependent on the corn crop's stage of development. Hail affects yield primarily by reducing stands and defoliating plants. Defoliation usually results in most of the damage. The corn plant is generally little affected by hail prior to the 6 to 7 leaf stage because the growing point is below the soil surface and in the leaf whorl. However, once the growing point is elevated above the soil surface due to inter-node elongation, the plant grows rapidly and becomes increasingly vulnerable to hail damage with the tasseling stage being the most critical period.
Leaf damage by hail usually looks much worse than it really is, especially during the early stages of vegetative growth. Shredded leaves still have some capacity to contribute to plant growth. Plants not killed outright by hail usually show new growth within 3 to 5 days after injury occurs (i.e. if damage occurs prior to tasseling). For this reason, estimates of hail damage should be delayed several days to allow for this period of regrowth.
Based on estimates of the National Crop Insurance Association, at the 11-leaf stage if 50% of the leaf tissue is destroyed by hail, a corn plant loses only 7% of its yield potential; if 100% defoliation occurs, a corn plant loses 22% of its yield potential. At the 17-leaf stage, which occurs shortly before tasseling, a corn plant may lose up to 72% of its yield potential with 100% defoliation.
Strong winds and heavy rain can also lodge or knock plants over, especially if the nodal root system is not fully developed. The dry weather conditions we experienced recently in some parts of Ohio may have inhibited good nodal root formation and predisposed plants to such wind injury. This is a common problem in corn plants just prior to the rapid stalk elongation phase of development. (Note my C.O.R.N. article on "rootless corn" from last week). Plants usually recover from such wind damage quickly with little impact on performance as nodal roots are established. If winds cause plant stalks to snap and break over, then the plants are usually killed outright. During the rapid phase of vegetative growth the stalk tissue can be fairly brittle and stalks can be snapped by wind, especially when environmental conditions are very favorable for rapid stalk elongation. Certain hybrid genetics and herbicides can contribute to stalk "brittleness."
For more information on evaluating hail injury, consult "Assessing Hail Damage to Corn," National Corn Handbook Chapter 1. Please contact my office (614/292-2047) if you would like a copy of this bulletin.
The possibility of an earlier maturing wheat crop has raised questions about the possibility for double cropping. The two primary requirements for success are: 1) an early wheat harvest made even earlier by harvesting wheat at 19 percent to 22 percent moisture, and 2) lots of soil moisture to get the beans germinated and growing quickly. The cultural practices needed for successful double cropping can be found in the Agronomy Guide.
Wheat will likely be harvested one to two weeks early, satisfying one of the primary requirements. Most wheat fields are so dry as of June 9th that they will need 2 to 5 inches of rain to have enough moisture to get soybeans started well (as of June 11-14, this requirement has also been met). An important rule-of-thumb is "IF JUNE IS DRY, DON'T TRY" to double crop soybeans after wheat. Finally, most f our wheat crop is located in northwestern Ohio and grown on soil with high clay contents such as the Hoytville and Toledo soils which are droughty. Double cropping is rarely profitable on these soils. If a field won't produce a consistently good corn crop (150-180 bu/ac) with normal rainfall, it is probably too droughty for double cropping.
The 1998 OSU Weed Science Field Day will be held on July 8, 1998. This is a self-guided tour of the OSU weed management research plots, including evaluations of new herbicides, herbicide- tolerant crops, and research on narrow rows and weed interference. We will start at 9:00 at the OARDC Western Branch, break for lunch at noon, and resume the tour in the afternoon at Farm Science Review. A lunch will be available at Western Branch at a cost of approximately $10 per person. Reservations are not required, but we would appreciate a phone call to let us know how many in your organization are attending so we can plan for lunch.
The OARDC Western Branch is located about 5 miles south of I-70 on SR 41. Our plot area at Farm Science Review is located north of I-70 on SR 38, directly across SR 38 from the large red storage shed (about 1 mile north of the I-70 overpass, and after the road makes two sweeping turns). There is no exit from I-70 for SR 38 - you must exit I-70 at SR 29 and go west to SR 38, or you can exit at SR 42 or 56 and take SR 40 to SR 38. We look forward to seeing you on July 8.
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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 and Anne Dorrance (Plant Pathology), Jim Beuerlein (Wheat and Soybean Production), Hal Willson (Entomology), Peter Thomison (Corn Production), and Mark Loux and Jeff Stachler (Weed Science); District Specialist: Curtis Young (IPMP); Extension Agents: Roger Bender (Shelby), Clark Hutson (Seneca), Dave Jones (Allen), Larry Lotz (Fayette), Barry Ward (Marion), Howard Siegrist (Licking), Steve Prochaska (Crawford), Gary Wilson (Hancock) and Ray Wells (Ross).
Editor: Curtis Young Web Editor: Nathan Watermeier
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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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