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For Week of August 3, to August 9, 1998
C.O.R.N. 98-21
In This Issue:
A) Late Season Soybean Diseases
B) Update! Rootworm Activity in Ohio
C) Update! Corn Leaf Aphid on Field Corn
D) Update! Soybean Defoliator Activity
E) Results from Wheat Management Studies
There are several soybean diseases that may appear during and following pod fill in Ohio: Sclerotinia white mold, Phytophthora stem rot, Sudden Death Syndrome, Brown Stem Rot and Diaporthe stem canker. Sclerotinia white mold has been discussed extensively in earlier C.O.R.N. newsletters. Symptoms for each of these soybean diseases can be diagnostic to some extent. Brown stem rot and Sudden Death Syndrome both have tan to brown areas on the leaves surrounded by yellow while the veins remain green. Sudden Death Syndrome tends to have more circular yellow spots on leaves. The key difference between Brown Stem Rot and Sudden Death Syndrome is the discoloration of the pith in brown stem rot. The pith in soybean plants with brown stem rot is brown whereas the pith in soybean plants with sudden death syndrome is white. Soybean plants with Sudden Death Syndrome will have few if any roots and plants that are severely infected may have the blue spores of the fungus present on the tap root.
Soybean plants that appear wilted in fields may have Phytophthora stem rot or Diaporthe stem canker. Diaporthe stem canker appears as a small reddish-brown lesion on the lower third of the stem. The canker will girdle the soybean stem but the stem will remain green below. This characteristic will separate this disease from Phytophthora. The fungus that causes Diaporthe stem canker is the same fungus involved in seed decay. The incidence of stem canker rarely exceeds 1% in any field. Phytophthora stem rot will have very few if any roots and the stem will be discolored from the tap root up several nodes on the stem.
For fields that have high numbers of plants with these diseases, a good crop rotation is essential to reduce fungal populations. During years when soybean plants are planted in those fields, use resistant varieties. Some form of tillage will also reduce fungal populations that survive on soybean residue for some of these diseases.
Trap counts, sweep net counts and visual counts taken in corn and soybeans across the state during the past week demonstrated rootworm activity ranging from significant first year corn rootworm activity in the northwest counties bordering Indiana to minimal signs of rootworm activity in central Ohio. Collections of adult western corn rootworm from Pherocon AM traps stationed in soybean fields in Shelby, Logan, Clark, Union, Delaware, and Knox counties (to mention a few sites which I personally inspected) were minimal (i.e. usually zero). Sweep net samples of soybean fields also generated no adult rootworms. However, Multiguard sticky traps near corn fields sometime collected ten or twenty adult rootworms, but not the traps situated some distance away from corn exhibited not catch of rootworm beetles. Multiguard traps in first year corn at survey sites in Shelby, Logan and Clark counties exhibit rather high rootworm beetle counts in first year corn, but the movement of such activity into adjacent soybean fields is not evident to date. Corn silks in many corn fields are beginning to turn brown and if first year corn rootworm is present in an area, movement of the adult rootworms into soybean fields should become evident during the next few weeks when rootworm beetles migrate from their field of origin.
Curtis Young (IPM specialist in the northwest district) has reported significant activity both in traps and sweep net collections in Van Wert and Paulding counties plus some neighboring areas. Jim Jasinski (IPM specialist in the southwest district) is reporting moderate to low levels of rootworm activity. Results of the northwest and southwest districts will be reported in more detail next week.
Infestations of corn leaf aphids have been occasionally observed during the past week, but infestation levels are generally low. Corn leaf aphid abundance is generally proportional to the proportion of stand exhibiting colonies of CLA. When 75% or more of a stand exhibits abundant colonies of CLA, heavy infestations of CLA will be observed on numerous plants and pollination may be disrupted by excessive production of honeydew which tends to tie up the pollen. However, infestations of CLA are often cleaned up by a complex of beneficial insect predators such as Syrphid fly larvae and Coccinellid lady beetles. During the past week, I observed more lady beetle larvae on corn plants than rootworm adults. Thus, I tend to believe that nature may be taking care of most infestations of CLA.
As we sweep net sample soybeans around the state for adult corn rootworm, we are also noting the relative abundance of various soybean defoliators. Bean leaf beetle (BLB) is currently in the 1st generation of adult activity and numbers have been running from about two to eight per ten sweeps. Collection of only two to three BLB adults results in a level of defoliation of about 2%. As BLB collections approach one beetle per sweep, defoliation will be about 6 to 8%. The only field observed having one beetle per sweep was an early planted field. This particular field currently does not have enough BLB activity to cause significant pod injury, and will likely be harvested before the second generation of BLB become active. BLB activity in late planted soybeans is generally observed in low numbers, since such fields were probably planted too late to attract colonization by the over-wintering generation last spring.
Japanese beetles are more active than normal and are contributing to levels of defoliation observed in the field. In some corn fields, one may find more Japanese beetles feeding on corn silks than adult rootworms.
Studies were conducted over the past growing season to evaluate various production methods on maximizing wheat yields in Ohio. Two studies were conducted at each of three locations to evaluate the effect of early planting, seeding rate and nitrogen management (both fall and spring) on wheat yields. Dates of planting were 14 days before the Hessian fly-safe date, on the fly- safe date and 14 days after the fly-safe date; seeding rates were 16, 20 and 24 seeds per foot of row. Plots received either no nitrogen at seeding or 40 pounds per acre. Spring nitrogen treatments were either 60 or 100 pounds per acre in March or 50 pounds in March followed by another 50 pounds in mid- April. Most treatments were designed to increase the amount of fall growth, to improve winter hardiness and increase the length of the grain filling period.
Although there were some differences due to treatments among locations, the average yield of all plots was 83.5 bu/A. The average yields at sites in Wayne, Wood, and Clark County were 106, 73, and 71 bu/A, respectively. Seeding rates of 16, 20 and 24 seeds per foot of row produced yields of 83, 83 and 85 bu/A, respectively. Planting before, on and after the fly-safe date produced yields of 80, 82 and 76 bu/A, respectively. However, at the Wood Co. location the early planted plots were severely affected by the aphid transmitted virus disease Barley Yellow Dwarf resulting in a yield reduction of 15 bu/A. Applying 40 pounds of nitrogen in the fall increased yield 3.1 bu/A. Splitting the spring application of 100 pounds of nitrogen reduced yield by 3.7 bu/A as compared to applying the full amount one time in early spring.
A second management study was conducted on six commercial farms in Hardin, Henry, Highland, Fairfield, Pickaway and Wood Counties. These large plot studies were conducted by the farmers and were designed to evaluate the effect of seeding rate (20 vs. 30 seeds per foot of row), fall nitrogen fertilization (0 vs. 40 pounds nitrogen) and spring nitrogen application (60 pounds vs. 50 pounds in March and 50 pounds mid-April split application). The data from the six locations were combined for data analysis. The 30 seeds per foot of row seeding rate produced higher numbers of tillers in the spring than the 20 seeds per foot of row seeding rate (33 tillers vs. 26 tillers per ft., respectively). Applying 40 lbs nitrogen at planting increased spring tillers numbers as compared to no nitrogen at planting (31 tillers vs. 28 tillers per ft.). The 50 lbs N early / 50 lbs N later split application had significantly higher yield than the one time 60 lbs N application in early spring (81.3 bu/A vs. 76.6 bu/A, respectively).
Results from this years studies indicated that yield was improved by planting the higher seeding rates on or soon after the Hessian fly-safe date, and applying nitrogen in the fall at planting. Based on economics, the most profitable nitrogen treatment (based on $3.00 wheat price and $0.25/ lbs for nitrogen) was 40 lbs N in the fall and 60 lbs of N as a one time application at green up. Of course this year, weather conditions were most favorable for overwinter survival and tiller development in the spring. The results may be quite different in a more normal year with a more severe winter and a cold, wet spring. More studies are planned for the 1999 season.
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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: James Beuerlein (Crop Impr, Soybeans, & Sm Grain), Ann Dorrance (Plant Pathology), Pat Lipps (Plant Pathology), Mark Loux (Weed Science), and Hal Willson (Entomology); District Specialists: Ed Lentz (Northwest Agronomist); Extension Agents: David A. Jones (Allen Co.), Steve Prochaska (Crawford Co.), Larry Lotz (Fayette Co.), Gary Wilson (Hancock Co.), Clark Hutson (Seneca Co.), and Woody Joslin (Shelby Co.).
Editor: David A. Jones; Web Editor David Etzkorn
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.
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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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