Volume 4, No. 3 , July 2001

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Index


Troubleshooting the Soybean Crop

Problems with soybean growth and development will occur every year. Diagnosing these problems can sometimes be a challenge but is always a learning opportunity. Keen observation and close attention to details is imperative. Visual diagnostic aids such as insect, weed, and disease identification or herbicide injury sheets are valuable and can be obtained from various sources. But most important are the questions that are asked. Symptoms do not always follow the textbook and obvious patterns do not always exist, but historical information on the field may give solutions to the problem. Below are some guidelines for diagnosing problems with the soybean crop.

Preliminary Fact Finding. Obtain information on each of the following:

The Field Visit. After obtaining as much information as possible before going to the field, following the generally guidelines listed below:

Analysis of Data

Drawing a Conclusion. Review the facts and data. Evaluate the data regarding what is normal and abnormal. Eliminate unlikely causes. Validate the likely causes (for instance, streaks in the field are related to spraying, tillage, or planting equipment). Remember, a conclusion may not be able to be drawn in the field, especially if laboratory analysis is needed to confirm your suspicions. However, be prompt with you diagnosis. Solve the problem as soon as possible, so remedial actions can be taken.

Follow Up. If the problem was identified, did corrective actions resolve the problem? Re-visit the field. Gather and read any relevant information such as Extension publications, labels, journal reprints, etc. Forward to appropriate personnel.

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Seedling & Vegetative Soybean Diagnostic Outline

The guide below offers some possible causes of the injury symptoms described. However, one should not assume that the causes listed are the one that is being experiences unless clear proof is available via pest identification or laboratory analysis.

Seedling (Emergence to V4)

Plants are dead, dying, or stunted in isolated or widespread areas

Plants have spots, damage or discoloration of leaves and/or stems; plants may appear stunted with abnormal growth in isolated or widespread areas.

Vegetative Stages (V5-R1)

Plants are wilted or dead

Plants have damage to leaves or stems

Plants have spots, discoloration of leaves and/or abnormal stem growth; roots may also show stunted or abnormal growth

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Micronutrients

Direct application of micronutrients to soybean is usually not needed in Virginia. Most soils have inherent trace amounts. Micronutrients (Mn, Fe, B, Cu, Zn) availability is closely related to soil pH. For most cases, a soil with a pH in the range of 5.6 to 6.2 is generally satisfactory for providing all micronutrients, with the exception of molybdenum (Mo), needed for normal soybean growth. Usually deficiencies in micronutrients can be related to too high of a soil pH. Although deficiencies of most micronutrients are rare, there are a few that may occur in Virginia and are listed below.

Molybdenum. In contrast to most micronutrient deficiencies, Mo deficiencies become apparent at low, not high soil pH. If the soil pH is less than 5.8, deficiencies can occur. If deficiencies are suspected and soil pH is low, a foliar spray at _ elemental Mo per acre can be made. Mo, can also be applied as a seed treatment at 1 ounce of elemental Mo per acre. But remember, lack of Mo is not the problem, low soil pH is. Therefore, a long-term solution is to correct the soil pH.

Manganese. Over-liming (pH > 6.5) can induce manganese (Mn) deficiencies, and sometimes iron (Fe), zinc (Zn), and copper (Cu) on very sandy soils. Where these deficiencies occur, foliar applications may be necessary. Mn deficiencies have been reported on soils with an average pH as low as 6.0 to 6.2; some fields consistently show deficiencies. While correcting soil pH can be a long-term solution to some elements such as Mo, this is not always a practical solution for Mn. Soils should be maintained at a pH of 5.8 to 6.2 for optimal nutrient use and nodule development in soybean. Occasionally, especially the season following lime application, pH can temporarily rise above 6.2. Therefore, Mn deficiency is a real possibility.

Mn deficiency is recognized as chlorosis (yellowing) between the leaf veins of the new leaves. Mn is not a mobile nutrient, therefore the symptoms show up in the newly emerged leaves. Symptoms can also appear on lower leaves, but upper leaves are usually affected too. Tissue samples may reveal Mn deficiencies earlier that visual observation. Yield loss will depend on the stage of development that the soybean is affected. Early deficiencies will slow vegetative growth, but vegetative growth will resume if deficiencies are alleviated with Mn applications. Deficiencies at the later reproductive stages will reduce yield more because this will affect actual yield components (flowers, pods, and seed). Therefore, correct Mn deficiencies before flowering.

To correct Mn deficiency, apply _ lb chelated Mn (elemental basis) or 1.0 lb inorganic Mn (elemental basis) per acre to foliage upon appearance of symptoms and prior to bloom. More than one application may be needed to correct the deficiency.

Boron. Some research in the U.S. has shown that applications of boron (B) to R3-stage (early pod development) soybean will increase yield. However, experiments conducted in Virginia during 1997-1999 have not revealed any response to these "late-season" applications. These studies concluded that native B levels of 0.1 to 0.2 ppm as determined by the Mehlich I extraction appeared to be adequate to achieve high yields. This was validated by tissue analysis showing B content within or above the sufficiency range set forth for soybean production. Therefore, there is no reason to apply B to soybean in Virginia unless tissue analysis indicated a deficiency.

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Leaf-feeding Insects Pest Management

Ames Herbert, Extension Entomologist Leaf feeding insects can attack soybeans at almost any time during the season. Usually leaf feeding occurs continually throughout the season resulting in cumulative leaf damage. Recent research has shown that full-season and double-crop planting systems may react differently to this leaf damage. Soybean yield appears to be highly related to total leaf area, as measured by LAI (leaf area index), which is the area of leaves above a given area of ground. To achieve maximum potential yield, soybean must develop an LAI of 3.5 ­ 4.0 or above by the late-pod to early-seed development stage. To visualize LAI, think of a field with an LAI of 4 having four complete layers of leaves that can capture 95% of the sunlight; little sunlight is hitting the ground. Any leaf area above the 4.0 level can be removed (i.e., by leaf feeders) without reducing yield potential. Most full-season plantings achieve larger leaf canopies and LAIs, regardless of the climatic conditions (temperature, cloud cover, rainfall, etc.) during the season and are much more tolerant of leaf feeding. However, double-crop plantings do not always achieve as large a leaf canopy and therefore can be more sensitive to defoliation by insects.

With the above discussion in mind, we feel that the "traditional" percent defoliation thresholds (40% prior to bloom, 15% from flowering to pod fill, 35% after pod fill) should only be applied to full-season plantings, or double-crop plantings that, because of good growing conditions, achieve large canopies. Be more conservative with double-crop systems that do not achieve large canopies due to late planting, dry conditions, poor soil, or other factors that result in less than optimal growth. With double-crop plantings, allow lower levels of leaf loss before making insecticide treatments. Suggested thresholds for poor-growth double-crop plantings are currently: 20% pre-bloom, 10% from flowering to pod fill, 15% after pod fill.

When rating defoliation, sampling should start as soon as plants emerge, and continue throughout the season. Casual visual field surveys for "ragged" insect eaten leaves can suffice until a problem is suspected. If a problem is suspected, determine the percent defoliation of the field by checking several whole plants, not just the upper canopy, in several representative locations in the field. To "calibrate" your estimate of percent defoliation, pick several leaflets from all levels of the caopy and percentages of all leaflets sampled and divide that sum by the number of leaflets rated.

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Diagnosing Soil Compaction

Soil compaction reduces yields annually in Virginia, but diagnosing when and where compaction is occurring can be challenging. The above-ground crop symptoms can be very similar to other problems and include: slow plant emergence, irregular plant height, and multiple nutrient deficiencies. But the telltale sign is below ground ­ the roots. If the roots are growing laterally, then compaction should be suspected. In most cases, our sandier soils that undergo lots of tillage are more prone to compaction.

There are three main diagnostic methods and they are described below. However, before proceeding to the center of the field, take samples at two places before: 1) a field edge such as a wooded area or fencerow that should not be compacted; and 2) at the entrance of the field where equipment movement should insure compaction. These two points will serve as good references.

Diagnostic Methods

  1. "Knife-blade" technique. Dig a hole 24" deep and gently insert the knife into the soil profile. Note differences in resistance as you insert the knife in various places in the soil profile

  2. Soil probe technique. Using a standard soil sampling probe, insert the probe in several areas of the field and note any change in the amount of downward pressure needed to push the probe into the soil. This can also be done with a wire flag, a steel rod, or other similar type of probe.

  3. Soil Penetrometer. This technique is similar to the soil probe, except that a penetrometer graphically shows differences in resistance. Difference size points usually come with the penetrometer, so be sure to read the correct line that relates to that size of point.

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American Soybean Association News

ASA Members Urged To Call Senators in Support of Biodiesel Tax Legislation

ASA members should contact their Senate offices to encourage co-sponsorship of the Hutchinson/Dayton bill (S. 1058). Contact information for U.S. Senators is available under Capitol Hill at the SoyGrowers.com website, along with talking points on this issue.

The legislation introduced by Senators Tim Hutchinson (R-AR) and Mark Dayton (D-MN) will provide a partial exemption for biodiesel from the diesel fuel excise tax. The biodiesel tax incentive would provide a partial exemption to the federal excise tax in the amount of 3 cents for diesel fuel that contains 2 percent biodiesel. The bill also provides a 20-cent reduction of the excise tax for blends of 20 percent and higher. The approach is similar to the partial tax exemption for gasoline provided to ethanol.

Biodiesel State Legislative Wrap-up

Most state legislatures have closed shop for the summer, and they leave behind them significant biodiesel legislation in several states. Biodiesel bills, or alternative fuel bills that could have a positive impact on biodiesel, were successful in Washington, Hawaii, Nevada, Arizona, Montana, South Dakota, North Dakota, Iowa, Missouri, Arkansas, Indiana and Georgia. Although the statewide B2 effort in Minnesota will have to be taken up again next year, the legislature there did pass a bill that requires the evaluation of developing energy sources from resources derived from agricultural production, including biodiesel.

A bill passed in Montana establishes a revolving loan fund for alternative energy systems, which includes biodiesel use. Several states passed various biodiesel tax exemptions, including Hawaii, where a bill passed establishes the tax rate for biodiesel at half the rate for diesel. Successful bills in Missouri and Iowa establish a biodiesel revolving fund, which pays the cost of biodiesel fuel used by state agencies through a self-sustaining fund generated by the sale of banked EPAct credits.

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Sincerely,

David L. Holshouser


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