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ISSUE 10   July 9, 1998



    White mold in soybean has become an increasing problem. In addition to control of a wide spectrum ob broadleaf weeds, Cobra has recently received a supplemental label for suppression of soybean white mold. You must read the new label if you plan to use Cobra to suppress white mold in soybeans. There are two important considerations on the label: 1) the recommended application timing for white mold suppression use is at or near soybean blooming, and 2) the application rate is 6 to to 12.5 fluid ounces per acre. The label also states that the effect of Cobra on soybean white mold is not a fungicidal response but rather one that involves systemic acquired resistance.

    Cobra has been observed to suppress white mold for years by growers and plant pathologists. In a project funded by soybean check off dollars, Cobra and chemicals of the same family have been evaluated in fields and laboratories for white mold control. Field tests in the last two years in difference states show that 4 to 6 oz/A of Cobra applied at the R1 growth stage (just blooming) consistently reduced white mold infestation. White mold control was reduced if applied at the V4 (4th trifoliate) stage. Studies show that Cobra may enhance soybean resistance to white mold. Yet application is done too early, the suppression of the disease may not be sufficient.

    On effects of Cobra in soybean canopy and yield, it is known that Cobra can cause leaf burn and may reduce yield with late applications. When used for wide mold suppression, yield response to Cobra can be variable, especially if the disease incidence is low or absent. In the white mold project mentioned above, yield increase was consistent in the first-year experiments across states but less consistent for the second year experiments. Generally, if beings are under stress at application time (flowering, pod fill) any further stress will have a negative impact on yield. This is especially true when white mold is absent. Some of the Wisconsin data demonstrated this when white mold was not actually a problem. Therefore, as data is interpreted, if you have white mold, Cobra may offer some protection. If you do not, Cobra can be a problem. The key is knowing that you will have the problem before it shows up. Although there is no white mold forcasting system available, the risk level of this disease can be assessed by a few simple rules that can be found in the Iowa State University Extension Publication PM -- 1731, Soybean White Mold. You can order this publication through the Iowa State University Extension Distribution Center by calling 515-294-5247.

    About a surfactant. Currently, no research data are available to provide guidance as to whether or not to include a surfactant when Cobra is applied for white mold control. In the Cobra supplemental label, a surfactant is listed. In the first two years of the regional white mold research project, treatments without surfactant were not evaluated. However these treatments have been added in 1998 field tests.

    To apply Cobra on white mold tolerant varieties. Some people ask about applying Cobra to white mold tolerant varieties. Research results on a tolerant variety (NK 1990) showed no yield increase with Cobra. The tolerant variety out yielded the susceptible one with yield equivalent to that with Cobra application. Use of white mold tolerant cultivars can achieve equal or superior effects to that of chemical strategies.

    Adapted from the Iowa Integrated Crop Management Newsletter, June 15, 1998 page 107.



    The Weed Science Society of America has published a Herbicide Handbook for many years and the 7th edition was completed in 1994. Since then, many additional herbicides have been registered and the WSSA has just published the1998 supplement to the 7th edition of the Herbicide Handbook. The full handbook contains information on 140 products registered up to 1994 and the supplement gives information on 20 new herbicides and herbicide modifiers, plus three bioherbicides registered since 1994.

    The handbook and supplement are arranged alphabetically by common name of the active ingredient and for each one you find its chemical names and information on its herbicidal use, use precautions, behavior in plants, behavior in soil, toxicological properties and information sources. The handbook also has tables on chemicals that have been discontinued, a pronunciation guide to common names, a list of adjuvants, a list of terms and definitions, and company addresses. The supplement has four appendix tables with information on (1) transgenic crops with engineered herbicide resistance, (2) and analysis of herbicide registration dates, use rates, and acute toxicity by decade, (3) a herbicide company geneology, and (4) a list of Web sites useful to those interested in weed science.

    The Handbook and supplement can be purchased for $35, or if you already have the handbook, the supplement is available for $11. Shipping and handling are extra. For additional information or to order these publications, contact the WSSA, P.O. Box 1897, Lawrence, KS 66044-8897 (phone number = 800-627-0629.)



    The following information is summarized a paper a paper given at the Weed Science Society of American meetings that described a comparison of viable weed seed load in manure from four organic and three conventional farms. The expectation was that the composed manure used on organic farms would have much less weed seed than that from conventional farms. Manure samples were collected four times during 1996, dried, mixed with growing medium, watered, placed in a greenhouse, and then weed seedlings were counted and identified as they emerged.

    Weed diversity (number of species) in the manure was much smaller than that in the field. Common lambsquarters was the most common weed seed in both organic in conventional farms. Weed seed populations very greatly among farms: one have less than 2,000 seeds/ton of manure while another had more than 250,000 seeds/ton (1332 seeds/pound of manure). Manure from some of the organic farms have less weed seed than that from conventional farms, but not always. This shows that composting is not a fool-proof method of destroying weed seed in manure and that certain criteria have to be met for this technique to be highly effective.

    The full reference of the abstract is, Weed Seed Content of Manure: A Comparison Between Organic in Conventional Farming. Quebec, Canada. WSSA Abstract 9:15, 1998, Chicago IL.



    Herbicide drift use a recurring problem in agriculture, and the growing use of postemergence herbicides will increase the potential for off target injury. Due to the nature of herbicide applications, it is virtually impossible to eliminate drift. However, applicators can greatly reduce this risk by using appropriate sprayer equipment and common sense. And article by Mueller and Womac in a 1997 issue of Weed Technology 11:639-643 provides useful information on factors that influence drift with glyphosate. The article is entitled, "Effect of formulation and nozzle type on droplet size with isopropyl amine and trimesium salts of glyphosate."

    In the article, researchers used a laser or drop/particle size analyzer to determine the impact of several factors on droplet size with glyphosate applications. Three formulations (Roundup, Roundup Ultra, and Touchdown), three nozzle types (Extended Range flat fan [XR], Drift Guard [DG], and Turbo TeeJet [TT], and three spray pressures (20, 40, and 60 PSI) were evaluated.

Factors effecting the percentage of spray volume contained in droplets <191 microns in diameter.


     Roundup Ultra
Nozzle Type
Spray Pressure (psi)

Percentage of volume in droplets <191 microns.




   The data presented in the Table are pooled over all other factors (i.e., the data for the XR nozzle represent the average of the three glyphosate formulations sprayed at three pressures). The table shows the percentage of spray volume present in spray droplets less than 191 microns in diameter. The spraying systems catalog states that any droplet less than 200 microns is a potential contributor to drift.

    All three factors studied had a significant effect on the percentage of volume found in small droplets. The two new nozzle types designed to minimize drift (DG and TT) reduced the volume of small droplets from 45 percent with the XR tip to 32 percent with the DG tip and 27 percent with the TT tip. As one would expect, increasing spray pressure resulted in a greater volume of small droplets.

    The most interesting finding was that glyphosate formulation influence droplet size distribution. With Roundup, 29 percent of the spray volume was in droplets less than 191 microns compared with 43 percent with Roundup Ultra. Touchdown was intermediate with 32 percent of the volume found in the small droplets range.

    The authors also reported that several interactions were observed among the factors studied (data not presented here). For example, at 40 PSI the TT nozzle was most effective at reducing small droplets with Roundup Ultra, whereas the DG nozzle was best with Roundup.

    When Roundup Ultra was introduced in 1996 there were numerous problems reported in the mid-west from drift of glyphosate onto adjacent corn fields, resulting insignificant crop injury. Because of the number and severity of problems, many people concluded that the change in formulation, Roundup to Roundup Ultra, was responsible for the drift problems. The data supported this theory becomes Roundup Ultra resulted in a greater percentage of the spray volume in droplet size prone to drift than other glyphosate formulations. However, this finding does not necessarily mean that the change in formulation was a major factor in the 1996 drift problems. Several other factors were involved that are just as likely to have played a role in this problem. First, much of the corn crop was under considerable stress due to unseasonably cool conditions, thereby making it more prone to injury. The cool, humid weather also would favor greater off-target movement and allow small droplets to stay intact longer prior to evaporation.

    Experience with Roundup Ultra in 1997 showed that this product can be used with minimal drift problems. However, as with any product, applicators must take steps to insure that the sprayer is set up properly (proper nozzles selection, spray pressure, and boom height) to minimize off-target movement, they also must use common sense when monitoring weather conditions to avoid drift problems.

    Adapted from the Iowa Integrated Crop Management Newsletter, March 23, 1998 pages 27-28.

Richard Zollinger
NDSU Extension Weed Specialist

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