Monthly Archives: July 2013

Application Equipment for Effective Insect Pests and Foliar Disease Control

Posted on July 2, 2013 by David Holshouser

Bobby Grisso, Ext. Agricultural Engineer and David Holshouser, Ext. Agronomist

Several years ago, we stressed that proper application equipment, specifically nozzle selection and spray volume, was critical to obtaining good disease control with foliar fungicides. At that time, we were trying to prepare for an invasion of soybean rust. The problem was that soybean rust (and most other diseases) would begin establishing itself in the middle and lower leaves of a full-canopied crop. Getting enough spray droplets deep into the canopy was required for adequate control.

Although we have yet to see soybean rust early enough in the growing season and at great enough levels to cause a yield loss, we think our efforts to educate the agricultural community on proper application paid off. Foliar fungicide sprays, now being used to control other soybean diseases, are reaching their target and penetrating to the middle and lower canopy. Most importantly, it takes spray volumes of 15 to 20 gallons per acre (GPA) and medium-sized droplets to obtain good coverage.

Now we have another pest, the kudzu bug, which will likely require a similar spray strategy in order to get the pesticide down into the canopy where it needs to be. Although this pest can be found on upper leaves and stems, past observations from states to our south indicate that it likes to bury itself well into the canopy on stems and lower leaves. Therefore, revisiting nozzle and sprayer strategies for optimum control of this and other pests are warranted.

The single most important factor affecting prevention of disease and/or control of insect pests is good coverage of the plant with pesticide. Fortunately, the technology is available. You may however incur additional expenses with adaptation to current application equipment.

Remember the five major principles that result in satisfactory and economic control of the problem: 1) Positive identification of the insect pest or disease; 2) Correct pesticide; 3) Selection of the right equipment, particularly the right type and size of nozzle; 4) Timely application; and 5) making sure of the accuracy of equipment to confirm correct application amounts based on label recommendations.

First and probably most important, apply 15 to 20 gallons per acre of spray solution when using ground applicators. Only with enough volume can we penetrate a large soybean canopy and cover the leaves with pesticide. With aerial applications, this can be reduced to 5 gallons per acre (see the pesticide label for specific requirements).

Just as important, select the nozzles and pressures that will result in medium sized droplets (226 to 325 microns). Pressure is not as important. Depending on the nozzle selected, too high of pressure will just create fine and very fine droplets that are prone to drift. Too low of pressure may create large droplets that may penetrate the canopy but not coat the leaves.Flat-fan pattern nozzles are still the best choice as long as most of the droplets from these nozzles are categorized as medium. A flat-fan nozzle will normally be operated between 30 and 60 pounds per square inch (psi), with an ideal range of 30-40 psi. Extended-range flat-fan nozzles are a modification of the flat fan that allows the applicator to spray at lower pressures and still maintain uniform distribution. Regardless of the type of flat-fan nozzle selected, insure that the selected nozzle size will produce a medium-sized droplet at the operating pressure range and speed that the sprayer will be traveling. Most nozzle catalogs will contain this information. An example is shown below.The twin orifice flat-fan nozzle produces two spray patterns: one angled 30 degrees forward and the other directed 30 degrees backward. Such a nozzle may provide better penetration and coverage of plants with fully developed canopies. This is mainly due to being able to double the gallons per acre while still maintaining the desired droplet size. Still, hitting the target from two different angles, with one forward and one backward spray pattern may also provide a more effective coverage than spraying with just one spray pattern shooting down. Twin orifice nozzles can be designed with a twin spray pattern from one tip or special fittings/caps that allow the producers to place two nozzles in the same cap, one pointed forward, and the other one pointed backward. Nozzles producing cone pattern are not recommended for foliar soybean diseases; droplets are usually too fine and will not penetrate the canopy.Here are recommendations to help achieve the best coverage and control when spraying for soybean rust.

Keep spray volume above 15 gallons per acre for best results (20 gallons per acre is better).
Choose the appropriate size and type of nozzles and operate them at a pressure that will allow them to produce medium-size droplets (200-300 microns).
Nozzles producing a flat-fan pattern provide better coverage than the nozzles producing cone pattern when there is full canopy.
Use directed spraying, if applicable, to improve coverage.
Use twin nozzle/pattern technology. Two spray patterns, one angled forward and one angled backward, generally perform better than single nozzles spraying in one direction. This is primarily due to being able to increase the spray volume, while maintaining medium-sized droplets.

For more information on sprayer technology, see the following VCE publications:

Nozzles: Selection and Sizing. Virginia Cooperative Extension publication 442-032, May 2013; http://pubs.ext.vt.edu/442/442-032/442-032.html (accessed: June 2013)

Droplet Chart/Selection Guide. Virginia Cooperative Extension, publication 442-031, May 2009; http://pubs.ext.vt.edu/442/442-031/442-031.html (accessed: June 2013)

Asian Soybean Rust – Frequently Asked Questions VI: Sprayer and Nozzle Technology. Virginia Cooperative Extension publication 450-306, May 2009; http://pubs.ext.vt.edu/450/450-306/450-306.html (accessed June 2013)

Kudzu Bug Update

Posted on July 2, 2013 by David Holshouser

Ames Herbert, Extension Entomologist

The map below lists the progression of kudzu bug in Virginia from 2011 and 2012 (blue and orange counties) though this year (purple counties). As of June 27, 2013, we have documented kudzu bug (KB) infestations in soybean fields in 21 of those counties (Accomack, Amelia, Appomattox, Brunswick, Campbell, Charles City, Culpeper, Dinwiddie, Franklin, Greensville, Goochland, Hanover, Isle of Wight, Middlesex, New Kent, Orange, Prince George, Southampton, Suffolk, Sussex, and VA Beach). The problem is spreading quickly and almost daily I get word of an infestation in another county. If you find KBs in a soybean field in a county that is NOT listed, please contact me with that information. If you are growing soybeans (or crop advising) in a county on the list, you should make the effort to check fields. Although adults are still present, nymphs are hatching from eggs masses and dispersing to stems and petioles. Adult KBs have a strong aggregation pheromone that results in clusters on individual plants with many plants not infested. This will begin to change as nymphs emerge.

Their tendency is to disperse to new feeding sites, new plants or areas of plants which will result in a more widespread and more uniform infestation. As of the last week in June, the nymphs we are seeing are quite small. You can see them with your naked eye, but it takes either really good vision (those days are over for me) or a hand lens to see that those tiny light colored things on stems are indeed KB nymphs. This too will change as they gradually grow and molt into larger nymphal instars. Based on all that we know, we should try to keep the management recommendations as simple as possible, trusting those that have done the research—that using their recommendations will result in the best possible outcome: control at the least cost. As we move forward in the season, the best advice is to treat fields that are flowering or developing pods when an average of one nymph (big enough to see, see image below) is captured per sweep net sweep—or, 15 nymphs in a 15-sweep sample. If this situation is encountered, we are advised to treat that field. Remember, this insect is a slow feeder—gradually drawing down a plant’s vigor. This is good in a way, as this gives us plenty of time to sample fields and react with a treatment if needed. KBs do not eat holes in leaves and do not take bites from pods or seed. You may find nymphs and second generation adults on pods, but the damage is not direct like a corn earworm that eats the seed or a stink bug that punctures the seed. This is a new pest for us and we will all have to learn how best to deal with it. For now, we should abide by the recommendations above. Given the number of infested fields, I fully expect that some will have to be treated, eventually.
What about product choice. We have covered this in an earlier advisory (http://www.sripmc.org/Virginia). There are many good choices. Below is an insecticide efficacy chart that was developed by researchers at Clemson University and University of Georgia. They (and I) do not recommend using any product that falls below 80% control and the higher the better. I have been asked about a lot of products, some on this chart, some not. I go with the chart.

Manganese Deficiencies

Posted on July 1, 2013 by David Holshouser

Mark Reiter, Extension Soils Specialist, and David Holshouser, Extension Agronomist

Manganese (Mn) deficiencies are common in Virginia soybean, but these deficiencies are not necessarily due to low Mn levels in the soil. Instead, like many micronutrients (nutrients that are needed by the plant in small amounts), Mn availability to the soybean crop is directly related to soil pH.

When pH levels reach 6.5 or above, Mn deficiencies will likely appear, especially on sandy soils. However, some soils with a pH of 6.2 and lower can show deficiencies if soil Mn is low. Generally, Mn deficiencies are more common on our sandier soils as pH changes more rapidly and sandy soils typically have a lower Mn concentration.

Manganese deficiencies will also reveal themselves with dry soil conditions, especially on tilled soils. This is because Mn becomes less available under oxidizing conditions. Oxidizing conditions occur under dry environments where there is more oxygen and less water available in the soil pores. In this situation, Mn oxides form (basically a rock) and Mn oxides are not available for plant uptake. In places with more soil compaction, such as wheel tracks, or under wetter conditions (more pore space is occupied by water), Mn oxides will be reduced to Mn²⁺, the form of plant uptake. This is why we often see Mn deficiencies throughout a field but not in wheel tracks where the soil is more firm. Firmer soils don’t have as much pore space; therefore, they have less oxygen to form Mn oxides. Shown below is an example of this.Finally note that other problems can cause look-alike symptoms similar to Mn deficiencies. In particular, inter-veinal yellowing is a common symptom of soybean cyst or other nematode damage. Therefore, it may be prudent to further investigate the problem, especially the root system.

Use the following guidelines for Mn applications:

Scout your fields. Mn deficiencies may or may not materialize. The only sure way to determine a deficiency is to observe the deficiency symptoms through visual observation or tissue tests. The characteristic visual symptom is yellowing between the veins on the new leaves. Mn is an immobile nutrient. Therefore, it will not move out of older leaves to the new leaves. Symptoms will appear when the plant can no longer extract sufficient amounts of the nutrient from the soil.

Take a tissue sample. If Mn deficiencies are suspected due to high pH and/or a field history of Mn deficiencies, but no symptoms have yet appeared, you should consider taking a tissue sample. Tissue samples can reveal deficiencies before symptoms appear (hidden hunger). We suggest a tissue test if lime, lime stabilized biosolids, or an ash product was recently applied.

Manganese application. To overcome a deficiency, apply ¾ lb. chelated Mn (elemental basis) or 1 lb. inorganic Mn (elemental basis) per acre to foliage upon appearance of symptoms and prior to flowering. More than one application may be required to correct a severe deficiency.

Don’t use low rates to correct a deficiency. Note that many Mn products recommend applying lower rates of Mn. However, the label usually states that these are maintenance rates. Once a deficiency occurs, these lower rates will not correct the deficiency and the rates stated above will be needed.

Split Mn application on deficiency-prone soils. An alternate method of application can be used before a deficiency is evident on soils that commonly show a deficiency, especially on soils that have a high pH (above 6.8 or so). A lower rate (~ ½ of that listed above) can be combined with another scheduled application, such as a postemergence herbicide or insecticide. This may be a sufficient rate to prevent a deficiency from occurring. But, continue to scout the field and take future corrective measures if visual deficiencies appear. If a visual symptom appears, you need to use the full rate. I will remind you that this is a preventative treatment. A deficiency may not occur. Furthermore, these are only maintenance rates and another application will likely be needed if the field is truly deficient.

Use EDTA chelated Mn formulations when mixing with glyphosate. Be reminded that some Mn formulations in combination with glyphosate herbicide (Roundup, Touchdown, many generics, etc.) will result in reduced weed control of certain weeds. Other herbicides have not shown to interact. If including Mn with glyphosate, use the EDTA chelated formulation as it has shown not to interact.

Don’t spray if you don’t need it. Mn can be toxic to soybean. Spraying greater than recommended rates or spraying as a preventative spray when soil pH is relatively low (5.7-5.9) could lead to toxicity problems