Unlocking Pigweed

from Peanut Grower June 2020

Well-Deserved Recognition

A team of researchers, including Clemson University geneticist Chris Saski, has determined how pigweed changed its DNA to resist glyphosate herbicide.

Have plant geneticists found the key to Palmer amaranth’s glyphosate resistance?

Just hearing “pigweed” can cause fear in row-crop farmers, but Clemson University geneticist Chris Saski and other researchers believe they have discovered the armor this dreaded weed uses to protect itself against glyphosate herbicide.

According to researchers, a specific genetic feature — the extrachromosomal circular DNA (eccDNA) replicon — gives pigweed, or more specifically glyphosate-resistant Palmer amaranth, its resistance to the often-used herbicide, making this weed difficult to control.

The discovery could help refine herbicide treatment strategies and combat the rise of resistant weeds, says Saski.

“The DNA structure of the eccDNA replicon is sophisticated,” says Saski, who is an associate professor of systems genomics in Clemson’s Plant and Environmental Sciences Department. “This circular molecule contains functional genes intermixed among complicated repetitive DNA that seems to have originated from different chromosomes to form the circle.”

He says the difficult sequencing was only possible with advanced single-molecule technology provided by Pacific Biosciences, a biotechnology company based in Menlo Park, California.

This discovery provides an explanation of how weeds evolve and adjust to become resistant to herbicides.

A DNA Change In Pigweed

Glyphosate was introduced in the 1970s and became the most widely used herbicide on many food and non-food field crops as well as non-crop areas where total vegetation control is desired.

Then in 1996, Roundup Ready crops were introduced. Being able to plant a Roundup Ready crop and then spray the whole

field to kill weeds but not the resistant crop worked well for about 10 years until weeds began to appear in sprayed fields. Older herbicides had to be reintroduced to control resistant weeds. Glyphosate-resistant pigweed is now found in 27 states.

Through this study, researchers found pigweed DNA in resistant plants had drastically changed to form a very large, self-replicating circular DNA in addition to its normal chromosomes that carried the gene for the protein that glyphosate attacked.

A Breakthrough Finding

“What this means is plants like pigweed that contain the eccDNA are able to survive and propagate where glyphosate is used frequently,” Saski says, adding that glyphosate stress might contribute and influence eccDNA formation and activity.

“Understanding the complete genomic structure and function of the eccDNA replicon will aid our knowledge of how plant genomes respond and evolve to stresses and help to further refine herbicide treatment strategies and combat the rise of resistant weeds,” he says.

Saski says future research will focus on how the eccDNA self-replicates in plant cells and on identifying the essential functional elements of the eccDNA.

An article about their research, “The eccDNA Replicon: A heritable, extra-nuclear vehicle that enables gene amplification and glyphosate resistance in Amaranthus palmeri,” appears in the current issue of The Plant Cell journal as a BreakThrough Report.

Clemson Extension Pigweed Expert

Clemson University Extension weed specialist Michael Marshall, who works at the Edisto Research and Education Center, says pigweed is an aggressive, invasive weed found throughout the United States. “Once it is introduced to an area, the weed rapidly adapts and produces many small seed. It can quickly become a major problem for producers.”

Pigweed plants can grow 2 to 3 inches per day and compete with crops for nutrients, water, sun and more. Pigweed’s emergence period extends well into the typical crop season and can produce viable seed after crop harvest in the fall.

Pigweed, annual morningglory and sicklepod are the three main weeds South Carolina growers must battle every year. Proper application of herbicides can help relieve some of the pressure.

Taking The Discovery Further

When the researchers discovered the mechanism that amplifies genes in pigweed, Saski applied for and was awarded funding from the Clemson University Research Foundation to advance this technology to the next level. The foundation helps explore the potential for moving research beyond Clemson laboratories and into commercial markets.

Research into pigweed’s glyphosate resistance mechanisms continues, but Marshall says this discovery could be a game changer for agriculture. PG

Article by Denise Attaway, College of Agriculture, Forestry and Life Sciences; Public Service and Agriculture.

Management Options for ALS-Resistant Palmer Amaranth in Peanut 1

Palmer amaranth populations exist in South Carolina that are resistant to several herbicide families including acetolactate synthase (ALS) inhibitors (i.e., Cadre, Classic, Strongarm), dinitroaniline (i.e., Prowl and Sonalan), and glyphosate (Roundup). The following table is designed to assist growers in developing an herbicide system to manage and/or prevent ALS-resistant Palmer amaranth populations in peanut.

Tillage

Strip Tillage

Tillage

Coventional Tillage

Preplant Burndown

Glyphosate or paraquat + 2,4-D

Preplant Incorporated 4

Prowl or Sonalan

Preemergence

Valor 3

Valor 3 + Dual, Outlook, Warrant, Prowl

Preemergence

Valor 3

Valor 3 + Dual, Outlook, Warrant

1st Weed Flush

paraquat + Storm 3 + Dual, Outlook, or Warrant

paraquat + Storm 3 + Outlook, or Warrant

1st Weed Flush

paraquat + Storm 3 + Dual, Outlook, or Warrant

paraquat + Storm 3 + Outlook, or Warrant

POST 2 (by 28d after Planting)

2,4-DB + Cobra 3 or Ultra Blazer 3

POST (by 28d after Planting)

2,4-DB + Cobra 3 or Ultra Blazer 3

1 ALS-resistant (CADRE resistant) Palmer amaranth biotypes have been documented in South Carolina. A program multiple soil residual and postemergence herbicides with differing modes-of-action will be required for optimum control. 2 CADRE and PURSUIT (ALS-inhibitors) may be tank mixed with COBRA or ULTRA BLAZER if control of other broadleaf weeds is needed. Because CADRE and PURSUIT are ALS-inhibitors, these tank mixtures are preferred over CADRE or PURSUIT alone. 3 VALOR, STORM, COBRA, and ULTRA BLAZER have the same mode of action (PPO inhibitor). Do not make more than 2 applications of these herbicides per year. 4 Use DUAL MAGNUM or OUTLOOK instead of SONALAN or PROWL if DNA-resistant Palmer amaranth biotypes are suspected and/or confirmed in your fields.

Q&A Soil Moisture Sensor Q & A Increase water-use e ciency with the aid of sensors for irrigation scheduling.

By Michael T. Plumblee, Clemson University Precision Agriculture Extension Specialist

Water is needed for the plant to grow, and for peanuts specifically, it is needed to move calcium from land plaster into the pegging zone and then into pods. It also improves the effectiveness of pesticides, such as activating herbicides and helping fungicides reach the soil line for improved activity against white mold. Without timely rain or irrigation, these inputs would be wasted. Rainfall or irrigation also lowers soil and canopy temperatures, which allow for normal peg development and reduces aflatoxin risk.

Using an irrigation scheduling tool is a better way to meet crop water demand than using the “rule of thumb” growth stage method or the standard checkbook method. Using a method to schedule irrigation to ensure the crop has the moisture needed to grow properly will reduce plant stress and maximize yield and quality. It also reduces the likelihood of over-watering, which can reduce yields and cost money.

Real-time, Site-Specific Data

Soil moisture sensors are one tool producers can use to schedule irrigation.

Soil moisture sensors can be separated into two categories based on how soil moisture is determined. The first category, volumetric sensors (volumetric water content and capacitance sensors) measure the amount of water in a given volume of soil providing a soil water content percentage.

The second category, which are soil water tension sensors, such as gypsum blocks and Watermark sensors, measure the force that the plant roots must overcome to extract water from the surrounding soil particles. These sensors provide readings in units of kilopascals

(kPa) or centibars (cbar). Several differences exist between the two categories of sensors including price, accuracy, recurring subscription costs and telemetry or how data is accessed. However, both categories are suitable for irrigation scheduling in row crops.

The following are commonly asked questions in regards to soil moisture sensors: Q. How many moisture sensors

do I need?

A. At least one sensor or set of sensors if multiple sensors are needed for multiple depths per irrigation management zone (i.e. under each center pivot) will aid in irrigation decision making. Other scenarios where more than one sensor per irrigation management zone may be warranted include changes in soil texture across the field in areas that can be managed separately or with the use of a variable-rate irrigation system. Furthermore, if a particular system takes several days to make one revolution, consider placing sensors at the start and stop of the irrigation cycle to determine if the system needs to continue on to another irrigation cycle at completion of the prior cycle.

Q. Where do I put my sensors

within the field?

A. Several factors should be considered when placing sensors in the field to ensure a representative reading will be obtained. Consider soil texture differences; try to manage irrigation based on the soil texture that represents the majority of the field. Avoid putting sensors in areas that are more susceptible to drought or hold water during the growing season.

If yield data is available, yield maps can be used as another tool to evaluate areas of the field to avoid or try to stay in with placement. Try to place sensors in the field after planting and in areas where a representative stand exists. Avoid traffic rows and minimize damage to plants when installing sensors. Due to the limitations on irrigation sprinkler packages on center pivot systems avoid placing sensors near the center point of the system. It is recommended to try to install sensors a tower or two from the end of system to ensure irrigation uniformity.

Q. Do I install sensors in the row

or row middle?

A. Install soil moisture sensors within the planted row of plants. By installing sensors within the row, accurate measurements of soil moisture within the crops rooting zone can be achieved. With all soil moisture sensors, sensor-to-soil contact is essential in order to accurately read soil moisture. Therefore, the correct installation of soil moisture sensors is critical to the sensors working correctly.

Q. How do I know when to irri

gate based on the soil moisture sensor?

A. Most sensor manufacturers have generic threshold values associated with the crop and soil texture that the sensor is being placed into. Typically, these threshold values reflect allowing the plant available water of a specific soil texture to deplete 25% to 50% before irrigation is applied to recharge. Ongoing research at

Clemson University is evaluating sensor thresholds in multiple crops to develop recommendations based on South Carolina soil textures and crop.

If soil water tension or Watermarktype soil moisture sensors are being used to schedule irrigation, Clemson

University has a web-based application, accessible with a smartphone or computer, to take sensor readings and assist in making irrigation decisions based on predefined or manually entered thresholds.

The web-based calculator can be found online at https://precisionag. sites.clemson.edu/calculators or www. irrometer.com/thresh.html. PG

Soil Moisture Sensor Benefits:

• Real-time site-specific monitoring of soil moisture. • Accurately depicts crop-water use and, thereby, water-sensitive periods. • Quantifies the actual amount of rainfall that enters the soil rooting zone.

dual modes of action and broad-spectrum activity of Elevest insect control should be deployed.”

For more information, visit your FMC Star Retailer or FMCcrop.com.

FMC Launches Elevest Insecticide

The U.S. Environmental Protection Agency recently granted FMC Corp. registration for Elevest insect control in a broad range of crops including peanuts, soybeans, sweet corn, cotton and potatoes. The new insecticide combines full rates of Rynaxypyr active ingredient for lepidopteran control with full rates of bifenthrin, a pyrethroid.

According to FMC, Elevest insect control provides growers enhanced activity on a broad spectrum of more than 40 insects that plague southern row crops, including corn earworms, armyworms and yield-robbing stink bugs.

Following a mild winter, the potential exists for high lepidopteran and plant bug populations. Scouting will be key to selecting the best tools for the current and late-season conditions.

“Scouting ensures you use the right insecticide at the right time. A premix product only fits certain situations,” says Bruce Stripling, FMC regional technical service manager. “If growers are fighting lepidopteran insects, Prevathon insect control with the single active ingredient Rynaxypyr is the best agronomic choice for effective control, while also being cognizant of the value of conserving beneficial insects.

“However, later in the season, when growers may have to fight lepidopteran pests plus other insects, that’s when the

Reinke Precision Irrigation For Corners

Reinke Manufacturing recently introduced a patent pending technology to increase variable-rate irrigation efficiency for swing arm corners — SAC VRI. Growers will benefit from the user-friendly RPM Preferred Touch or Annex PFTM panels and the ability to upload and store several prescriptions, saving both time and money.

This latest irrigation innovation from Reinke has performed well in field tests, enabling growers to apply the highest probable prescription for a swing arm corner path. Uniform water application increases yield potential while eliminating the expense for outside sources to create and adjust prescriptions.

Used on the whole irrigation system, variable-rate irrigation can segment water application into more than 300,000 zones, which growers can optimize over time. A predictive prescription is provided for SAC VRI to maximize yield based upon results of swing arm corner tests across numerous fields.

“We believe this is the most advanced and easiest to use swing arm corner in the industry,” says Chris Roth, Reinke president. “Our team at Reinke is dedicated to making sure producers can save time when they use our precision irrigation products in their operations.”

SAC VRI is available on all new swing arm corner configurations.

Visit reinke.com/variable-rate to learn more about variable-rate irrigation, SAC VRI and to find a Reinke dealer in your area.

Southern States Partners With Heliae

Southern States Cooperative has partnered with Heliae Agriculture to bring innovative microalgae products to farmers that enhance soil health and improve yield.

According to the company, their product, PhycoTerra, can improve crop yields and soil health by feeding and multiplying soil microbes. Plants benefit directly from increased bio-available nutrients and stronger root and shoot growth. The environmental improvements include better water permeability and less soil erosion and runoff.

For more information on PhycoTerra and how it is driving regenerative agriculture forward, visit PhycoTerra.com.

AGCO Recasts Vision, Commitment

Agco recently announced the change of vision as a commitment to bringing farmers sustainable high-tech solutions.

“Our new vision — sustainable hightech solutions for farmers feeding the world — better represents our thoughtful approach to helping farmers and our businesses continually find better and more sustainable ways to raise the food people need all around the world,” says Martin Richenhagen, president and chief executive officer of Agco Corp.

Agco’s sustainable high-tech solutions enable farmers to do more with less. Advanced farm machines require less fuel and make fewer passes in the field. Precision technology and agronomic solutions enable farmers to grow and harvest more per acre. Better sprayer technology enables farmers to apply products only where needed and reduce inputs to grow healthy food.

For more information on Agco, visit https://agcocorp.com.

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Pointers

Watch For Yellow Peanuts

In June, growers should continue to implement strategies to control weeds and prevent thrips from causing injury. Hopefully, in-furrow systemic insecticides have provided adequate control, and follow-up sprays of insecticide have been made to clean up thrips infestations.

Depending upon weather conditions, residual herbicides applied at planting or early postemergence should have been effective. If not, matching the correct herbicide with the weed complex is critical.

Make timely applications to effectively control of weeds. A good practice is to include more residual herbicide with the contact herbicide in a single application, especially in fields that routinely have weed issues throughout the season. The contact herbicides control escaped weeds that have emerged, and the residuals prevent other weeds from emerging for several weeks. In most instances, contact herbicides (paraquat plus Basagran, Cobra, Ultra Blazer, Storm) can be mixed with residual herbicides (metolachlor products, Outlook, Warrant, Zidua, Anthem Flex.) Keeping peanuts weed-free for the first 6 weeks is important in capturing yield potential.

Minimizing thrips injury, especially in the upper Virginia-Carolina region, also helps maintain peanut yield potential.

For Virginia-market types and large-seeded runners, applying gypsum at flowering is an important input. Addressing any micronutrient deficiencies (boron and manganese) ensures optimal plant nutrition.

DAVID JORDAN

North Carolina State University Extension Agronomist

Thrips injury Yellow row of peanuts

Keep an eye out for yellow peanuts and determine the cause. Sometimes this is from wet conditions and poor root growth. At other times, it can be a sign that nodulation is not effective and the plants need nitrogen. If a nitrogen deficiency is because of inoculant failure, getting nitrogen applied to plants as quickly as possible is a must. It won’t be cheap, but it will be important to maximize yield.

Take a look at the southern corn rootworm index while making decisions on applying chlorpyrifos. When you need it, you need it. If you don’t need it, an application of this insecticide can do more harm than good, such as flaring spider mites.

The major task for the mid and latter portion of the season is establishing a solid leaf spot and stem rot fungicide spray program. Protecting peanuts from the pathogens that cause these diseases starts early. In North Carolina, applying fungicides at R3 to start protection from these pathogens is important. Then, make applications at twoweek intervals or possibly extending those intervals based on weather-based advisories. Rotating chemistry and making sure fungicides are applied at the correct rate, on time and using the right equipment are essential elements of an effective disease management strategy. Each state has a lot of good information on fungicide programs, and growers and their advisors are encouraged to spend time putting in place an effective program.

Keep in mind that we have a significant number of cases where certain groups of fungicides are no longer effective. Make sure you are covered with the most effective chemistry each time you spray. In this column, I seem to have used “important” a lot. There are many things to do in June that are important and doing them on time helps set you up for optimum yields this fall.

Use Gypsum On Dryland Crop

Most farmers have finished planting or are almost done by now. Hopefully, everyone has a solid stand of peanuts and the crop is off to a good start. Scout for early season weed escapes and clean those up while they are small and easy to control.

Then, it’s time to begin a fungicide program. Disease control in peanuts is one of the most expensive production costs. Everyone wants to make the highest yield and save money by reducing the number of sprays. My advice is to start spraying peanuts

KRIS BALKCOM

Auburn University Extension Specialist

for leaf spot at 35 to 40 days of age. We don’t want to be behind and spend more money trying to play catch up, while still possibly suffering yield losses in the end.

Apply fungicides before disease is established. Use the recommended rate and volume of spray for good canopy coverage. Also, I think banding some of the high-end fungicides early is an excellent way to reduce costs and still provide adequate protection. There may be opportunities to delay sprays later in the season that will wind up saving a trip across the field, but the first one is not the one to skip or delay.

One last tip is to pull a pegging-zone sample about 2 to 3 inches deep in the row to ensure you have adequate calcium levels for pod fill. Gypsum is a great product when you have a high pH but a low calcium level. Also, gypsum is important on the dryland acres because it is so water soluble and will enter the soil solution faster than lime when rains are scattered and sparse.

Late Planting, Crop Fertility

Rainfall during May has been inconsistent and that will cause some growers to have to plant peanuts in June. By no means is June planting a deal breaker, but obtaining optimal yield and grade becomes riskier. Increased leaf spot disease affects late-planted peanuts but can be managed with a good fungicide program.

The biggest risk to Juneplanted peanuts is maturity. Maturity at 145 to 150 days after planting puts the crop well into October when it starts to cool down. Temperatures can be cold enough to stop normal maturation of pods, reducing yield and grade potential. On average, Tifton has experienced late October temperatures around 50 degrees, and Georgia has been in the low 40s in mid- to late-October a few times in the last 10 years.

For those with peanuts in the ground, calcium and boron are two important fertility components to be mindful of in June. Decisions on calcium, in the form of gypsum, and boron applications need to be finalized over the next few weeks. Currently, about a third of the crop is moving into the 40 to 60 days after planting range. According to Glenn Harris, UGA fertility specialist, traditional calcium needs are accomplished through either lime at planting or gypsum at early bloom time. If your soil test results indicate at least 500 pounds per acre calcium in the top 4 inches of soil, the pegging zone, and a Ca:K ratio of 3:1 or higher, then you are not likely to see a yield response to calcium fertilization.

However, Harris says, “All seed peanuts should automatically receive 1,000 pounds per acre of gypsum, or calcium sulfate, at early bloom. A lot of gypsum is applied ‘just in case’ or as insurance. This is a good strategy. Put your fertilizer dollars into gypsum rather than into some unproven or untested product.”

In regard to boron, growers need to apply 0.5 pounds of boron per acre. Boron applications can be achieved in a number of ways. The easiest method is by foliar feeding at 0.25 pound boron per acre tankmixed with your first two fungicide sprays. Make product decisions wisely. Beware of products recommended at very low rates. For example, 6 ounces of a 5% liquid boron only gives you 0.025 pounds of boron per acre.

If you need help in any way, call your county Extension agent. I hope everyone has a great production year.

SCOTT MONFORT

University of Georgia Extension Agronomist

Check For Active Nodulation

Most of the Southwest peanut crop has been planted by June. Although rainstorms in early May helped increase planting moisture, many growers had to irrigate fields prior to planting.

Herbicides applied preplant and at planting have been effective, but new weed flushes following rainfall or irrigation are likely as soil-applied herbicides are dissipating. Herbicides applied at early postemergence that do not have soil activity are also prone to allow new weed flushes. In other words, good early season weed control may need some attention because previously applied herbicides have dissipated over time.

Peanut forms a symbiotic relationship with Rhizobium spp. to fix atmospheric nitrogen if an inoculant is properly applied at planting. It is time to assess whether the nodulation was successfully achieved. Carefully dig plants and count the number of active nodules. Active and healthy nodules should appear reddish-pink inside due to the reaction of leghemoglobin.

Supplemental nitrogen is not required if an average of 20 active nodules per plants is observed. If supplemental nitrogen is required, 10 to 20 pounds per acre of N should be sufficient depending on the number of active nodules/ plant and residual N in soil and irrigation water. Excess N can increase disease problems. PG