Time to Scout for Weeds

Kurt M. Vollmer, Postdoctoral Researcher – Weed Science, University of Delaware; kvollmer@udel.edu

With most of the corn and soybeans planted, now is the time to start scouting for weeds. Doing so will prevent major headaches later in the growing season. While scouting, be sure to note the weed species present, height, life-cycle, and severity of the weed infestation. When looking at fields this year, pay attention to those areas that were drowned out last summer. The weeds in many of those spots produced seed and now have very high seed banks. So while weed pressure in the rest of the field may not be too heavy, weeds present in these spots may be at unacceptable levels.

In particular, Palmer amaranth can quickly become unmanageable if not spotted early. Many herbicide labels suggest spraying this weed when it is less than 4 inches tall, but the UD Weed Science program recommends applying postemergence herbicides before its 3 inches tall. Our research with soybean shows that the best time for this second application is no later 28 days after applying a residual herbicide. Furthermore, Palmer amaranth can quickly exceed 4 inches, and research at the University of Maryland has shown that delaying the postemergence application to 32 days or longer can result in reduced levels of control. Remember, the earlier Palmer amaranth is spotted the better. Furthermore, keep in mind there could be several days between scouting and actually getting the sprayer into the field, allowing Palmer amaranth to reach heights that prevent complete control.

Glyphosate and Liberty Tank Mixtures

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Glyphosate plus Liberty has been a pretty good treatment to burndown weeds prior to planting double-cropped soybeans in the past few years. With some of the newly released soybean traits, tankmixing glyphosate with Liberty is also an option for postemergence applications. We have found this combination to work very well in many situations but just a few things to keep in mind.

  • Neither of these herbicides will provide residual control, so if weeds with long germination periods are present (i.e. Palmer amaranth) consider including a residual herbicide such as Dual, Zidua, Anthem, or Warrant when spraying postemergence.
  • Liberty needs good spray coverage to maximize effectiveness, including when its tankmixed with glyphosate. A minimum of 20 gallons per acre should be used with medium to coarse spray droplets.
  • I will occasionally see poor control of fall panicum with this combination. I am assuming the Liberty is interfering with glyphosate providing complete control. This is more likely to occur when spraying for burndown with large fall panicum plants, but I have seen it when spraying postemergence to fall panicum that is 3 to 4 inches tall. I have not seen this reduction of control with other annual weed species, but giant foxtail and large crabgrass are the only other grasses in most of my trials. I suspect an increase in glyphosate rate would help reduce the likelihood of this happening, but I have not tested this.

Irrigated Soybean Seeding Rates

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu and Cory Whaley, Sussex Co. Extension Ag Agent; whaley@udel.edu
Research was conducted on irrigated soybean seeding rates at the Warrington Research Farm in 2017 and 2018. Soybeans were planted in 15” rows at rates of 80, 100, 140, and 170 thousand seeds per acre in irrigated and dryland plots. No yield advantage was observed at higher seeding rates, while the use of irrigation improved yields by an average of 7 bu/acre over both years. With no difference in yield by seeding rate, projected income was greater at the lowest seeding rate of 80k seeds. This represents only two seasons of research on one soil type, but does indicate that Delaware soybean producers could potentially lower their seeding rates and not see a decrease in yield.

To read the full report see: https://www.researchgate.net/publication/329811290_2017-2018_Irrigated_Soybean_Seeding_Rates

Treating Soybeans with New Herbicide Traits

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

A recent article in an ag newsletter raised the question of what herbicide brands can be sprayed on the new herbicide traits. The article asked about use of glyphosate on soybean varieties that are “glyphosate-resistant” but the soybeans are not designated as “Roundup Ready”. Many brands of glyphosate are labeled specifically for “Roundup Ready” crops. So with the help of industry contacts and Delaware Department of Agriculture we have sorted out the issues. It is important that crops are treated only with the registered herbicide brands, so it may require re-reading labels to be sure they can be applied to new varieties.

Liberty Link crops are stated on the label of most brands of glufosinate, the active ingredient in Liberty.

Roundup Ready crops are stated on most brands of glyphosate.

Roundup Ready 2 Xtend soybeans can be treated with most glyphosate brands because of the “Roundup Ready” designation. But these can only be treated with dicamba formulations approved for “Xtend”-branded crops (Engenia, Fexapan, or Xtend).

Enlist E3 soybeans are resistant to glyphosate, glufosinate, and 2,4-D choline. Enlist One (2,4-D choline alone) and Enlist Duo (2,4-D choline plus glyphosate) can be applied to these soybeans. But Enlist soybeans do not carry the brand name of “Roundup Ready” or “Liberty Link”. If the glyphosate brand says only use on “Roundup Ready” crops then is cannot be applied to Enlist soybeans. These soybeans can only be treated with glyphosate brands that allow application to “glyphosate-resistant” or “glyphosate-tolerant” crops. Likewise, if the label specifies use on “Liberty-Link” soybeans then it cannot be used. Only glufosinate brands that say they can be applied to “glufosinate-resistant” or “glufosinate-tolerant” crops are allowed.

Enlist corn is resistant to 2,4-D choline, glyphosate and registered postemergence grass herbicides. Currently, Assure II (quizalofop) has a special label for use with Enlist corn.

LLGT27 Soybeans are resistant to glyphosate, glufosinate, and an HPPD herbicide. These soybeans are branded as “Liberty Link” and can be treated with most glufosinate brands. While these soybeans are resistant to glyphosate they are not “Roundup Ready” and must be treated only with glyphosate brands labeled for “glyphosate-resistant” or “glyphosate-tolerant” soybeans. The HPPD herbicide product currently is not approved by the EPA.

Many herbicide companies are changing their labels to allow application to these new soybeans, so more brand options will soon be available. But be sure to read the label of the brand you intend to use to be sure it is labeled for use.

If using crops with herbicide-tolerance traits be sure you keep detailed records of which fields are planted with which traits. Likewise, when switching varieties, you may need to clean out your planter to avoid mixing traits.

Field Crops Disease Management Resources and 2019 Guides

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu

The 2019 Fungicide Efficacy for Control of Corn Diseases table is now available through the Crop Protection Network https://cropprotectionnetwork.org/download/5214/. This table is produced annually by the Corn Disease Working Group and provides efficacy ratings for fungicides to manage corn foliar diseases. A number of other resources for disease management in corn, soybean, and small grains are available through the Crop Protection Network https://cropprotectionnetwork.org/library/.

Insecticide Trial Results for Vegetable and Agronomic Crops

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Summaries of last season’s insecticide trials in peas, sweet corn, watermelon, field corn, soybean, and wheat can be viewed at https://cdn.extension.udel.edu/wp-content/uploads/2013/12/15092741/Delaware-Field-and-Vegetable-Crop-Insect-Pest-Management-Trials.pdf.

Potential Hurricanes and Flooding

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

We avoided most of the flooding seen in the Carolinas with Florence, but hurricane season lasts until the end of November. Some later planted corn is still drying down, so saturated soils and winds may cause lodging, but there are no hurricanes on the horizon that may cause those issues. Full season and double crop beans are more likely to have issues if another storm heads for the Delmarva. Depending on development stage, storm conditions could increase disease pressure, cause lodging and shattering. For more detailed information, check out NC State extension as they dealt with the aftermath of Florence (https://soybeans.ces.ncsu.edu/2018/09/soybean-considerations-following-hurricane-florence/)

For fields along tidal streams and shorelines, hurricanes could bring salt water across fields. It may be necessary to perform soil tests in these fields to check for salt levels prior to next year’s crop. In general, if Na makes up more than 15% of the cation exchange capacity, lower yields could be observed. Total salts (which can include Ca and Mg) may also cause issues in fields flooded with tidewater. Gypsum works well if Na is the only issue, but irrigation is needed to leach soils high in Ca, Mg and Na.

Soybean Insect Scouting Update

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Insect populations are generally lower this week. Loopers can still be found in some fields, as can stink bugs and bean leaf beetle. Defoliation thresholds for R6 beans are considerably higher than earlier R-stage beans. Now that our earlier planted fields are starting to senesce or have dropped leaves completely, this is a good time to take stem samples for Dectes stem borer, especially in fields that had large numbers earlier or have a history of Dectes problems. If your field has a large stem infestation, prioritize that field for as timely a harvest as possible. Lodging loss potential increases with the percent of infested stems and late harvest.

Soybean Insect Scouting Update

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Continue to scout for corn earworm and soybean looper. Last weekend’s storm activity from the south could have brought up more loopers from that region. I have received a couple of reports of pockets of loopers in fields. They often show up in localized sections, typically in drier, less robust parts of fields. Defoliation thresholds prior to R6 are 15%, at R6, plants can tolerate greater defoliation. R7 beans are safe. Also, be sure to check the lower canopy if you do see significant looper activity, they sometimes start first in the lower canopy unlike our other defoliators that concentrate in the upper canopy. NC only recommends Intrepid Edge, VA recommends Intrepid Edge and Steward. Both products are ‘worm’ products and will clean up corn earworm. Steward has some, but not great activity on bean leaf beetle, and neither has stink bug activity. We also had a report of armyworm activity in some isolated fields. Armyworms can feed on pods, but are not as aggressive as corn earworm. There doesn’t seem to be much consensus in different states’ recommendations; some combine them with corn earworm thresholds, others treat them as defoliators, and others use a threshold in between.

Guess the Pest! Week #23 Answer: Sudden Death Syndrome of Soybean

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Lamar Witmer for correctly identifying the disease as sudden death syndrome of soybean and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #23 Answer: Sudden Death Syndrome of Soybean
by Nancy Gregory, Plant Diagnostician; ngregory@udel.edu

Sudden death syndrome of soybeans (SDS) is caused by the fungus Fusarium virguliforme. We started seeing this disease in Delaware in 2002 in cool and wet seasons, but have seen it more often in the past few years. SDS can be confused with other stem diseases such as Phomopsis stem canker and charcoal rot. Leaf symptoms of yellowing and browning between the veins are typical, and leaves shrivel and fall off, leaving petioles still on the stems. If stems are pulled up and placed in a plastic bag overnight, blue spore masses of the fungus may be seen at the base of stems. The internal stem tissue (cortex) may show dark discoloration. There is a toxin produced by the fungus that is responsible for the symptom pattern showing up at the top of the plant. The fungus overwinters in debris, and disease is most severe when infection occurs early. Improving drainage, alleviating compaction, and treating seed may help get seedlings established.