More Is Better When it Comes to Pigweed Control in Wheat Stubble

Weed control in wheat stubble following harvest has become more difficult with the proliferation of glyphosate-resistant Palmer amaranth and waterhemp - which we are lumping together here as pigweeds.  A tour near Salina, Kansas, on August 3 demonstrated the effectiveness of a few common weed treatments after wheat harvest. 

There is some good news in the fight against pigweeds: There are options that provide good control of these nasty weeds. The bad news is this: Glyphosate, sadly, no longer appears to be one of them.
"We're finding more and more disasters," admits Dallas Peterson, weed control specialist at Kansas State University. "We cannot continue to keep doing the same weed control treatment we've always done."

Cooperator Taylor Currie, who farms near Gypsum, Kansas, worked with KSU Extension to develop five postharvest herbicide treatments on a field of clean wheat stubble harvested in June and sprayed July 20. He used Turbo Tee nozzles, triple-rinsing the sprayer after each treatment. Prior to wheat planting in fall 2014, the field was disked because glyphosate treatments weren't providing weed control. Currie also spread manure on the farm last year.

• Crop rotation and chemistry selection for pigweed control

These were the treatments, with approximate cost per acre (not counting cost of application) and the weed specialists' comments following:

1. 32 oz/acre Buccaneer 5 Plus (generic glyphosate)/13 pounds dry AMS. $7.44 per acre. Peterson comments: This provided about 50% control of pigweeds, but those that survived were healthy and producing seed. Each pigweed can produce nearly .5 million seeds.
   
2. 32 oz/acre Buccaneer 5 Plus/27.5 oz 2,4-D Amine 4/13 pounds dry AMS. $11.05 per acre. Peterson comments: This provided slightly better control of pigweeds than the first treatment, but still plenty of healthy pigweeds remained. Healthy pigweeds can grow 2 inches (or more) per day.
   
3. 32 oz/acre Buccaneer 5 Plus/27.5 oz 3,4-D Amine 4/1 ounce Sharpen/12 oz MSO/13 pounds dry AMS. $18.52 per acre. Peterson comments: The Sharpen dinged the pigweeds pretty well, but they were beginning to come back. Whether they produce as much viable seed as the other treatments remains to be seen. Sharpen is inconsistent. It burns down weeds fast, but two weeks later you may see new sprouts coming on. It works well on small pigweeds, but two ounces may be better. Two weeks after spraying, there are some weeds with green stems; these will survive.
   
4. 3 pts/acre Gramoxone SL/1 qt per 100 gallons non-Ionic Surfactant (NIS). $15.20 per acre. Peterson comments: The Gramoxone smoked the pigweeds, although there is still some green in the stem, and these weeds could regrow. Growers need to use 15 gallons of water per acre. Rate is important; 3 pints per acre works better than 2. This can be mixed with atrazine or metribuzin to heat it up a bit. It is a contact herbicide and is more dangerous to handle: Use gloves and a mask.
   
5. 3 pts/acre Gramoxone SL/2 oz Sharpen/1 qt per 100 gallons NIS/12 oz MSO. $30 per acre. Peterson comments: The results of Treatment 5 are very similar to Treatment 4. We hoped that by adding Sharpen we would get a bit of residual control. We did get a little additional effect, but not sure if it is worth nearly $15 more per acre. 

Peterson told producers in attendance that mixing modes of action is paramount when trying to control pigweeds. Prior to planting a spring crop, growers must use a preemerge herbicide, hit pigweeds when they are young - preferably 3 to 4 inches tall - and be sure to use a postemerge herbicide.

Soybeans for Sudden Death Syndrome

 

Sudden death syndrome (SDS) was detected in mid-April-planted soybeans at the University of Illinois' Northwestern Research Center in Warren County, Illinois. This confirmation comes three weeks earlier than the first sign of the disease in 2014.

Weather throughout the 2015 planting and growing season has been favorable for SDS, according to Purdue Extension. The disease is worst when soybeans are planted early into cool, wet soils and when soils are saturated from ample rainfall throughout the summer. SDS affects soybeans in many Midwest states and some areas to the south, including Illinois, Indiana, Iowa, Kansas, Kentucky, Minnesota, Wisconsin, Mississippi, Missouri, Nebraska, Ohio, and Tennessee. 

On July 15, Arkansas Research and Extension announced a confirmed case of SDS in Desha County, Arkansas. Since 2012, 38 counties have had confirmed distribution of SDS in Minnesota. 

Symptoms
The onset of SDS usually occurs early in the season, beginning by infecting the roots. Symptoms usually don’t surface until late July or August.  SDS is a soilborne fungus Fusarium virgulifome, which overwinters in the soil and may survive for an extended period of time.

SDS infects the roots and sends a toxin up through the plant that causes yellowing and dead tissue between veins. When scouting, look for yellow between leaf veins, and split open the lower stem and taproot. The lower stem and taproot will appear tan to light brown compared to a healthy plant, and the middle portion of the stem will remain white or cream-colored.

Yellow spots become larger, brown lesions as the disease progresses, and leaves may cup or curl. Eventually, leaves will detach from petioles, and brown to grey-color areas will form in the tissue of the lower stem.

Management
While few in-season management options exist, there are things you can do to protect future soybean crops.

University of Minnesota Extension suggests planting soybean varieties with the highest level of SDS resistance soybean cyst nematode (SCN). Minimizing soil compaction and improving drainage to reduce excess soil moisture will also help ward off the disease. Crop rotation and tillage practices can reduce the risk of SDS in soybeans.

“While there is some evidence that changing planting dates or tillage practices may have an effect on SDS control, we don’t recommend farmers change these practices solely for this reason,” said Daren Mueller, ISU plant pathology assistant professor. “Farmers should use the most effective tillage practices and planting dates for their entire operation, not in an attempt to control one disease because of all the factors involved.”

This year, a new fungicide seed treatment, ILeVO, was introduced to the market with the potential to protect against SDS. The On-Farm Network is testing ILeVO in a collaborative project with Iowa State University and Bayer CropScience. Learn more here.