Tar Spot Of Corn: What To Know And New Research

by: Dan Quinn and Darcy Telenko, Purdue University

Due to its relatively recent U.S. discovery and its ability to cause significant production and economic losses, tar spot is often a topic of angst and anxiety amongst corn farmers and agronomists in Indiana. For example, a severely infected field can reach yield losses upwards of 60 bushels per acre! Yield losses are often a result of reduced photosynthetic capacity (green leaf area) of the corn plant during grain fill resulting in poor grain fill, kernel abortion, and reduced kernel weight. In addition, severe infection can reduce corn stalk integrity and cause significant lodging later in the season. Tar spot was first confirmed in northwest Indiana in 2015 and the first significant yield-reducing event of the disease was observed in 2018. Similarly, severe outbreaks and large areas of infection of this disease were observed in Indiana in 2021. Tar spot is caused by the fungus known as Phyllachora maydis and can be identified by small, raised black and circular spots present on corn leaves, stalks, and husks (Figure 1). These black and circular spots are known as fungal fruiting structures called stromata, each of which can produce thousands of spores. Overall, tar spot infection and severity can vary based on environmental conditions, the total amount of the pathogen present in the field, and corn hybrid chosen.

What Conditions Cause Tar Spot? Continue reading

Double-Crop Soybean Weed Management

by: Dr. Mark Loux, OSU Extension

It’s been a tough summer in parts of Ohio to do anything on a timely schedule and there are some weedy fields.  The best advice we have for big weeds in full-season soybeans is to increase rates and the complexity of POST herbicide applications, while still adhering to cutoffs for the application of certain herbicides as much as possible.  Dicamba products, XtendiMax, Engenia, and Tavium, cannot legally be applied to Xtend and XtendiFlex soybeans after June 30.  This cutoff date pertains to use in double-crop soybeans also.  If you are planning on planting Xtend or XtendiFlex soybeans in double-crop fields and using dicamba as a burndown, apply before Friday.  There isn’t a cutoff date for most other POST soybean herbicides – it’s based on either crop stage (eg R1) or days before harvest.

Double crop soybeans usually need some type of weed control program, although how weedy they get depends upon weeds surviving down in the wheat that can take off once they receive light; how much rain we get in July, which drives additional weed emergence and rate of soybean growth; and how fast the soybeans grow and develop a canopy.  Control can occur via the use of pre-plant/preemergence burndown herbicides, followed by POST as needed.  It’s also possible to accomplish this with one early POST application in Enlist soybeans, using Enlist Duo or a combination of Enlist One with glyphosate or glufosinate.  And also in LLGT27 soybeans with a combination of glyphosate and glufosinate.  Herbicides need to address marestail in many fields, which is often lurking in the wheat ready to regrow.  Marestail that are taller and get cut off by the combine will be more difficult to control than the smaller intact ones below the cutter bar.  Herbicide options vary depending upon the weeds and what type of soybeans are planted.  More effective options include:

  • Glyphosate or glufosinate + Sharpen (1 oz) + MSO – any soybean, prior to emergence
  • Glyphosate or glufosinate + 2,4-D – any soybean, at least a week before planting
  • Enlist Duo; glyphosate or glufosinate + Enlist One (Enlist soybeans) – PRE or POST, no wait to plant
  • Glyphosate + XtendiMax or Engenia (Xtend or XtendiFlex soybeans) – PRE, apply by June 30
  • Glyphosate + glufosinate – PRE in any soybean, PRE or POST in LLGT27 soybean

​​​​​​​It is possible to include residual herbicides with a PRE burndown treatment, but their value in this situation is questionable.  Residual herbicides with long recrop intervals to corn should be avoided.  POST options in double-crop include glufosinate, glyphosate, Enlist One/Duo, and conventional herbicides, depending upon the type of soybean planted.  One caution here is to avoid excessive injury to soybeans that slows growth and development since this is likely to reduce yield due to the short season.  Weed emergence is reduced and variable in July compared with May and June.  Where burndown herbicides are used, but there is still a need for POST herbicides to control a flush of late-emerging weeds, consider reduced rates.  Research we conducted back in the 1990s demonstrated that weeds up to 2 inches tall can usually be controlled with half of a typical labeled rate.  When we planted soybeans in early June, the single application of a half-rate provided adequate control, versus early May when a second application was needed.  So this should be a suitable approach for double-crop soybeans.  Just be sure to start with an effective burndown at planting, and apply when weeds are well within the 2-inch size.

Take Action Weed Management Webinars

By: Dr. Mark Loux, OSU

Given how messed up the whole herbicide supply and price thing is right now, it might be a good time to take advantage of free resources to improve your herbicide and weed management acumen. The USB Take Action program and university weed scientists are once again conducting a series of webinars to cover several key topics in weed management. Three webinars occur this month, and will be followed by the release of videos covering other pertinent weed-related subjects. January webinars include the following:

Value of Residuals in Herbicide-Resistant Weed Problems – Thursday, January 20, 11 am EST

Harvest Weed Seed Control Practices – Thursday, January 27, 11 am EST

Registration information can be found here.  Videos of the webinars will be made available following their broadcast.

Another great resource is the “War Against Weeds” podcast.  This podcast features guests with expertise in a variety of aspects of weed science, and discussions on integrated weed management, herbicide resistance, and other timely topics. The podcast is hosted by Sarah Lancaster, Kansas State Extension Weed Science Specialist, Mandy Bish, Extension Weed Scientist at the University of Missouri, and Joe Ikley, Extension Weed Scientist at North Dakota State.  Podcast episodes are available at https://waragainstweeds.libsyn.com/ and also on Spotify, iTunes, and Google Podcasts.

 

 

Save the Dates – Central Ohio Agronomy School

Due to COVID uncertainties the 2022 Central Ohio Agronomy School has been pushed to March. 

Monday March 7 – 6:30 – 9:00p.m.

Monday March 14 – 6:30 – 9:00p.m.

Monday March 21 – 6:30 – 9:00p.m.

Monday March 28 – 6:30 – 9:00p.m.

The School will be at the new Ramser 4-H Activity Center (on the fairgrounds)

700 Perimeter Dr.  Mount Vernon, OH  43050

More details to come

Herbicide Resistance in Ohio Waterhemp Populations

Source:  Mark Loux, OSU Extension

Waterhemp populations across the Midwest continue to develop more complex variations of herbicide resistance.  Multiple resistance to an increasing number of herbicide sites of action is the norm in many populations in states west of Ohio.  Waterhemp has on the whole developed resistance to seven sites of action, including the following:

Group 2 – ALS inhibitors – chlorimuron, imazethapyr, etc

Group 4 – Synthetic auxins – 2,4-D, dicamba, etc

Group 5 – Photosystem II inhibitors – atrazine, metribuzin, etc

Group 9 – EPSP synthase inhibitor – glyphosate

Group 14 – PPO inhibitors – fomesafen, flumioxazin, sulfentrazone, etc

Group 15 – long chain fatty acid inhibitors – metolachlor, pyroxasulfone, etc

Group 27 – HPPD inhibitors – mesotrione, isoxaflutole, topramezone, etc

Individual populations with resistance to three or more sites of action are common.  Mutations are occurring that confer resistance to several of these sites of action simultaneously, through a resistance mechanism that enhances the metabolism and inactivation of the herbicides by the plant.  For example, there appears to be a linkage in the resistance to mesotrione and atrazine, where resistance to one means it’s likely that resistance to the other occurs also.  Weed scientists have concluded that this weed is capable of developing resistance to any herbicide site of action used against it.  We aren’t actually sure what the correct recommendation is for stewardship of herbicides once a single mutation can confer resistance to multiple sites of action.  Which is the reason we stress the need to take steps in mid to late season to prevent seed from plants that survive management strategies.

Continue reading