2005 Season Review: Weed Science

Bill Johnson, Assistant Professor, Botany & Plant Pathology, Purdue University
Glenn Nice, Weed Diagnostician, Botany & Plant Pathology, Purdue University

We received a number of corn and soybean herbicide injury reports during 2005.  In some cases, the cause of the injury was related to stressful weather conditions which reduced the plant’s ability to metabolize or degrade the herbicide. 

Herbicide Injury Issues

Growth Regulator: Several cases of soybean growth regulator injury were sent into the P&PDL in 2005.  This is a common situation in Indiana and the surrounding states or for that matter anywhere where corn and soybean are grown in close proximity.  Soybean leaf puckering, cupping, and strapping are a common symptom when exposed to low doses of a growth regulator such as 2,4-D, Banvel, Clarity, Crossbow, Distinct, etc.  In many cases if the injury occurs early in the season there is not a yield response.  However, if the growing season leads to delayed corn applications and injury occurs later in the soybean development yield effects may be seen. For more information on growth regulator injury on soybean, see the following publication (http://ipcm.wisc.edu/pubs/pdf/dicamba2004.pdf).

ALS Injury on Corn: We noticed several cases of ALS herbicide injury to corn from applications made during the extremely hot weather we experience in early July.  The injury symptoms consisted of leaf yellowing, particularly in the whorl area of the plant, crinkling of the leaf edges near the chlorotic tissue and stunting.  In most cases we are aware of the corn grew out of the injury and did not suffer yield loss.  During stressful weather conditions, corn plants are not able to metabolize herbicides as efficiently and crop injury is more likely.  Some postemergence ALS herbicides also contain statements on the label that warn users that crop injury is more likely when applications are made in daytime air temperatures of 90 degrees F or higher.

Giant Ragweed and Common Lambsquarter: We received several reports of poor giant ragweed and lambsquarter control with glyphosate in 2005.  In many cases, glyphosate applications were made in early June during a hot dry spell and herbicide activity was compromised.  Although resprays were effective in many cases, we have received a number of reports of fields with giant ragweed that were sprayed three times with glyphosate and giant ragweed was still not controlled.  We are currently investigating about 20 different sites to determine the sensitivity of giant ragweed and common lambsquarter to glyphosate.

Common Pokeweed:  Common pokeweed continues to be problematic to manage in no-till soybean and corn production.  We believe that infestation levels are increasing due to lack of management in the fall when control is best with herbicides.  Although herbicides applied during the growing season can reduce competition by controlling the above ground growth, we believe it is not controlling the underground tap root of this perennial.

Giant Hogweed: A new invasive plant by the name of giant hogweed (Heracleum mantegazzianum) was identified in Indiana by an employee of JFNew (a Natural Resource Consulting agency, www.jfnew.com).  Giant hogweed is a problem invasive in Australia, Europe, and Canada.  In the US it has been found in Maine, Michigan, New York, Pennsylvania, and Washington.  It is a large plant in stature resembling a large cow parsnip (Heracleum maximum).  For more information about giant hogweed see the following article “The Infamous Giant Hogweed” (http://www.btny.purdue.edu/weedscience/2004/articles/gianthogweed04.pdf)

Greenbrier: Several homeowners that have woods on their property have sent in vines with or without spines that belong to the Smilax genus.  Plants from this genus are fairly common in Indiana’s wooded areas and have been also known as “Jacob’s Ladder”, “Bristly Sarsaparilla”.

Horseweed/marestail: In 2004, 19 counties that had been screened had glyphosate resistant horseweed/marestail.  In 2005, we identified glyphosate-resistant marestail in 9 additional counties.  For more information about our marestail work, see this website http://www.btny.purdue.edu/weedscience/marestail/index.htm.