Strobilurin-resistant Frogeye Leaf Spot Confirmed in Mississippi

Frogeye leaf spot of soybean as was observed in many soybean fields throughout MS during the 2012 season.

For whatever reason, generally speaking, frogeye leaf spot has been more prevalent throughout MS and the nation this growing season.  In some cases in MS, extensive acreages of frogeye-susceptible soybean varieties were planted.  However, I’ve observed frogeye leaf spot in almost every soybean field I’ve stepped in this season regardless of the county.  As additional information for the soybean farming community, ratings from the soybean variety trials regarding susceptibility to frogeye leaf spot (Cercospora sojina), and several other important diseases, were made this season and will be contained in the variety trial publication released by Variety Testing Services.

A total of 15 separate field samples, containing frogeye infected soybean leaf samples were submitted to Dr. Carl Bradley at the University of Illinois in Champaign-Urbana to be screened for resistance to the strobilurin fungicides, or more properly the Quinone outside inhibitor fungicides (QoI; FRAC Group 11).  The standalone strobilurin fungicide products labeled for use in soybean are sold under the trade names of Aftershock, Aproach (DuPont’s strobilurin fungicide that has not yet received a label as of 10/5/2012), Evito, Gem, Headline, and Quadris.  In addition, numerous products are available that contain either a strobilurin + triazole as a pre-mix (e.g. Avaris, Evito T, Quadris Top (azoxystrobin + difenoconazole), Quilt, Quilt Xcel, Stratego, Stratego YLD), or the new novel pre-mix that contains a strobilurin + amide (BASF’s Priaxor; pyraclostrobin + fluxapyroxad). Out of the 15 total submitted samples, two samples were determined to be resistant to strobilurin fungicides in poison plate trials.  At present I am not aware of the specific field history associated with the two fields in question.  However, several of the samples that were determined to be susceptible to strobilurins were from Bolivar, Coahoma, Issaquena, Leflore, and Webster counties.

Since their introduction, the strobilurin fungicides have proven to be beneficial in preventing yield loss to numerous soybean diseases; one of those has been frogeye leaf spot. However, strobilurin fungicides have been used extensively in soybean production systems to provide a yield benefit, typically in the absence of foliar diseases.  Moreover, the Fungicide Resistance Action Committee, or FRAC, has classified the QoI fungicides as being high risk to the development of fungicide resistance.

Fungicides are excellent management tools and provide a reduction in overall yield loss attributed to foliar diseases in soybean.  But, there are certain scenarios that could potentially increase the likelihood of the development of resistance to the strobilurin fungicides:

1)      Reduced rate applications of strobilurin products.  Even though I realize that ultra-low, reduced rate applications are not commonplace, I’ve heard several folks allude to their uses in non-irrigated soybean fields.  Reduced rate applications (I’m speaking of reduced application rates of strobilurin fungicides with a label rate of 6 fl oz/A), such as 2 fl oz/A @ R3/R4 followed by 2 fl oz/A @ R5 should be avoided.  Remember, some strobilurin fungicides have a label rate range of 2 – 5.7 fl oz/A (e.g. Aftershock, Evito), or 3 – 3.5 fl oz/A (e.g. Gem), or 4 – 4.65 fl oz/A (e.g. Stratego YLD).  Applications of such products as Headline or Quadris made to add up to 6 fl oz/A in a single season should be avoided.  In addition, fungicide applications of 1 fl oz/A should be completely avoided since these types of fungicide applications, if made in the presence of disease especially frogeye leaf spot, greatly increase the likelihood of resistance developing within a fungal population.

2)      Applying fungicides in a reduced water volume, such as 3 gal/A as opposed to 5 gal/A are also potential sources of resistance development since the overall leaf/plant coverage may be reduced when water volume is reduced.  However, data are lacking on the potential of this particular situation attributing to resistance.  While I don’t have any information/data to speak to the source of resistance in MS it is always possible that resistance could develop in situations where poor plant coverage occurred.

3)      Lack of crop rotation.  Continuous soybean planting situations with continuous strobilurin fungicide applications as either an R3/R4 fungicide management strategy or strobilurin fungicides predominantly used to manage frogeye leaf spot could greatly increase the risk of fungicide resistance developing.

4)      Planting susceptible varieties and applying strobilurin fungicides as a management tool for frogeye leaf spot.

Preventing further development of strobilurin-resistant frogeye will be one method of reducing the likelihood of yield loss as a result of the disease.  But, in situations where frogeye has been a historical problem choosing a resistant variety or applying a triazole fungicide when the disease has been observed will be the best options for reducing yield loss.

At present, the number of states with confirmed strobilurin-frogeye confirmations stands at seven and includes (state (year of documented resistance)): Arkansas (2012), Illinois (2011, 2012), Kentucky (2010, 2011, 2012), Louisiana (1 parish; 2011), Mississippi (2012), Missouri (2011), and Tennessee (2010, 2011, 2012).  See (http://deltafarmpress.com/soybeans/strobilurin-resistant-frogeye-leaf-spot-confirmed-arkansas; http://www.uaex.edu/news/august2012/0820ArkResistantFrogeye.html; http://agfax.com/2011/10/19/4-states-find-fungicide-resistant-frogeye-leaf-spot-in-soybeans/; http://www.arkansas-crops.com/2012/08/16/strobilurin-resistant-frogeye-leaf-spot-confirmed-in-arkansas/) for further information regarding the situation in other states.