Foliar Soybean Disease Update: August 31, 2013

Soybean rust

Soybean rust on the underside of a soybean leaf. Note clustering of raised, sporulating pustules.

Over the past 7 to 10 days additional counties have been confirmed to contain soybean rust (SBR) infected plant material (either kudzu or soybean).  Unlike last year at this time we are struggling to find infected plant material at most locations.  As an example, in the situations where I have detected SBR this week I have only been able to find a single leaf in numerous plots observed.  On Tuesday, a fungicide trial was rated in Stoneville.  The trial contained 64 plots and all I could manage to find was two SBR-infected leaves.  Ultimately, this is good news for farmers that have full-season soybean fields since fields that have reached R5.5 and beyond are out of the woods when it comes to SBR.  Several weeks will be required for enough inoculum to build up and reduce yield in any soybean field situation.  However, those fields that are still in vegetative growth stages need to be monitored.  But, with that said, fungicides should not be applied to vegetative soybean fields for several reasons.  First and foremost, applying a fungicide to vegetative soybean fields even with the threat of soybean rust will not likely be economically beneficial.  Moreover, for vegetative soybean fields to become infected with SBR a heavy source of inoculum needs to be present in the particular area.  Since 2007, I have only observed vegetative stage soybean plants infected with SBR on two occasions.  One of those was during 2009 when conditions were optimum for the development of SBR in MS, SBR was detected in all 82 counties, and fields of volunteer, vegetative soybean plants were heavily infected in late November.

Over the next several weeks I expect the number of MS counties with SBR to increase dramatically.  Much like last year, I expect that we’ll detect SBR in all 82 counties.  But, since SBR has been observed in the U.S. (2004 was the first year) extremely limited yield losses have been recorded in MS due exclusively to SBR.  In most situations where yield loss has been observed in MS soybean fields the farmer didn’t have a consultant and the disease wasn’t observed until foliage was heavily infected or severe defoliation had already occurred.

Stay tuned to the Crop Situation Blog, the SBR website (www.sbrusa.net), as well as the Disease Monitoring maps provided on the Crop Situation Blog by visiting https://www.mississippi-crops.com/disease-monitoring/ for current information regarding the distribution of the disease.

Frogeye

Frogeye leaf spot. Note smaller lesions indicative of secondary infection.

A tremendous number of calls continue regarding frogeye leaf spot.  In the years since I have been in MS this is the most frogeye we’ve observed.  Some fungicide products have done a good job of delaying additional spread of the disease.  However, products that we have relied on in the past to manage the disease, such as stand-alone strobilurin fungicide products, don’t appear to have done as good a job managing frogeye as they have in the past (see: https://www.mississippi-crops.com/2013/08/29/frogeye-leaf-spot-fungicide-preliminary-data/ for additional information on how products have observationally performed this year at two locations on a susceptible soybean variety).  Numerous field samples have been collected this season from fungicide treated as well as non-treated fields to determine how widespread strobilurin-resistant isolates of Cercospora sojina (frogeye leaf spot fungus) may be in MS.  At present, I am not prepared to call the C. sojina strains present in Starkville and Stoneville resistant.  But, I do think this suggests the fungal strains at those two locations have lost their sensitivity to the stand-alone strobilurin fungicide products.  For a strain to be “resistant” a specific molecular shift will need to have occurred within the amino acid structure of the fungus.

For doublecrop soybean fields, weigh fungicide decisions on overall yield potential as well as soybean variety.  Susceptible soybean varieties will likely continue to become infected as evidenced by observing the doublecrop soybean trials in Stoneville, MS.  Frogeye leaf spot is heavy in the field of irrigated Armor DK 4744 soybeans that were treated with additional fungicide trials on Wednesday (8/28/2013).  Susceptible soybean varieties in high-yielding situations may benefit from a pre-mix or tank-mix combination fungicide application (e.g., strobilurin + triazole or other preventive + curative fungicide combination) or, if frogeye leaf spot is already present in the field a triazole fungicide application.  But, keep in mind that not all triazoles are as active on frogeye leaf spot and some products will perform better than others.  In addition, at some particular locations products will perform differently than the trials presented on the blog in the above mentioned post since strains of the fungus are extremely variable between locations.