Title:  Development of an effective program to manage strobilurin-resistant frogeye leaf spot in Arkansas
Principle Investigator:  Travis Faske and Burt Bluhm
Production System:  Full Season and Double Crop
Status:  First of three years (2013 to 2015)
(Fig. 1) Soybean leaflets from nontreated and fungicide treated plots with rating of 3 and 1, respectively.
Goal:  Develop practical management tactics to control strobilurin-resistant frogeye leaf spot and characterize traits in these fungicide-resistant pathogens that may increase disease incidence.  Determine the potential of DMI- resistant development and develop guidelines to reduce the impact of fungicide-resistant diseases.
Objectives:
  1. Evaluate the efficacy of fungicides, labeled and experimental, to control strobilurin-resistant frogeye leaf spot.
  2. Determine if biological traits in strobilurin-resistant frogeye leaf spot contribute to an increase in disease development.
  3. Investigate the potential development of DMI-resistance in frogeye leaf spot and other fungal diseases of soybean.
  4. Develop fungicide resistance management strategies to delay or prevent the population increase of strobilurin-resistant pathogens in Arkansas
Objective 1:  Several foliar fungicide trials were established and inoculated with strobilurin-resistant frogeye leaf spot (S-R FLS, 1,400 conidia/ft of row) at the Newport Extension Station.  Disease development was relatively uniform across plots and several new fungicides were effective at suppressing S-R FLS.
Data from foliar fungicide trials supported the importance of using a mixed fungicide or triazole to manage S-R FLS on a HS soybean variety (Asgrow 48R40) rather than a strobilurin-alone program (Fig. 2 and 3).
(Fig. 2) Effect of foliar strobilurin (blue bar) and triazole + strobilurin (green bars) fungicide on suppression of FLS.
(Fig. 3) Effect of foliar strobilurin (blue bar) and triazole + strobilurin (green bars) fungicide on suppression of FLS.
However, FLS suppression did not always contribute to higher yields.  On modernly susceptible variety (Asgrow 4993) disease suppression and yield response were similar among fungicide and non-treated plots (data not shown).  Numerically, a fungicide treated plots had an average yield increase of 2.25 bu/ac.  Yield loss averaged 6% among four small plot trials with fungicide treatment providing a yield increase over the non-treated control 50% of the time.
Given these trials were inoculated with S-R FLS additional data is needed to determine the benefit of one fungicide between highly susceptible and moderately susceptible varieties.  Similar experiment will be repeated at Newport station thus providing more meaningful data.
Objective 2:  A host differential set of 10 soybean accessions with various genes for resistance to FLS were inoculated on July 25, 2013 with blend of five isolates confirmed to be S-R.  Heavy rains shortly after application did limit disease development; however, FLS was observed on 8 of 10 soybean accessions.
None of the soybean lines containing a single dominant gene  Rcs 2, 3, or Peking had any infection, thus resistance is still effective on S-R FLS (Fig. 4).
(Fig. 4) Effect of S-R FLS infrection on a group differential soybean genotypes with resistance to FLS.
Seed was limited for this season so, hopefully all 12 soybean accession will be available in 2014 to determine the most common race in Arkansas.
Objective 3:  Several isolates have been evaluated for variation in sensitivity to triazole fungicides.  To date, the triazoles are consistently efficacy at suppress hyphal growth, with the new generation triazoles expressing a little higher suppression of hyphal growth.  Future trials will expand by isolate number and fungicide combinations.  These data have provided supportive information for fungicide efficacy trials.
The EC50 values for Proline, Quadris top, Topsin, tebuconazole, Echo, Quadris xcel, Stratego YLD, and Quadris, were 0.8, 1.2, 1.5, 2.5, 2.5, 2.6, 6.0 and 9.7, respectively.  Thus, the new generation triazoles (Proline and Quadris top) were the most effective in-vitro; followed by other labeled fungicides classes and older generation triazole mixed with strobilurin fungicides.
Objective 4:  As management strategies are updated, information will be presented at producers’ meetings and posted on the row crops blog.  As additional data is gathered for more meaningful analysis additional recommendations for fungicide-resistance management and control of FLS will be available and updated on the UAEX website and fact sheets for S-R FLS management.