Latent Infection of Soybean rust


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  • 1-15-11
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  • Overwinters on kudzu throughout the lower gulf southInfection spreads throughout kudzu during springSoybeans do not develop symptoms until seed setBrazil – infection begins in cotyledon stageLA – symptoms usually not observed until R4-R6Photo Tom Isakeit, 2008
  • In laboratory and greenhouse settings, pustules usually develop 7 to 10 days after inoculation.Even under humid conditions, disease symptoms do not occur in the field until a particular growth stage
  • During the winter 2008/2008, sporulating pustules were confirmed in 4 southern statesBy July, the disease was moving rapidly
  • Sentinel plots are…Despite disease overwintering on kudzu, SBR was not detected on soybeans until mid-R stagesSentinel plots were not sprayed with fungicides
  • DNA from leaf samples was analyzed from field plots. The next slides will demonstrate the timelines using green/negative, stripe/latentInoculum was present, kudzu overwintered in Baton RougeIn the early part of this study, I sampled April-planted soybeans from our research farm beginning at R1SBR was detected on the first sampling. Levels were high, so infection occurred much earlierRust symptoms never developedLatent period of +60 days
  • In the second field, we detected P.p. in soybean at flowering, but symptoms did not appear until 43 days later, R6
  • Late-july beans were infected in the mid-V stages, and latent period was 33 days
  • Comparing 3 fields from our research farm in Baton Rouge…Green stem study produced first symptoms on soybean in the Baton Rouge area, Aug 20
  • 2010 was quite differentOverwintering kudzu leaves were destroyedSBR did not move until late summer/early fall
  • Winter of 2009/2010 was unseasonally cold. A winter storm in early January resulted in lower-than-normal temperatures for nearly 2 wksIn south florida, low temps in early January 10degrees below normalIn baton rouge, we also saw lows dipping below 20 degreesFThese below average winter temperatures throughout south resulted in destruction of green kudzu from which SBR overwinters
  • As compared to 2009, when low temperatures in January averaged in the 30’sOnly 3 nights during the 2008/2009 winter resulted in temperatures below 20F
  • In an effort to determine whether P.p. was overwintering systemically or within woody tissue, we sampled kudzu from an area where where SBR typically overwinters. We analyzed leaf, stem, and petioles separately from late March (after green-up) through mid-May. P.p. was not detected in kudzu. SBR was never detected at this site in 2010A similar experiment was conducted on kudzu in Baton Rouge, and we got similar results.
  • Kudzu in Florida was also sampled in 2010. Leaves onlyOnly one location yielded positive results, however, it was July and symptoms were already present in that patch of kudzuNo latent infections were detected in kudzu in 2010 (LA or FL)
  • Soybeans planted in July in Quincy, FL were sampled weeklyLatent infection was detected at R4 and symptoms occurred at R6 – 18-day latent periodInoculum load was low, so even after SBR was reported at the research station, we did not detect DNA of P.p. until 43 days later
  • 2010, limited rust 2009, rust overwintered on kudzu and we detected DNA of P. pachyrhizi early in the season.Fungicide Time-of-Infection trials were conducted to determine ideal fungicide application times
  • Field experiments show that fungicides applied soon after infection yield the best controlTOI Field planted late July, latent infection detected V4Fungicide applied within days of infection – R15th day of the 33-day latent period
  • In both years of the study, P.p. was detected in soybeans long before symptoms developed. Infection period occurred as early as V4 and as late as R2/3, but symptoms occurred between R4 and R6 (sentinel and research farms)
  • Latent Infection of Soybean rust

    1. 1. Documentation of an Extended Latent Infection Period with Phakopsora pachyrhizi in SoybeanN.A. Ward, R.W. Schneider LSU AgCenter
    2. 2. •Phakopsora pachyrhizi•Still a major disease ofsoybean in the southeasternUS•Preventative fungicideapplications result in largegrower inputs•Effective timing of fungicidesis the most effective means ofcontrol and/or prevention ofthe disease
    3. 3. Overwinters on kudzuSporulating pustules are oftendetected in January and FebruarySoybean symptoms at seed set (R5)
    4. 4. HypothesisLatent infection period can be longer than 10 days, regardless of weather conditions or inoculum load In laboratory and greenhouse, pustule development 7-10 days after inoculation Even under cool temperatures and humid conditions, disease symptoms do not occur in the field until mid-reproductive stages
    5. 5. ApproachUse qPCR to quantifyPhakopsora pachyrhiziHigh specificityDetects as little as 1 picogramof DNA of P. pachyrhiziin 10ng total DNA
    6. 6. Experimental 2009 – Baton Rouge, 3 soybean fieldsProcedure (April, May, and July) 2010 – New Iberia, kudzu (April) 2010 – Quincy, FL kudzu (March to October)
    7. 7. 2009
    8. 8. 2009 Parish Date Growth Stage St. Martin June 5 R4 Iberia June 24 R5 Acadia June 16 R5 Assumption July 27 R7 Near Baton Rouge Patricia Bollich, IPM Pipe
    9. 9. 2009 April SoybeansApr 17 June 4Planting R1 <43 days 60+ days qPCR negative qPCR positive Symptoms
    10. 10. 2009 May SoybeansMay 20 Jul 23 Sept 4Planting R1/R2 R6 64 days 43 days qPCR negative qPCR positive Symptoms
    11. 11. 2009 July SoybeansJuly 21 Aug 20 Sept 25Planting V4 R5 30 days 33 days qPCR negative qPCR positive Symptoms
    12. 12. 2009 SummaryApr 29 June 4Planting R1 60+ days May 20 Jul 23 Sept 4 Planting R1/R2 R6 43 days July 21 Aug 20 Sept 25 Planting V4 R5 33 days Aug 20 Green Stem
    13. 13. 2010
    14. 14. 2010 Weather
    15. 15. Compare to 2009
    16. 16. Sample Date +/-Stem April 3 - 2010 Results New Iberia Kudzu April 11 - Collection by Tomas Rush April 18 - April 27 -Leaf April 3 - April 11 - April 18 - April 27 - May 11 -
    17. 17. Latent Month County Symptoms Infection 2010 Gadsden KudzuMay Holmes - - North Florida Research Jackson and Education Center University of Florida Gadsden Quincy, FLJune - - Jackson Collection by Kelly O’BrianJuly Gadsden - - HolmesAugust - - Jackson GadsdenOctober Holmes -/+ -/+ Jackson
    18. 18. 2010 July Soybeans July 4 Aug 25 Sept 12Planting R3? R6? 52 days 18 days 43 days July 13 SBR Reported on nearby kudzu
    19. 19. 2009 Fungicide Time of Infection StudyR.W. Schneider, C. L. Robertson
    20. 20. Field Views of FungicideTreatments Nonsprayed Topguard, 7 0z, control R1
    21. 21. Summary:2009 – early sources of inoculum early infection2010 – late or no inoculum late or no infectionInfection detected V4 to R2/R3Disease symptoms R4 to R6
    22. 22. Conclusion:Latency is afunction of plantgrowth stage.Infection can takeplace long beforediseasesymptoms occur.Fungicides shouldbe applied atonset of latentinfection.
    23. 23. Acknowledgements