Dispelling the Myths: Biology, Epidemiology and Risk

1. Dispelling theMyths: Biology, Epidemiology and Risk X.B. Yang Department of Plant Pathology Iowa State University

2. Conditions for occurrence: are weather conditions in US production areas suitable to the epidemics of soybean rust?Establishment: After entry, can the pathogen survive winter in the U.S.? If so, where? Dispersal: Can rust pathogen establish a pathway to the north during the season? Entry: natural pathways and 2004 risk Questions Addressing the Risk of Soybean Rust Slide 2

3. Data Source I: Research in USDA Foreign Disease and Weed Science Research Unit Has a Biosafety Level 3 laboratory and greenhouse - only place in the US mainland to study soybean rust experimentally Critical studies: temp and moisture effects on infection, germination, spore production, pathogenicity, alternate hosts, resistance Slide 3

4. Summary on Infection Components - Germination, 46-97F, optimum 61-75F, minimum 6 hr dew - Infection, 52-82F, optimum 66-75F, minimum 6 hr dew - Uredia mature in 6-7 days after infection in 72-81 F - Period for urediospore production 10-11 days Slide 4

5. Data Source II: Field Experiments in AVRDC, Taiwan Taiwan, latitude 22 - 25N, has tropical climate and soybean grows year roundEpidemiology studies were conducted 1978-1982 by Tschanz, A. & Wang, T.C.Disease development wasquantified from 71 plantingexperimentsA simulation model was developed Slide 5

6. Estimated yields measured and predicted with and without rust in Florida (Yang et al 1991) Slide 6

7. Conditions for occurrence : are weather conditions in production areas suitable to the epidemics of soybean rust? Establishment: After introduction, can the pathogen survive winter in the U.S.? If so, where?Dispersal: Can rust pathogen establish a pathway to the north during the season? Entry: natural pathways and 2004 risk Questions Addressing the Risk of Soybean Rust Slide 7

8. Year-round rust survival potential in America 2003 2002 2001 Rust has to survive in green leaves of hosts Conditions are more suitable to rust in BR than in the U.S. Risk to NC regions depends on seasonal northward spread Slide 8

9. 2003 2002 2001 Chronological expansion of soybean rust in South America. 2003 estimated loss in BR was $1.3 Billion USD Slide 9

10. Kudzu as a host Slide 10

11. Kudzu in Charlotte, NC 10-Acre Kudzu in central Florida (from S. Rizivi) Slide 11

12. Conditions for occurrence : are weather conditions in production areas suitable to the epidemics of soybean rust? Establishment: After introduction, can the pathogen survive winter in the U.S.? If so, where? Dispersal: Can rust pathogen establish a pathway to the north during the season? Entry: natural pathways and 2004 risk Questions Addressing the Risk of Soybean Rust Slide 12

13. Over-winter in Mexico and southern TX, LAS-N spread of inoculum in a growing seasonThe earlier spores arrive, the severe the disease is Wheat Stem rust pathway in US from USDA Cereal Disease Lab Slide 13

14. 45 N 30 N 20 N Predicted over-wintering Occurrence of Soybean Rust in China (modified from Tan, 1994) Slide 14

15. Occurrence in China and Survival in N. America 25oN China North America Slide 15

16. Cautious points for comparison More survival in China, reducing US risk U.S. does not have three planting seasons, reducing US risk. Seasonal winds are different: Low level-jet in US is faster, adding risk. Large areas of kudzu in U.S., adding risk. Slide 16

17. Comparison of Wind Patterns in May Asia North America Slide 17

18. Conditions for occurrence : are weather conditions in production areas suitable to the epidemics of soybean rust? Establishment: After introduction, can the pathogen survive winter in the U.S.? If so, where? Dispersal: Can rust pathogen establish a pathway to the north during the season? Entry: natural pathways and 2004 risk Questions Addressing the Risk of Soybean Rust Slide 18

19. Possible natural pathways From West Africa From South America Direct airborne Land bridging Risk in 2004 growing season Slide 19

20. Possible entry route 1: Airborne from West Africa to mainland, or to Caribbean, then to mainland Region with soybean rust Slide 20

21. History: Entry of sugarcane rust to Americas 1978 1978-1979 1986 1st report in 1970s Slide 21

22. Possible entry route 2: Airborne from South America to mainland, or to Caribbean then to mainland Region with soybean rust Slide 22

23. West Hemisphere Winds Directions in March Slide 23

24. Prediction of Relative Spore Pathway for Feb 15 to March 15 (Prediction made in January, 2004) Figure 1. Reported 2004 soybean rust occurrence in Argentina (left) and spore movement predicted in January (right), which is consistent with prediction. For left figure, blue = PCR confirmed, yellow = confirmation under process. Slide 24

25. Possible entry route 2: Airborne from South America to mainland, or to Caribbean then to mainland Low level winds along equator were East to west during growing seasons of S.America Spores are ineffective when land in the US during a winter Dispersal during summer month is under analhysis Region with soybean rust Slide 25

26. Possible entry route 3: Through land-bridging to North America Region with soybean rust Slide 26

27. History: Entry of barley stripe rust into the Americas 1991-1994 1987 From Europe 1975 The chronological reports of barley stripe rust in Americas 1976-1982 Slide 27

28. Natural Pathways and likelihood 1) Direct airborne from S.America to N. America, no previous example, - less likely 2) W. Africa to Caribbean to N. America, possible with an example 3)Land-bridging from S.America to Central America to N. America, two examples - more likely Slide 28

29. Possible natural pathways From West Africa From South America Direct airborne Land bridging Risk in 2004 growing season Slide 29

30. Year round survival in North and Central America Slide 30

31. Likely seasonal dispersal pattern Northward spread) (May onward) Inoculum increase (Before May) Slide 31

32. July 25 June 18 May 20 Sept 1 Aug 15 Worst case scenario - southern leaf blight of corn in 1970 (Moore, Plant Disease:1970). Quick spread was due to a tropical storm and favorable weather in corn belt in that year Slide 32

33. For the known cases of windblown diseases that previously entered the U.S. from S. America, none reached the U.S. without being first reported in countries in Central America or the Caribbean Islands.No diseases became severe and region-wide epidemics in their first year of detection in the U.S.It is unlikely to see soybean rust impact soybean production in NC Region this year. Risk of 2004 Season Slide 33

34. USDA-ARS-FDWSRU United Soybean Board, North Central Soybean Research Program Shimon Pivonia, ISU Zaitao Pen, Bob Pasken & Joe. Pietrowicz, SLU Acknowledgement Slide 34