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Powdery Scab in SA: What’s New?

Powdery Scab in SA: What’s New?. Jacquie van der Waals Department of Microbiology and Plant Pathology. Problem in SA. Project Aims. Incidence, severity and distribution has increased substantially over past few years No longer restricted to one or a few growing areas

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Powdery Scab in SA: What’s New?

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  1. Powdery Scab in SA: What’s New? Jacquie van der Waals Department of Microbiology and Plant Pathology

  2. Problem in SA Project Aims • Incidence, severity and distribution has increased substantially over past few years • No longer restricted to one or a few growing areas • No truly resistant cultivars available (internationally) • Confusion with common scab • Appearance of problem in tunnels and virgin soils

  3. South African Potato Production: Hectares per Production Region Limpopo LoskopValley Mpumalanga North West Gauteng Eastern Free State KZN Western Free State Northern Cape South West Free State North Eastern Cape Eastern Cape Sandveld Ceres ± 2 million tonnes annually ± 58 000 ha Southern Cape South West Cape B. Pieterse, 2009

  4. Sandveld = Sandy field

  5. Project Aims Project Aims • Optimise detection methods for Spongosporasubterranea(Sss) from South African tubers and soil • Test various rotation crops for susceptibility to infection by Sss • Evaluate cultivar susceptibility to Sss (prelim. trial) • Identify the types and variation among Sssisolates from various growing regions • Determine infection period of Ssssporeballs (using tomato seedlings) – investigate possibility of tomato seedlings as pre-plant risk assessment indicators • Determine relationship (if any) between initial soil inoculum and final disease severity

  6. Detection and quantification Project Aims • PCR and qPCR optimised for South African samples • PCR – Primers Sps1 and Sps2 (Bell et al., 1999) • qPCR – Primers and probes as in van de Graafet al. (2003) • Accuracy of quantification technique confirmed in international ring testing procedure + - → sporeballs / g soil (increasing [])

  7. Alternate hosts Project Aims • Five crops were tested for their ability to host Sss • Cabbage (cv. Copenhagen market) • Mustard (cvs. Florida Broadleaf and Southern Giant Curled) • Soybean (cv. PAN737R) • Wheat (cv. SST882) • Tomato (cv. Rodade) • Galls containing sporeballs formed on roots of tomato and mustard. Hosts for Sss! • Zoospores infected cabbage and wheat, but did not form galls on roots. Trap crops? • Soybean was not infected by Sss

  8. Alternate hosts Project Aims

  9. Cultivar susceptibility Project Aims • Argos, Caren, BP1, UTD, Buffelspoort and Valor were tested • Soil was inoculated with three different concentrations of sporeball suspensions • Plants were harvested four months after emergence • Evaluation – root galls and tuber lesions

  10. Root infection

  11. Lesion severity

  12. Typing of isolates Project Aims • 28 Sss samples were collected from throughout South Africa (Sandveld, Ceres, KZN, Mpumalanga and one sample from 1936 from Sandveld) • ITS sequencing was done to determine genetic variability of samples • All samples were shown to be Group Type II • This finding has implications for import and breeding of new material • More Sss samples need to be collected to confirm results

  13. Tomato seedlings as indicator plants? Plant in contaminated soil 3 wk: Root galls and + AgriStrip test 6 d: Root infection 12 d: Zoosporangia

  14. Initial inoculum and final disease severity Project Aims

  15. Initial inoculum and final disease severity Project Aims • Results from two pot trials and two field trials: appears to be littleorno relationship between initial inoculum in soil and final disease severity • Final amount of disease is therefore determined by environmentalconditions and other riskfactors

  16. Overall objective – future work Project Aims To develop an integrated and sustainable management strategy for powdery scab of potatoes in South Africa

  17. Management measures Project Aims • Treatments: • Fluazinum (soil fumigant) • Chloropicrin (soil fumigant) • Calcium cyanamid (soil amendment) • Zinc oxide (soil amendment) • Biocontrol agent (Trichoderma) • Untreated control • Commercial field • Strip application of treatments • Three replicates • Naturally infested soil • Cultivar Nicola (?)

  18. Trial layout Project Aims

  19. Closing thought “Powdery scab will never be a problem in South Africa.” • Dr JE vanderPlank

  20. Acknowledgements Project Aims Potatoes South Africa THRIP Colleagues in the International Potato Diagnostics Collaboration (Alison Lees) Students in the Potato Pathology Programme @ UP

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