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

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

4. Sandveld = Sandy field

5. Project Aims Optimise detection methods for Spongospora subterranea (Sss) from South African tubersand soil Test various rotation crops for susceptibility to infection by Sss Evaluate cultivar susceptibility to Sss (prelim. trial) Identify the types and variation among Sss isolates from various growing regions Determine infection period of Sss sporeballs (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. Project Aims Detection and quantification PCR and qPCR optimised for South African samples PCR – Primers Sps 1 and Sps 2 (Bell et al., 1999) qPCR – Primers and probes as in van de Graaf et al. (2003) Accuracy of quantification technique confirmed in international ring testing procedure + - → sporeballs / g soil (increasing [])

7. Project Aims Alternate hosts 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. Project Aims Alternate hosts

9. Project Aims Cultivar susceptibility 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. Project Aims Typing of isolates  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 12 d: Zoosporangia6 d: Root infection 3 wk: Root galls and + AgriStrip test

14. Project Aims Initial inoculum and final disease severity

15. Project Aims Initial inoculum and final disease severity Results from two pot trials and two field trials: appears to be little or no relationship between initial inoculum in soil and final disease severity Final amount of disease is therefore determined by environmental conditions and other risk factors

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

17. 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 (?) Management measures

18. Project Aims Trial layout

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

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