Powdery Scab in SA: What’s New?

Jacquie van der WaalsDepartment of Microbiology and Plant

Pathology

Project AimsProblem 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

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 CapeSouth West Cape

South African Potato Production:Hectares per Production Region

B. Pieterse, 2009

± 2 million tonnes annually± 58 000 ha

Sandveld = Sandy field

Project AimsProject Aims

•Optimise detection methods for Spongospora subterranea (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 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

Project AimsDetection 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 [])

Project AimsAlternate 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

Project AimsAlternate hosts

Project AimsCultivar 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

Root infection

BP1

Care

n

UTD

Valo

r

BP13

Arg

os

0

0.5

1

1.5

2

2.5

3

3.5

4

aa a a

a a

Mean

of

root

infe

cti

on

severi

ty s

core

(0

-4)

Lesion severity

BP1

BP13

Caren

Argo

s

Valo

rUTD

0

0.5

1

1.5

2

2.5

3

3.5

4

a

a a

a

a a

Mean

s o

f tu

ber

lesio

n

severi

ty s

core

(0

-10

)

Project AimsTyping 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

Tomato seedlings as indicator plants?

Plant in contaminated soil

12 d: Zoosporangia6 d: Root infection

3 wk: Root galls and + AgriStrip test

Project AimsInitial inoculum and final disease severity

Project AimsInitial 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

Project Aims

To develop an integrated and sustainable management strategy for powdery scab

of potatoes in South Africa

Overall objective – future work

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

Project AimsTrial layout

1 2 3 4 5 6 7 8 9 10 11 12 11 8 2 4 5 10 3 12 1 9 7 6 5 7 12 8 10 3 1 9 2 11 6 4

100m

12m

144m

432m

SUB

PLO

T 3

SUB

PLO

T 2

SUB

PLO

T 1

Closing thought

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

Project AimsAcknowledgements

Potatoes South AfricaTHRIP

Colleagues in the International Potato Diagnostics Collaboration(Alison Lees)

Students in the Potato Pathology Programme @ UP