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