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Powdery scab in France
Past and current research
Karima Bouchek-Mechiche1,2, Catherine Chatot 3 &
Yves Le Hingrat1
1 French Federation of Seed Potato Growers
2 INRA -IGEPP
3 Germicopa
1
Powdery scab studied as part of a potato tuber blemishes program
Rhizoctonia R. solani
Silver scurf H. solani
Black dot C. coccodes
Powdery scab S. subterranea
Common and netted scab * CS : S. scabies et Streptomyces spp. * NS: S. reticuliscabiei
5 pathogens/diseases studied
Objective : How to manage several soil-borne pathogens together?
Powdery scab context
• Powdery scab is not a major problem in French potato production areas
• French certification threshold is 0.2% infected tubers
• Occasional contamination in fields and peat or other culture substrates
– Main question : what is the origin of the contamination, remaining unsolved so far ?
PS activities over time
• Diagnostic & detection tools
2000-2003
• Cultivar assessment trials
2004-2005
• Epidemiology trials
2008-2010
• Improving soil detection
• Epidemiology
2016-2018
From tubers
ELISA Test AgriStrip
TaqMan/PCR PCR/Nested PCR
Development of detection tools
From soil
tomatoes
Tomato roots
PCR
Baiting test
TaqMan-PCR
diseased
Not yet achieved
AgriStrip is helpful for the diagnosis
Lots Symptoms Type Sss
AgriStrip
qPCR
Sss
Streptomyces
marquers
1
2
3
4
PS
PS
PS
PS
Typical
Typical
Typical
Typical
+
+
+
+
+
+
+
+
-
-
-
-
5a
5b
6a
6b
7a
7b
PS
?
PS
?
PS
?
Typical
Atypical
Typical
Atypical
Typical
Atypical
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
8
9
10
11
12
13
14
15
16
17
?
?
?
?
?
?
?
?
?
?
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
18
19
CS
CS
Typical
Typical
-
-
-
-
+
+
Efficient detection of S. subterranea
Atypical powdery scab symptoms
PCRq=+ AgriStrip = +
PCRq=+ AgriStrip = +
PCRq=+ AgriStrip = +
PCRq=+ AgriStrip = +
PCRq=+ AgriStrip = +
Web site potato tuber blemishes
http://www.potato-tuber-blemishes.com
Are French environmental conditions conducive to powdery scab?
3 years trials 2008, 2009 2010
Contaminated field
Measures
Air T° and rainfall, soil humidity
Scoring roots for galls and tuber for powdery scab At tuber initiation and each 15 days intervals
Two varieties Nicola & Kennebec
Powdery scab expression in 2008
Powdery scab observed early after tuber initiation
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70 80 90
21 may 10 july 17 july 24 july 31 july 07 august 18 august 18 september
Se
ve
rity
(in
de
x)
incid
en
ce
(%
)
prélèvements
incidence and severity of PS over the season
Kennebec incidence Nicola incidence
Tuberisation
P1
No galls observed on roots
Powdery scab expression in 2009 & 2010
-1
1
3
5
7
9
11
13
15
0
10
20
30
40
50
26 june 06 july 16 july 21 july 28 july 3 august 11 august 10 september
Ind
ex
inc
ide
nce
Sampling
2009
Kennebec incidence Nicola incidence Kennebec Index Nicola Index
harvest
0
2
4
6
8
10
12
0
20
40
60
80
100
28 june 05 july 12 july 19 july 26 july 2 august 11 august 09 september
Ind
ex
Inci
den
ce
Sampling
2010
Kennebec incidence Nicola incidence Kennebec index Nicola index
Tuberisation
Tuberisation
Rainfall & tuberisation during the three years trials
Tu
beri
sa
tio
n
20
09
/20
10
Tu
beri
sa
tio
n 2
00
8
0
10
20
30
40
50
60
70
80
90
d1 d2 d3 d1 d2 d3 d1 d2 d3 d1 d2 d3 d1
mai mai mai juin juin juin juillet juillet juillet aout aout aout sept
Rain
fall
(m
m)
2008
2009
2010
0
2
4
6
8
10
12
14
28-mai 04-juin 11-juin 18-juin 25-juin 02-juil. 09-juil. 16-juil. 23-juil. 30-juil. 06-août 13-août 20-août
Soil
hu
mid
ity
(b
ar)
Y 2010
Y 2009
Dry period
Summary of the main data for PS expression
Years 2008 2009 2010
Plantation date 22/05 05/05 12/05
Tubérisation 10-17/07 26/06 28/06
Precipitations around tuber
initiation period
97,5 28.5 28
Temperatures at tuber initiation 15.5 16 16
Mean T° during the trial 16 °C 18,5 16.5
Total of precipitation during the
trial
394 mm 265 mm 290 mm
Powdery scab incidence 22-81% 3-7% 10-38%
2 years trials 2005 & 2006
Scoring At harvest for powdery scab on tubers
One month after planting for root galls
Susceptibility of French cultivars
Number of cultivars tested 2005: 15 2006: 21 References varieties in both trials: 7
Susceptibility of French varieties Incidence *
*Standard varieties
Different ranging in 2004 and 2005
0
10
20
30
40
50
60
70
80
90
100
110
Incid
en
ce
Varieties
Y 2004
Y 2005
* *
*
Less susceptible Moderately susceptible
Susceptible
* * *
Susceptibility of the varieties Severity
* *
0
1
2
3
4
5
6
7
8
9
Dis
ea
se
in
de
x
Varieties
Y 2004
Y 2005
Susceptible
Moderately susceptible
Less susceptible
*
* Control cultivars
* * * * *
Sclerotia of R. solani on roots
No powdery scab galls were observed on roots
Susceptibility of the varieties
Susceptibility to other potato blemishes
0
2
4
6
8
10
12
14
16
18
20
Rosanna Lady Roseta Nicola gladiator Désirée Estima Saturna Agria Kennebec
Sca
b i
nd
ex
Powdery Scab
Rhizoctonia
common scab
Gladiator with common scab and few powdery scab
Gladiator with Rhizoctonia solani
Susceptibility to other potato blemishes
Conclusions
The French certification standard is severe in order to avoid the dissemination of the pathogen in the territory
AgriStrip ® is an efficient tool to distinguish PS symptoms from CS, it helps the certification procedure
Detection tools are very efficient to detect the pathogen from tuber and plant roots but not optimal for routine detection directly from the soil
The French environmental conditions may be favorable to PS if soils become contaminated
The French varieties are classified in different categories ranging from susceptible to fairly resistant
Ongoing research
Develop detection tools for soil and other substrates samples (peat, other …)
Routine test
Understand the origins of the contamination in the substrates
Develop a routine test to screen different control methods (eg: biocontrol), particularly to secure the quality of early seed generations
Objectives are to :