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Role of Rhizoctonia solani AG 3-PT inoculum source on disease
development on potatoes in South Africa
J A C Q U I E VA N D E R WA A L S 1, N O R M A N M U Z H I N J I 1, M A R I E T T E T R U T E R 2, J A M ES W O O D H A L L 3
1D E PA R T M E N T O F P L A N T A N D S O I L S C I E N C E S , U N I V E R S I T Y O F P R E TO R I A ; 2A R C - V O P ; 3U N I V E R S I T Y O F I D A H O
@ J A C Q U I E V D W
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Rhizoctonia solani
(A. Pitman, New Zealand)
Muzhinji et al. 2017 Plant Disease
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White stem collar can develop early in season if sexual stage is present
Pathogen overwinters as sclerotia and mycelia
Infected seed and volunteer potatoes
Roots & stolonsattacked any time during season
Late season damage may result in aerial tuber formation
Sclerotia form on tubers when still attached to mother tuber; continue to develop in storage
R. solaniDisease Cycle
Mycelia and sclerotiaon infected tubers infect developing sprout, root and stolon
BACKGROUND
• Inoculum: seed tuber- and/or soil-borne sclerotia or mycelium• Basidiospores during crop closure >> sexual reproduction >> polycyclic
nature of disease
• Role of soil- vs seed tuber-borne inoculum in disease development: Contentious issue
• Mark-release-recapture experiments can provide answers• Allow tracking of inoculum sources during disease development >>
determine origin of inoculum
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Objectives1. To investigate the contribution of R. solani AG 3-PT seed tuber-
and soil-borne inoculum to disease development
2. To investigate the evolution of an experimental population of R. solani AG 3-PT under field conditions
VS
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MOTIVATIONThe role of each source of inoculum is a contentious issue
Understanding the contribution of each source of inoculum is an important step in implementing effective disease management strategies for R. solani
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Materials and MethodsoUniversity of Pretoria experimental plots
oRCBD
oTreatments: (4 treatments x 4 reps = 16 plots)
Contaminated seed, clean soil
Clean seed, clean soil
Contaminated seed, infested soil
Clean seed,infested soil
VS
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Two distinct MLG groups of R. solani AG 3-PT isolates(five isolates per group)
PCR-RFLP profiling(Ceresini et al. 2002)
Disease assessments:21 & 60 days after planting;At harvestIsolations from diseased material
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Materials and Methods
Contaminated seed, infested soil
From previous trial –not reinoculated
Disease assessments:60 days after planting;At harvestIsolations from diseased materialPCR-RFLP Microsatellite analysis (Ferrucho et al. 2009)
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Root infection Stolon canker
Inoculum source
2013 2014 2013 2014
Control 0.0 c* 0.0 c 0.0 d 0.0 d
Soil 14.5 b 10.5 b 8.5 b 15.0 a
Soil and tuber
17.5 a 12.5 a 10.2 a 12.0 b
Tuber 13.0 b 6.0 c 5.0 c 8.0 c
Results: Disease indexVS
*Duncan’s Multiple Range test P = 0.05
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Stem canker Black scurf
Inoculum source 2013 2014 2013 2014
Control 0.0 d* 0.0 0.0 c 0.0 d
Soil 22.5 c 14.5 c 13.4 b 5.8 c
Soil and tuber
37.0 a 22.5 a17.6 a 11.3 a
Tuber 29.0 b 17.4 b 12.6 b 7.8 b
VSResults: Disease index
*Duncan’s Multiple Range test P = 0.05
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Seed (60%)
Soil (36%)
Non Released(4%)
Isolated from plants in plots inoculated with both seed and soil-borne inoculum
Suggests recombination in field
Genotypes of isolates from symptomatic tissue (n = 151)
Results
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0
5
10
15
20
25
30
35
40
Root infection Stolon Canker Stem Canker Black Scurf
No.
of
isol
ates
/sym
pto
m
R. solani symptom
SoilTuber
PCR-RFLP genotyping to correlate isolates obtained from symptoms to inoculum source
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Conclusions
Black scurf: Seed-borne inoculum more important
Root & stolon infection: Soil- and seed-borne inoculum equally important
Stem canker: Seed-borne inoculum more important
VS
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Recombination may occur in field under conducive environmental conditions >> gradual increase in genetic diversity
Production of basidiospores >> initial inoculum
May gradually result in changes in genetic structure of field populations from year to year
Ensure the use of pathogen-free seed tubers to eliminate seed-borne inoculum and the introduction of new
genotypes of R. solaniApply to control measures to limit development of sexual
phase in field
Conclusions
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AcknowledgementsFunding bodies:
o Potatoes South Africa
o National Research Foundation
o British Society for Plant Pathology (Travel Award)