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Rhizoctonia Onion Stunt
Simon Anstis, Trevor Wicks & Sue PederickSouth Australian Research & Development Institute
Factors affecting severity and management options
Onion Stunt• Widespread in Mallee, SA &
found Columbia basin, USA• Causes undersized bulbs and
economic loss• Onions rotated with cereals
increase Rhizoctonia• Root damage similar
to cereals - spear tips- sunken cortex
Onion Stunt- aerial view
Onion nurse crops• Associated with cereal nurse crops
that protect onion seedlings from sandblast
• Cereal nurse-crops used because: - rapid germination in cool soils
- easily killed with herbicide
- availability and cost
• Barley/wheat/triticale
• Alternative: above and non-host R. solani????
• Timing of sowing influences stunt severity
Timing of sowing:
• Some growers are moving away from sowing into established cover crops
• Claim better establishment• Risk of wind damage • June/July minimal, varies with
location• Late plantings prone to wind-
blowing • Stunt risk less, late plantings
Onion stunt severity
• Temperature• Moisture• Soil type
Method• Onion seedling bioassay • Mallee onion soils
Effect of Temperature on Onion Stunt
22 C
15 C
Con ConAG 8 AG 8
Soil moisture and stunt severityIncreasing soil moisture increases plant growth in the presence of
Rhizoctonia solani AG8
0
50
100
150
200
250
300
0 1 3
Dry
Wt/p
ot (m
g)
Rhizoctonia propagules/pot
25
50
75
Onion seedling growth in 5 soil types at different moisture
%WHCLsd (0.05) 39.4
Onion Stunt across different soils
• Over different soils proportion of coarse sand generally related to stunt severity
• Other parameters variable pH, mineral nutrients 0
100
200
300
400
500
Caloote Kea Keb Tailem Bend
Dry
Wt
(mg/
pot)
No pathogenR. solani
49.6%
Coarse sand fraction
72.4%
86.5% 69.0%
Soil Fumigants
• Both Metham & TeloneC35 reduce
soil inoculum
Onion metham fumigation response
Fumigated strip through non-fumigated plot
Onion crop 2 seasons after fumigation
• Aerial photograph onions 2 years after metham
• Patches of onion stunt
Patches:448 ± 58 pg AG 8 DNA/g soil
Non-stunted areas:45 ± 21 pg AG 8 DNA/g soil
Onion Stunt: Case Study• Site preparation:
• Deep ripping• Clay spreading• Deep cultivation• Levelling• Compost, lime
Time of sampling
R. solani pg DNA/g
Before 88 ± 18 After - with prep. 9.9 ± 1.4
After - no prep. 135 ± 22
• Pathogen reduction with pivot preparation
Pivot preparation Pivot preparation • Negligible stunting
observed in final crop
• Some drainage problems
• Not available to all growers
• Deep profile enabled this type of soil preparation
• Significant cost but disease control made up for expenditure
Beneficial microbes
Bacillus, Streptomyces, Trichoderma, Aspergillusbased formulations evaluated in field and pot experiments
• Variable response – growth of diseased plants never completely restored
Onion seedlings transplants• Seedling roots soaked in fungicide solutions or Trichoderma spore
suspensions • Transplanted into stunted patches• 4 weeks assessed for height, 16 weeks bulb width and weight
Seedling transplants
0
5
10
15
20
25
Control Amistar Rizolex Plantmate T. harzianum
Pla
nt w
eigh
t (g)
/ bu
lb w
idth
(cm
) Weight Width
Trichoderma based treatments
Biological treatments more effective than chemical treatments
Experimental seeder and chemical injection machineryExperimental seeder and chemical injection machinery
• Laboratory and glasshouse studies• Field evaluation
Fungicide evaluation
Fungicide response
• Response to soil fungicide application
• 10 weeks after sowing until plant maturity
• Up to 50% increase bulb wt per m seed row
Treated rows
Onion stunt can be managed by:• Soil working and pivot preparation• Timing of nurse crop sowing with regards
to early or late planting • Fumigation• Fungicide application
Management optionsManagement options
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