Rhizoctonia Onion Stunt

Rhizoctonia Onion Stunt
Factors affecting severity
and management options
Simon Anstis, Trevor Wicks & Sue Pederick
South Australian Research & Development Institute

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 windblowing
• 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
AG 8 Con
AG 8

Soil moisture and stunt severity
Increasing soil moisture increases plant growth in the presence of
300
Rhizoctonia solani AG8
250
Dry Wt/pot (mg)
200
150
100
50
Lsd (0.05) 39.4
%WHC
25
50
75
0
0 1 3
Rhizoctonia propagules/pot
Onion seedling growth in 5 soil types at different
moisture

Onion Stunt across different soils
• Over
different soils
proportion of
coarse sand
generally
related to
stunt severity
500
400
300
• Other
parameters
variable pH,
mineral
nutrients
200
100
0
Dry Wt (mg/pot)
Coarse sand fraction
69.0%
86.5%
49.6%
72.4%
Caloote Kea Keb Tailem Bend
No pathogen
R. solani

Soil Fumigants
• Both Metham &
TeloneC35 reduce
soil inoculum

Onion metham fumigation response
Fumigated strip through non-fumigated plot

Onion crop 2 seasons after fumigation
Non-stunted areas:
45 ± 21 pg AG 8 DNA/g soil
• Aerial photograph
onions 2 years
after metham
• Patches of onion
stunt
Patches:
448 ± 58 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
• 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, Aspergillus
based 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
Biological treatments more effective than chemical treatments
Plant weight (g) / bulb width (cm)
25
20
15
10
5
0
Weight
Width
Control Amistar Rizolex Plantmate T. harzianum
Trichoderma based
treatments

Fungicide evaluation
• Laboratory and glasshouse studies
• Field evaluation
Experimental seeder and chemical injection machinery

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

Management options
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