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