Rhizoctonia - New treatments and products

A. McKay¹, P. Bogacki¹, V. Gupta², J. Desbiolles³, R. Correll⁴, D. Roget⁵, D. Huberli⁶ and W. MacLeod⁶

¹SARDI, Urrbrae, SA 5064; ²CSIRO Ecosystem Sciences, Urrbrae, SA 5064; ³University of South Australia, Mawson Lakes; ⁴ Rho Environmetrics Pty Ltd, Highgate SA; ⁵David Roget Consulting Pty Ltd, Clapham SA; ⁶ DAFWA South Perth WA;

GRDC project codes : DAS00122, DAS00123, DAS00125, CSE00150

Keywords: Rhizoctonia

Take home messages

• New fungicides, Vibrance^® and EverGol^® Prime have been registered as seed treatments to suppress Rhizoctonia
• In SARDI/DAFWA field trials Vibrance® in wheat and EverGol® Prime in barley, produced yield responses averaging 5% higher than control.  Cereals and grasses are the main hosts for Rhizoctonia; inoculum increases most during spring.
• Non-cereals, especially canola, provide useful reduction in Rhizoctonia levels.
• Controlling summer weeds reduces risk from Rhizoctonia
• Rhizoctonia inoculum is reduced by frequent summer rainfall when summer weeds are controlled
• Early sowing and soil disturbance below seed encourages rapid root growth down soil profile
• Autumn “green bridge” needs to be controlled to prevent inoculum build-up
• N deficiency should be minimised; deep band N and leave stubble on soil surface
• Risk of Rhizoctonia in 2013 is potentially high:
  • following the dry 2012 spring
  • and continuation of dry 2012/13 summer/autumn conditions
  • risk will be further increased if season breaks late and soils are cold

Introduction

Vibrance® and EverGol® Prime, new succinate dehydrogenase inhibitor (SDHI) fungicides have been registered in 2013 for suppression of Rhizoctonia.  Both have been evaluated in field trials conducted by SARDI and DAFWA (Table 1).  (See Cereal Seed Treatments 2013 Factsheet for activity on other fungi.)

Rhizoctonia bare patch caused by Rhizoctonia solani Kühn AG-8 is a difficult disease to manage, but research projects in SA and WA funded by GRDC and SAGIT are making significant progress towards improving management options. 

Rhizoctonia occurs mainly in the low to medium rainfall regions of southern Australia, particularly in sandy soils, though its distribution extends into higher rainfall districts.  Annual losses are estimated to be around $59 million in wheat and barley (Murray and Brennan, 2009).

Rhizoctonia is generally considered to be a pathogen of seedlings and not greatly affected by rotation: however results from the current GRDC funded projects show it can attack crops throughout the growing season and is affected by rotation.  Cereals and grasses are the main hosts.    

The current research program aims to improve disease prediction, reliability of existing control strategies and support development of new methods of control including banding fungicides and where appropriate assist with label extensions. 

Results and Discussion

Vibrance® and EverGol® Prime have been evaluated as seed treatments to suppress Rhizoctonia in field trials conducted by SARDI and DAFWA in 2011 and 2012.

Vibrance® at 360 mL/100kg seed increased grain yield by an average 80kg /ha or 5.7% (range 0 to 17.7%) across 8 wheat trials.  The responses were significantly greater than the untreated at 2 sites.  In barley, the yield increases averaged 0.4% (range -4% to 6%) across 5 barley trials (Table 1).

EverGol® Prime at 80 mL/100 kg seed was evaluated only in barley; it increased yield by an average 120 kg/ha or 5.4% (range 4% to 7.2%) across 4 barley trials. The increase was significantly greater than the untreated in the trial at Minnipa in 2011(Table 1).

Table 1. Summary of cereal yield responses for Vibrance and EverGol Prime seed treatments in efficacy trials conducted by DAFWA and SARDI

* Significant (P < 0.05) or ** Significant (P < 0.001), compared to untreated plots

Rhizoctonia distribution in soil and multiplication

Rhizoctonia exists as a hyphal network, mostly in the top 5 cm of soil, but can extend down to 10cm during the growing season.  This fungus is adapted to dry conditions.

Rhizoctonia can attack a wide range of plants and grow on plant residues however the main hosts are cereals and grasses.  Inoculum levels increase during the growing season, especially during spring and reach maximum levels as the crop dries off. 

Rhizoctonia levels are reduced by: 

• Repeated significant summer rainfall events that result in the soil remaining moist, irrespective of rotation.  
• Weed control which prolongs moist soil conditions that cause inoculum to decline; summer/autumn weeds may also be hosts and increase inoculum. 

Rhizoctonia levels can recover following significant summer rainfall if there is a prolonged dry period, >4 weeks.  As the soil dries out Rhizoctonia can regain its competitive advantage over other soil biota and grow saprophytically on plant residues.

Inoculum levels remain high during summer if there is low infrequent summer/autumn rainfall.

Seeders soil openers

The type of soil opener used affects levels of Rhizoctonia during seedling establishment.  In 2010 inoculum levels along rows sown with knife points (high soil disturbance to 10cm) were much lower than those sown with a triple disc system with flat discs (minimum soil disturbance to 6cm).  Levels in rippled coulter sown plots with disturbance to 10cm were intermediate.  High Rhizoctonia levels in the flat disc sown plots helps explain why Rhizoctonia is more serious with disc sown crops.

Impact of rotations

Grass free canola, mustards, field peas, chickpea, medic pastures and fallow provide useful reductions in inoculum which can benefit the following wheat crop, increasing yields between 9% and 47%.  However the reduction in inoculum lasts for only one season as Rhizoctonia builds up on the cereal roots during spring.  The impact of this can be seen by examining the crown roots for spear tips.

Impact on crop growth

Rhizoctonia can attack crop roots throughout the growing season.  Damage is greatest when soil temperatures drop to around 10°C and or root growth down the profile is restricted.  Severe damage to seedling roots results in the characteristic bare patches.  In many early sown crops, root damage is delayed until around tillering when soil temperature drops to around 10°C.  This slows root growth and the fungus can attack seminal and crown roots causing uneven crop growth.

Seedling damage can be reduced by sowing early and reducing constraints to root growth e.g. nutrient deficiencies (especially N), compaction layers, etc.  Ensuring adequate N and micro nutrients during tillering will help reduce losses from this damage.   

Damage to the crown roots is much harder to stop, but increasing seeding density can reduce impact of lost tillers. 

Loss of crown roots during spring may cause lodging in some crops.

Identifying high risk paddocks

If sowing back to cereals, spear tipping on the crown roots of the previous plants is a good way to identify paddocks at risk that don’t display the classic bare patches.

The current PreDicta B risk categories are also a good guide* to disease risk in most districts and can identify other pathogens that don’t cause distinctive root symptoms.

(* note that risk categories need to be revised for the western slopes of NSW and some areas around Morawa WA and Pingelly WA).   

Disease severity depends on a combination of factors including Rhizoctonia level, soil disturbance and N levels at seeding, constraints to root growth (e.g. compaction layers, low temperatures, soil moisture etc) and activity of the soil biology community. 

Future work

Ongoing research will:

• further our understanding of the role of summer weeds and rotation crops
• evaluate banding fungicides
• improve disease prediction

Further reading:

GRDC Factsheet March 2012

Cereal Seed Treatments 2013 Factsheet by Hugh Wallwork available from: www.pir.sa.gov.au/factsheets

Contact details

Alan McKay
SARDI Plant & Soil Health, Adelaide
(08) 8303 9375
alan.mckay@sa.gov.au