Paddock Practices: First common root rot yield loss trials in 20 years on the Darling Downs

Author: | Date: 15 Feb 2018

Why and what growers need to know about CRR

Despite its name the disease common root rot (CRR) is often overshadowed by its better-known cousin crown rot, yet both present significant risks for grain growers in Queensland and New South Wales.

Now for the first time in two decades, a research investment by the Grains Research and Development Corporation (GRDC) and the University of Southern Queensland (USQ) is investigating the potential yield loss impact of varying CRR levels in the paddock.

Wheat and barley varieties are routinely assessed for resistance to common root rot as part of NVT and germplasm development programs. However the yield loss project located at Wellcamp on Queensland’s Darling Downs is investigating losses in current farming systems.

Person kneeling in paddock
USQ Crop Pathologist Dr Cassandra Percy is involved in the first CRR yield loss trials in two decades and says due to the indistinct nature of symptoms the disease frequently goes undiagnosed in the paddocks.

Caused by the fungus Bipolaris sorokiniana, CRR has the potential to cause chronic yield loss in wheat and barley.

However, USQ Crop Pathologist Dr Cassandra Percy says due to the indistinct nature of symptoms CRR frequently goes undiagnosed when causing lower levels of yield loss in paddocks.

“CRR is definitely the lesser known root rot, but it occurs in most soil types worldwide and throughout Australia’s wheat growing regions,” Dr Percy says.

“Yield losses of up to 40 per cent have been reported in bread wheat and average long-term losses are estimated to be about 10 per cent annually.

“Disease severity is often higher in barley, but yield loss is generally lower than in bread wheat.”

Testing of stubbles/plant residue and soil before seeding - using the DNA-based PREDICTA® B service that has been developed with GRDC investment - is the best way to assess the risk of CRR. Results can help to determine management strategies for future seasons.

“Damage is more likely in a situation where cereal crops are grown consecutively,” Dr Percy says.

“Yield losses are usually more significant when there are low soil moisture levels during grain-fill, but can also occur in wetter years.”

She says CRR losses were also exacerbated when crops were infected by other root and crown pathogens, such as crown rot.

Symptoms of common root rot

Dr Percy says symptoms of CRR include:

  • Crops may lack vigour.
  • Reduced tillering, pale colour, stunting and smaller heads. These symptoms can appear from tillering onwards, but usually are most obvious after flowering.
  • Dark brown to black lesions on subcrowns will be evident and seminal roots, and sometimes crown roots, will be pale to mid-brown.
  • The disease may affect a single plant or areas scattered through the paddock.

“Plants can become infected at any growth stage, but the earlier the infection the more severe the damage,” she says

CRR can cause significant losses in wheat and barley with yield losses usually more significant when there are low soil moisture levels during grain-fill.

“We have found temperatures greater than 20-30 degrees Celsius favour CRR, but the disease can develop at cooler temperatures, particularly where there is moisture stress. 

“The fungus has been known to survive in host debris for at least two years, and is unlikely to survive outside the host tissue for extensive periods.” 

Hosts include durum and bread wheat, barley, oats, triticale and rye. A wide range of non-cereal grasses are also thought to act as hosts.

Why test for common root rot pre-planting

  • To rank paddocks it is important to know inoculum levels before sowing to determine the potential risk of CRR to the next season’s crop. Testing can also be used to compare inoculum load in paddocks with different cropping sequences. Samples can also be collected in-crop to confirm diagnosis of CRR, as symptoms can be visually similar to crown rot and take-all.

Testing soil samples (with added stubble) using PREDICTA® B in the lead-up to seeding also provides information on the levels of a broad range of other pathogens. A modified version of PREDICTA B, distributed by Crown Analytical Services, has been developed for the northern summer rainfall cropping region. It includes tests and disease risk categories for the Pratylenchus thornei and crown rot.

PREDICTA B north also tests for a broad range of pathogens that don’t yet have disease risk categories developed; however, to provide a guide to growers the results are ranked against the rest of industry using categories based on population density.

These new tests are for cereal pathogens that cause common root rot (disease risk categories are close to being developed), rhizoctonia, pythium root rot, yellow leaf spot and white grain disorder.

New tests have also been added for ascochyta blight of chickpea and phytophthora root rot of chickpea (latter most useful for confirming disease in-crop). Also included are several tests for diseases of summer crops including charcoal rot and fusarium stalk rot of sorghum and long fallow disorder (caused by low levels of mycorrhiza).

How to test for common root rot

The PREDICTA® B service is provided by the South Australian Research and Development Institute (SARDI), a division of Primary Industries and Regions South Australia (PIRSA).

The PREDICTA® B laboratory is closed for an extensive renovation until early February, but stubble and soil samples can be submitted before this and will be stored until processing recommences.

PREDICTA® B north is delivered to growers via accredited consultants. Dr Alan McKay says growers can use PREDICTA B north soil kits to send samples direct to SARDI, but they should consult with an accredited agronomist first to ensure the correct sampling strategy is used. The reports will be emailed to the consultant who will interpret the results and consult with the grower to develop a cropping plan for the paddock.

In northern NSW and Queensland growers can contact Crown Analytical Services at crownanalytical@bigpond.com or 0437 996678 to get a list of nearby accredited PREDICTA® B consultants.

Management and planning

Limited research has indicated the highest CRR populations occur after a susceptible barley crop.

Dr Percy says management strategies that reduced inoculum levels included:

  • Reducing the frequency of cereals in rotations and planting non-host pulses, oilseeds and pasture species.
  • Ensuring break crops or pastures are grass free.
  • Selecting partially resistant varieties of wheat, barley and durum.
Dr Percy says management strategies to reduce CRR include reducing the frequency of cereals in rotations and planting non-host pulses, oilseeds and pasture species.

To minimise the impact CRR had on potential yield growers should:

  1. Avoid sowing susceptible varieties into paddocks with high pre-planting inoculum levels.
  2. Be aware disease severity is generally higher in barley, but yield impact is lower than wheat.
  3. Plant resistant varieties of durum, bread wheat and barley.
  4. Ensure soils have adequate moisture and fertility (particularly nitrogen and phosphorous).
  5. If moisture permits, reduce sowing depth to limit the length of the subcrown internode.

“The severity of common root rot in wheat and barley is associated with the population of B. sorokiniana in the soil at the time of planting, which is why it is invaluable to do a PREDICTA® B test,” Dr Percy says.

“Disease risk levels for CRR have yet to be developed, so on-farm management decisions need to be based on current industry best practice.” 

She says confirming the disease diagnosis allowed growers to make informed management decisions.

Contact

Cassandra Percy, USQ,
Cassy.Percy@usq.edu.au

Dr Alan McKay, SARDI,
08 8303 9375,
alan.mckay@sa.gov.au

Useful Resources

PreDicta B® testing service on the Primary Industries and Regions SA website.

GRDC-DPIRD Crop Variety Sowing Guides, northern region 2018.

GRDC GrowNotes™ Wheat, Barley, Canola and Oat.