Harvest measures show potential to help control sclerotinia stem rot

Author: | Date: 01 Sep 2018

Key points

  • Western Australian trials show grinding sclerotia to a fine powder at harvest can reduce - but not eliminate - its ability to germinate the next year.
  • Fungicide protection is the only in-season control option for sclerotinia stem rot.
  • Narrow windrow burning trials found, when stubble height is about 15 centimetres, more than 50 per cent of sclerotia are captured in the windrow for burning.
  • Burn temperatures of more than 250°C for 10 seconds have been able to kill 100 per cent of sclerotia less than 3 millimetres in diameter in WA trials.

Regular rainfall experienced since winter 2018 in a number of WA cropping areas led to the development of canola crops with high yield potential in some areas, but conditions were also conducive to disease.

Paddocks at risk of sclerotinia, caused by the fungus Sclerotinia sclerotiorum, included those that had had the disease before. Other risk factors were dense canola crops that had received regular rainfall.

Sclerotinia has been shown to reduce canola yields when there is enough late winter and spring rainfall to cause prolonged periods of leaf wetness in the presence of flowering crops.

Disease lifecycle

Regardless of seasonal conditions, sclerotinia fungus survives in soil across years as dormant structures known as sclerotia.

Prolonged cool and moist conditions trigger germination, when tiny mushroom-like forms called apothecia are produced. These release spores that are then spread by wind to infect canola crops (typically via the flower petals), causing stem rot and plant lodging if the weather during this time is humid and wet.

Symptoms

MyCrop has a canola diagnostic tool to help identify sclerotinia stem rot. It can be found here.

Affected paddocks typically have bleached, or light brown, dead plants among green healthy plants after flowering.

Symptoms of sclerotinia on the canola plant stem, branches and pods appear as bleached greyish white, or brownish, patches (with white fluffy fungal growth in later stages). Advanced infections have sclerotia (hard, black irregular shaped and rounded bodies) on the inside and bleached parts of the stem.

In moist weather, sclerotia can also form on the outside of infected stems and roots. Canola pods can become infected and will appear cream-white and seeds can show mould.

Information about protecting crops from sclerotinia can be found on the Department of Primary Industries and Regional Development (DPIRD) website here.

Windrow burning and weed seed destruction tactics

To complement in-season fungicide use for sclerotinia, WA growers can use tactics such as windrow burning and weed seed destruction at harvest. These will help destroy sclerotia bodies before sowing a crop next year.

To test this, DPIRD researchers carried out laboratory experiments in 2017, with GRDC investment, simulating weed-destruction technology at four levels of grinding ‘coarseness’.
Ciara says some sclerotia germinated in the laboratory after grinding - regardless of coarseness - but germination was slower and levels significantly lower when using the finest grind of less than 0.5 millimetres.

In 2018, the research is investigating whether the quantity of spores released is reduced by grinding - as the apothecia from the ground sclerotia were notably smaller than those produced by intact sclerotia.

Researchers at the GRDC and Curtin University supported Centre for Crop and Disease Management (CCDM) have also assessed whether narrow windrow burning of canola stubble after harvest can reduce sclerotinia burdens for the subsequent canola crop.

Findings from work carried out by former honours student Kyran Brooks - and supervised by CCDM’s Dr Sarita Bennett and Dr Michael Ashworth (who is now with the Australian Herbicide Resistance Initiative) - were presented to the 2018 GRDC Grains Research Update, Perth. The full paper Factors affecting sclerotinia stem rot infections in canola can be found here (Session 8).

These results indicate narrow windrow burning could be a potentially useful non-fungicide tactic to reduce disease incidence and severity in subsequent years when infection is severe.

Some key 2017 trial results from this project included:

  • When harvested at a crop height of 15 centimetres, more than 50 per cent of sclerotia were captured in the windrow for burning
  • Burn temperatures of more than 250°C for 10 seconds killed 100 per cent of sclerotia less than 3 millimetres in diameter
  • Temperatures of 350°C and 400°C for 10 seconds killed sclerotia at 3 to 4mm and more than 4mm in diameter respectively
  • Temperatures of more than 500°C could be maintained in the centre of the windrow for more than eight minutes
  • A temperature of 350°C was consistently maintained at the edges of the windrow for eight minutes.

Kyran, who now works at Landmark, says temperatures in the windrow consistently exceeded the required temperature and duration to kill all sclerotia-size fractions tested.

But he says temperatures 1cm below the soil surface did not exceed 65ºC, which indicated buried sclerotia would survive a burning treatment. Further research is needed in this area.

CCDM research is also continuing to investigate the process by which sclerotinia infects canola crops in WA; potential genetic tools to increase resistance in current varieties; and genetic diversity and behaviour of Australian isolates of Sclerotinia sclerotiorum compared to the rest of the world.

An app, called SclerotiniaCM, is being developed by DPIRD to help growers and advisers manage sclerotinia. It is based on comprehensive fungicide trial data from across Australia over multiple years and is being tested nationally by researchers, growers and advisers during the 2018 season.

This app will aid decision-making about optimal management tactics, especially fungicide use, and is expected to be available for public release in 2019.

More information

Ciara Beard, DPIRD, 0438 468484, 08 9956 8504, ciara.beard@dpird.wa.gov.au
Dr Sarita Bennett, CCDM, 0457 898199, 08 9266 2740, sarita.bennett@curtin.edu.au

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