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Sclerotinia risk assessment

Course: Oilseed rape diseases | Last Updates: 22nd June 2016

 
Dr. John Clarkson
Plant Pathologist
Warwick Horticultural Research Institute
Biography >>
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Disease is most likely to occur if spore release coincides with the oilseed rape crops' flowering and if the weather is conducive to spores germinating and infecting plants.

Many factors contribute to the risk level faced by individual fields, which growers can use to assess the potential threat this season and to optimise timing of a one- or two-spray fungicide strategy to protect crops.

osr in flower
Conditions during flowering play a big role in potential for crop damage.

Rotation

The more frequently oilseed rape is grown in the rotation, the more likely sclerotia will build up in the soil. Sclerotia spores are believed to remain viable for at least five years, with some possibly surviving longer. The highest risk will be for the first seasonn after oilseed rape and then potentially decline over subsequent years as a proportion of sclerotia germinate each year and the viability of remaining sclerotia decreases.

Other crops in the rotation can also host sclerotinia and increase numbers of sclerotia returning to the soil from disease outbreaks, including peas, beans, lettuce, potatoes and carrots. Many dicot weeds can also host the disease.

Establishment technique

Generally, sclerotia only germinate and produce spore-producing apothecia in the top 5cm of the soil profile. Ploughing down sclerotia could make a significant difference in reducing future spore production, providing they remain buried for a number of years until they lose viability. But ploughing every year will return a proportion of the sclerotia back towards the surface, from where they can germinate when conditions are right.

Broadcast establishment of oilseed rape during harvesting the preceding cereal crop, with no soil movement involved, could reduce germination.

Field location

In practice, most sclerotinia spores travel less than 50m. Fields that have a historical record of sclerotinia infection, or a recent crop of oilseed rape, clearly have the highest risk. Fields on the prevailing downwind of previously infected crops will also be at higher risk.

In dense oilseed rape crops with limited air movement at the soil surface, most spores will be intercepted by plants around the sclerotia source, leading to typical pockets of high localised infection. Spores produced on bare soil or in other short crops in infected around the farm may spread further.

Thin and more open oilseed rape crop canopies could allow increased air movement, which will enable spores to spread more easily and over a wider area.

Weather

sclerotinia apothecia
These sclerotinia apothecia survive longer in wet soils and cool temperatures

For sclerotinia, temperature and moisture are the most important factors for spore production. The cold weather required to "condition" the sclerotia for germination usually occurs during winter, but the requirement does generally prevent the disease going through more than one cycle a year and causing secondary infection.

Spore-producing apothecia survive longer when the soil is wetter and in cool temperatures. On free-draining soils rain may only trigger a short period of spore release, but on wet heavy soils sufficient moisture may remain to initiate continued spore release over several days. While previous thoughts were that apothecia produced puffs of spores under certain conditions, it is now recognised that spores are also released in a continuous stream.

Moisture is also necessary for spore germination and growth. Research has shown at lower levels of relative humidity the level of infection will be significantly reduced, but can still occur at a slower rate. Leaf transpiration can create a microclimate of humidity on the leaf surface and many mornings through the flowering period will experience a heavy dew, which is sufficient to allow disease development within a dense crop canopy.

Flowering

Sclerotinia spores land and stick to flower petals which, when they fall and land on leaves, create the food source for infection and disease development. Spore release at any stage during the flowering period can lead to new infection.

The duration of flowering can be affected by the variety and the growing season. Backward or pigeon damaged crops tend to have a more protracted flowering period as different plants and developing side branches come into flower at varying times through the season.

Variety

Research has shown there is no direct varietal resistance to the sclerotinia pathogen. If spores are present and the conditions are conducive to disease, then plants will be attacked.

But varieties do have different physical attributes and crop architecture that can alter conditions and influence the severity of any attack. Varieties with a naturally long flowering period, for example, will be exposed to potential infection for longer. Varieties that create lots of branches will have more axial points to trap falling petals, which may cause greater infection.

Fungicide strategy

stems infected with sclerotinia
Infected sclerotinia stems are at risk from lodging.

In seasons where establishment and over-wintering conditions have led to variable oilseed rape crops, growers will need to make continuous risk assessments for individual fields, balancing all the factors involved to calculate the optimum spray timing.

In a uniform crop at low to medium risk of infection, one fungicide application when most of the field is at the early to mid-flowering stage could provide sufficient protection on the petals and leaves. But in more variable crops, where forward plants could be through flowering before others are only just beginning, timing of application will prove more difficult.

Depending on the level of risk assessed, some crops could be more effectively protected by a two-spray programme. An initial application as most of the field comes into flower will protect the most forward plants, with a second application a few weeks later when most of the field is at full flower will boost protection on later-flowering plants.

Long-term strategy

If sclerotinia attacks continue at the level encountered in recent seasons, growers are going to have to adopt more effective strategies to minimise yield losses and to stop the disease increasing further. Measures to adopt include:

  • Extend oilseed rape rotations
  • Adjust cultivations
  • Change cropping patterns
  • Consider use of biological controls
  • Keep sclerotia returns to minimum through better control
  • Use predictive services to better time spray applications

Sclerotinia risk factors at a glance

  • Rotation
  • Establishment technique
  • Field location
  • Weather
  • Flowering
  • Crop architecture

Worst case scenario

Field location
  • Fields that contained an infected crop within past three years
  • Fields downwind of previous heavily infected crops
  • Thin crop canopies
Weather
  • Frequent rain showers during crop flowering
  • Heavy dew leaving wet leaf surfaces
  • Windy conditions blowing spores
Flowering
  • Crops with long flowering period
  • Crops flowering during periods of spore release
  • Uneven flowering within the crop
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