Appearance, impact and biology
The fungus, Sclerotinia sclerotiorum, survives in soil as sclerotia (shown in the photo below). These are dark, compact structures that can remain dormant for years but which normally germinate in the spring of the year after production.
The sclerotia can germinate and produce mycelia that infect plant roots directly but they usually form fruiting bodies (brown fungus shown in the next photo below) following a period of conditioning in the soil. For UK populations this normally comprises several weeks of moist cool conditions, followed by a few weeks of moist warm conditions. Conditioning often causes the fruiting bodies to coincide with flowering of oilseed rape. The fruiting bodies release wind-dispersed spores and they are encouraged by regular rain events interspersed by dry periods. Prolonged dry weather in spring prevents fruiting body production, unless the soil remains moist. However, dry weather may synchronise a large spore release event about 3-5 days after the drought is broken.
Airborne spores are deposited directly on leaves, stems and petals. Falling petals then may stick to wet lower leaves or branches and provide an undefended food source that allows the fungus to infect the healthy leaf and stem tissues of the plant. Infection is particularly encouraged if the temperature remains over 7°C and relative humidity in the crop is high. Strong rainfall can wash spores and petals off leaves to reduce infection risk. Senescing leaves may also provide an infection site.
Two to several weeks after infection, stems die and turn white around the infection point. New sclerotia are formed on and inside stems and are scattered into the soil at harvest.
Yield loss depends on how early the stem infection occurs and whether primarily on the main stem or side branches. Typically, yield losses are around 3% for every 10% of plants affected.
The AHDB provides a barometer of Sclerotinia risk at monitoring sites, using petal testing, air sampling and weather-based risk models. For weekly information on disease inoculum monitoring and risk alerts see http://cereals.ahdb.org.uk/monitoring/sclerotinia/sclerotinia-risk-report.aspx
Risk can also be assessed based on past history of sclerotinia, conducive weather (above 7°C, and humid or with frequent rain showers), and known observation of fruiting bodies locally. Risk may be reduced by lowering frequency of susceptible crops in the rotation (in addition to oilseed rape, carrots, beans and potatoes can become infected).
Fungicide timing is vitally important for good control, as products are protectants and have little or no curative activity. Generally most available sclerotinia fungicides (Amistar, Compass, Filan, Galileo, Proline, Prosaro, Tectura, Topsin) give good yield responses in infected areas if applied at the correct time. Information on fungicide efficacy is available from the AHDB: http://cereals.ahdb.org.uk/media/419394/3457-sclerotinia-summary-report-april-2014.pdf
For a single spray application, the optimal time is around GS 4.3 – roughly when a third of the way into flowering, based on flower production on the main stem. Fungicides are thought to provide protection for three weeks but some products may provide limited activity for longer. At high-risk locations, two applications may be necessary. Strategies are required to minimise the risks of selecting fungicide-resistant strains of sclerotinia and other pathogens. Do not use sclerotinia products containing single active ingredients more than once on their own. Use mixtures, co-formulations or products with a different mode of action.