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Appearance, impact and biology

The fungus survives in soil as sclerotia; dark, compact structures that can remain dormant in soil for years but which normally germinate in the spring of the year after production. They can germinate to produce mycelium that can infect plant roots directly but usually they form fruiting bodies 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. This 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. Fungicide sprays may be needed in high risk areas to give complete protection (see control options described later).

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.

Control Options

Information on fungicide efficacy is available from the AHDB in Fungicide resistance management in OSR and Fungicide performance in OSR.

Generally, most available fungicides give some protection if applied at the correct time. The AHDB website does show that some may be better than other fungicides – currently a mix of bixafen and prothioconazole (Aviator Xpro), or a mixture of dimoxystrobin and boscalid (Pictor) are reported to be the most effective, although straight azoles such as prothioconazole (proline) or propiconazole (shepherd) alone are also effective. 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 to four weeks but some products may provide limited activity for longer. This activity is based on the fungicide protecting leaves and stems onto which new infected petals may fall. However, at high-risk locations, a second application towards the end of flowering may be necessary to protect the whole of the flowering period.

For weekly information on disease inoculum monitoring and risk alerts see: Sclerotinia infection risk alerts (forecast) for oilseed rape | AHDB - which has both a weather-based risk indication and an airborne spore-based risk indication. This is only updated during the oilseed rape flowering period (April-May) as there is little infection risk before or after flowering and the website provides an indication of Sclerotinia risk at several monitoring locations over Great Britain, using petal testing and/or air sampling and weather-based risk models.

Risk can also be assessed locally based on past history of sclerotinia, conducive weather (above 7°C, and humid or with frequent rain showers), and observation of fruiting bodies. Risk may be reduced by extending frequency of susceptible crops in the rotation (in addition to oilseed rape, carrots, beans and potatoes can become infected).