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Forecasting Sclerotinia disease

Controlling Sclerotinia disease is difficult and control relies largely on the application of fungicides such as azoxystrobin and boscalid + pyraclostrobin which are targetted at killing the airborne ascospores. However, timing these applications for maximum efficacy is problematic and sometimes unnecessary sprays may be applied. A collaborative Defra-funded project with ADAS sought to address this problem by developing a disease forecasting system for S. sclerotiorum based on the response of the pathogen to environmental conditions.

Apothecia of S. sclerotiorum releasing ascospores Lettuce with Sclerotinia disease

S. sclerotiorum sclerotia have two different sets of requirements in order for germination to take place. Firstly, when sclerotia are formed on infected plants and incorporated into the soil they generally require a cold conditioning treatment before they can overcome a period of dormancy and secondly, germination only then proceeds when the soil is moist and the temperature is in the correct range. Experiments under controlled conditions have shown that S. sclerotiorum sclerotia require chilling periods before they can germinate rapidly and that subsequent germination can occur at 5-25°C with an optimum of 15-18°C in moist soil. Typically, appropriately cold-conditioned sclerotia will begin to produce apothecia after two weeks at 15°C but this process is slowed at 5 and 10°C. This response corresponds well with behaviour in the field where sclerotia formed on crops in summer or autumn, fulfil their chilling requirement over winter, with apothecia emerging in spring and early summer as soil temperature increases. The environmental requirements for germination of S. sclerotiorum sclerotia were incorporated into a model published in Phytopathology 97, 621-631 (2007). The model was further devloped with ADAS in HDC project FV394 to time fungicide sprays in lettuce using hourly inputs of rainfall and soil temperature and has now also been tested for use in oilseed rape in LINK project LK09130 (HGCA) led by ADAS with multiple research and industry partners.

Research has also shown that S. sclerotiorum ascospores require certain environmental conditions in order to germinate and infect plants. Wetness or high humidity (>97% RH) is required and at 15-25°C and 100% RH, germination can start in just a few hours with plants beginning to show disease symptoms one week later. However, reducing the temperature below 10°C slows down this process. In experiments using lettuce plants, infection was also seen in the absence of water and at humidities as low as 50-60% although disease appeared and progressed far less rapidly. This suggests that there are some areas on the lettuce plant where high humidities may still be maintained and ascospore germination can still occur, such as the stem base and this is where Sclerotinia disease nearly always begins for lettuce. This work has been published in Plant Pathology (2004) and PLOS ONE (2014).