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Forecasting Sclerotinia Disease on Lettuce: A Predictive Model for Carpogenic Germination of Sclerotinia sclerotiorum Sclerotia

First, second, and third authors: Warwick HRI, University of Warwick, Wellesbourne, CV35 9EF, UK; fourth author: ADAS UK Ltd., Defra Drayton, Alcester Road, Stratford on Avon, Warwickshire CV37 9RQ, UK; fifth author: ADAS UK Ltd., ADAS Wolverhampton, Woodthorne, Wergs Road, WV6 8TQ, UK; and sixth author: ADAS UK Ltd., ADAS Terrington, Terrington St. Clement, King's Lynn, Norfolk PE34 4PW, UK

A predictive model for production of apothecia by carpogenic germination of sclerotia is presented for Sclerotinia sclerotiorum. The model is based on the assumption that a conditioning phase must be completed before a subsequent germination phase can occur. Experiments involving transfer of sclerotia from one temperature regime to another allowed temperature-dependent rates to be derived for conditioning and germination for two S. sclerotiorum isolates. Although the response of each isolate to temperature was slightly different, sclerotia were fully conditioned after 2 to 6 days at 5°C in soil but took up to 80 days at 15°C. Subsequent germination took more than 200 days at 5°C and 33 to 52 days at 20°C. Upper temperature thresholds for conditioning and germination were 20 and 25°C, respectively. A predictive model for production of apothecia derived from these data was successful in simulating the germination of multiple burials of sclerotia in the field when a soil water potential threshold of between −4.0 and −12.25 kilopascals (kPa) was imposed. The use of a germination model as part of a disease forecasting system for Sclerotinia disease in lettuce is discussed.