Forecasting of Sclerotinia stem rot in winter oilseed rape

Sclerotinia stem rot (SSR) on oilseed rape (OSR) Brassica napus caused by Sclerotinia sclerotiorum (Lib.) has a growing importance worldwide as a consequence of the increased intensity in cultivation of this oil crop in the last decades. At present, control of SSR in winter oilseed rape predominantly relies on one fungicide spray at the time of flowering addressing the infection of stems by ascospores. As resistant varieties are not available, prophylactic fungicide sprays at full bloom have become a widely applied control practice. However, earlier analyses of a large number of field experiments conducted from 1981 to 1991 and from 1992 to 2004 have shown only 27 and 33 % of fungicide sprays at bloom to be cost-effective (Dunker & Tiedemann, 2004; Dunker, 2006; Wahmhoff, 2000).

This unsatisfactory situation has given rise to development of various approaches of decision support systems for this disease. Previous attempts of simple prediction systems have been based on field-site specific factors and the weather conditions during flowering. Such approaches have been extended by recording the emergence of apothecia and release of ascospores by means of sclerotia buried in depots in the field. However, attempts to predict the carpogenic germination of sclerotia based on air temperature and the soil water potential have been difficult and so far unsuccessful in being used for disease forecasting systems of SSR. Further approaches to forecast SSR disease incidence were based on a petal infestation test.

None of the previous efforts for implementing decision support tools for control of SSR have achieved a wider practical application. The main reasons for that are lacking reliability, requirement of too many input data to be manually provided by the user, a lack of an economic model predicting the monetary damage and a lack of infrastructure to deliver forecasting systems to the agricultural practice.

We therefore have developed a crop-loss related forecasting model for SSR in areas with typical OSR cultivation and hence abundant inoculum, enabling a field-site specific recommendation on fungicide sprays at flowering (Koch et al. 2007). This novel forecasting model, SkleroPro, provides an accurate and easy-to-handle, fully computerized decision support system based on weather and field-site specific data, and is accessible to growers and advisors since 2006 through the internet at www.isip.de.

SkleroPro provides decision support for the fungicide spray against SSR at bloom. Four weather variables, air temperature, relative humidity, rainfall and sunshine duration, are used to calculate the number of hours conducive for stem infection (Inh). SkleroPro consists of a two-tiered approach, the first providing a regional assessment of the disease risk, which is assumed when 23 Inh have accumulated after the crop has passed GS 58. The second tier provides a field-site specific, economy-based recommendation. Based on costs of spray, expected yield and price of rapeseed, the number of Inh corresponding to DI at the economic damage threshold (Inhi) is calculated. A decision to spray is proposed when Inh > Inhi. Crop rotation is included in the model as a field-site specific risk factor. The accuracy of SkleroPro has been evaluated with data from historical field experiments (1994 to 2004) and with field experiments specifically conducted in 2005 and 2006. In this evaluation, the predictive accuracy varied between 70 and 81%. Compared to the common practice of routine sprays this corresponded to savings in fungicides of 45 and 75%, and to increases in net return for the grower of 23 and 45 € per hectare, respectively. SkleroPro demonstrates that, particularly in areas with abundant inoculum, the severity of SSR can be predicted from conditions of stem infection during late bud/flowering with sufficient accuracy, and does not require the simulation of apothecial development and ascospore release. SkleroPro is the first crop-loss related forecasting model for a sclerotinia disease and is now widely used in the agricultural practice in Germany.

Selected Publications:

• Dunker, S. and Tiedemann A. v. (2004). Disease yield loss analysis for Sclerotinia stem rot in winter oilseed rape. IOBC Vol. 27 (19), 59-65.
  
• Dunker, S. (2006). Untersuchungen zur Schadwirkung von Sclerotinia sclerotiorum (Lib. de Bary) und Verticillium longisporum (comb. nov. Karapapa) in Winterraps (Brassica napus). Dissertation, University of Göttingen,
  
• Koch, S. (2006). Entwicklung eines schlagspezifischen und schadensbezogenen Prognosemodells zur Bekämpfung von Sclerotinia sclerotiorum an Winterraps. Dissertation, University of Göttingen,
  
• Koch, S., S. Dunker, B. Kleinhenz, M. Röhrig, A. v. Tiedemann (2007). A crop loss-related forecasting model for control of Sclerotinia stem rot in winter oilseed rape. Phytopathology 97:1184-1196.
  
• Wahmhoff, W. (2000). Integrierter Rapsanbau: Untersuchungen zur Entwicklung integrierter Produktionsverfahren am Beispiel des Winterrapses (Brassica napus L.). Erich Schmidt Verlag, Berlin, Germany.