Sclerotinia control in oilseed rape: Progress with quantitative diagnosis and development of a web-based forecasting scheme

Abstract

The aims of this four-year project were to improve the effectiveness of strategies for control of sclerotinia stem rot of oilseed rape by (a) development of a rapid diagnostic test for identification of crops at risk of infection to allow optimisation of fungicide requirement and timing (b) obtaining fundamental information on the relationship between sclerotinia inoculum, fungicide timing and final disease levels and yield and (c) development of a decision guide to assist with assessment of disease risk.

A number of monoclonal antibodies with potential for incorporation in rapid immunodiagnostic assay formats have been produced which are highly specific and sensitive to Sclerotinia sclerotiorum and appear to be significantly in advance of those produced in other countries.  As yet, the assay formats tested have not had the qualities of ease of use and accuracy required for a laboratory service or in-field methodology to be developed.  The report identifies strategies for overcoming or avoiding problems such as the inhibition of the test caused by the presence of oilseed rape petal extracts in test samples through further development of the current protocols.

Spatial analysis of sclerotinia infected crops showed that the disease can occur in gradients across a field indicating an external inoculum source, or in 'hot spots' within the crop indicating a source of infection within the field.  Data clearly emphasise the requirement for sampling at several points in each field for reliable determination of disease risk using agar or immunodiagnostic petal tests.  A sampling strategy involving taking samples on a 100 m grid pattern was able to detect the spatial variability present within the fields monitored.  This is equivalent to approximately 15 samples for a 10 ha field.  Further work is needed to confirm minimal, but statistically reliable, sampling strategies.  Accurate prediction of inoculuum and disease within a field may allow spraying to be targeted only in areas at risk.  Field experiments showed that fungicide treatments were very effective (95% control) for sclerotinia control but timing of applications was critical.  In 1999 and 2000, the optimum timing for sprays to control sclerotinia was between late April and early May.  However, the timing of infection was related to weather conditions favouring petal sticking and disease development and this was the critical factor determining the optimum growth stage within the flowering period for treatment.  A difference of only 10 days between treatments had a large effect on the level of disease control and this clearly identifies the value of a rapid test to guide spray applications.

A range of risk factors for sclerotinia stem rot have been investigated and together with data gathered during the project, have been incorporated into an web-based Decision Guide for determination of sclerotinia disease risk.  These factors include previous history of disease, weather and secondary crop factors.  The Guide also has options for inputting results of petal testing for estimation of inoculum potential.  The Decision Guide calculates a numeric value for each set of data entered and assigns a value for risk of sclerotinia, which is displayed as Low, Medium or High.  The Decision Guide is available and now requires validation on farms.