Under UK conditions, temperatures are usually not very favourable for sclerotinia epidemics. Soil temperatures are often too low (<10ºC) during early flowering for active germination and apothecia are produced in small numbers over a long period. (Dry soil conditions may also inhibit apothecial production in some years). Peak numbers of apothecia occur towards the end of flowering. Rapid germination of sclerotia, well synchronised with flowering, is typical of high risk areas in Europe (e.g. Sweden , Germany) where sclerotinia causes serious problems in oilseed rape. Details of apothecial production are available on a regional basis for the current year on www.agricentre.co.uk
Sclerotinia has a relatively high temperature optimum of 15-20ºC for infection, but it is capable of infecting plants at much lower temperatures. For a change to high risk conditions this sesson, look for above average temperatures combined with showery conditions during flowering.