Abstract: Sclerotinia stem rot is a major disease of oilseed rape in France. Severe yield losses usually occur two to three times per decade. For controlling the disease by spraying a fungicide only when it is needed CETIOM has improved and evaluated a diagnostic test based on a measured proportion of oilseed rape flowers infected by Sclerotinia sclerotiorum. The score of this test is used as a risk indicator. Receiver operating characteristic (ROC) curves are used to determine a decision threshold according to the sensitivity and specificity values. The threshold of 30% infected flowers generally leads to correct decisions except when disease pressure was too high, as in 2007.