Phoma stem canker (Leptosphaeria maculans/L. biglobosa) of oilseed rape (Brassica napus): is the G2 Disease Index a good indicator of the distribution of observed canker severities?

Several disease indices (DI) have been proposed to assess the severity of phoma stem canker at late growth stages. The DI commonly used in France (called G(2) DI) consists of integrating the distribution of the proportions of plants observed within six canker severity classes into a single index. This index uses arbitrarily chosen coefficients associated to each class of canker severity. A given value of this DI can therefore be obtained with different distributions among the canker severity classes. Consequently, it is important to assess to what extent the G(2) DI is representative of the distribution of plants in canker severity classes. We show in this study that the G(2) DI is a good indicator of the distribution of the observed canker severities, using a dataset from different years, sites and cultivars; this illustrates the robustness of the G(2) DI.