Ascospore discharge, leaf infestation and variations in pathogenicity as criteria to predict impact of Leptosphaeria maculans on oilseed rape

The aim of this investigation was to evaluate criteria for the forecast and targeted control of basal stem canker (blackleg) caused by Leprosphaeria m aculans on oilseed rape. Ascospore discharge, ratio of aggressive and nonag gressive isolates and leaf and stem infestations were determined during 199 1/92-1993/94 at 6-10 sites in Northern Germany. On a 1-9 scale, blackleg in tensity varied from 2.3 to 6.3 at BBCH 81 between different sites and years . Ascospore discharge started in September or October, and reached maxima 1 or 2 months later, without an apparent relationship to blackleg or leaf in festation. There was a positive relationship between leaf infestation and b lackleg, However, correlation coefficients were too low to be used as a bas is for forecasting. On plant residues from the stem base, aggressive isolat es were dominant (> 80%) on all sites. From higher parts of the stem and fr om leaves also, non-aggressive isolates were isolated with higher frequenci es on some locations, but the proportion of aggressive isolates was not rel ated to the blackleg intensity. Taken all together, the three criteria alon e seem to be insufficient for the development of a system of blackleg forec asting and targeted control. Further factors (e.g. climatic factors, seed- and soilborne inoculum, cultural practices) have to be included in models f or forecasting the impact of blackleg on oilseed rape.