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 Leptosphaeria maculans on oilseed rape. Ascospore discharge, ratio of aggressive and non-aggressive isolates and leaf and stem infestations were determined during 1991/92–1993/94 at 6–10 sites in Northern Germany. On a 1–9 scale, blackleg intensity 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 infestation. There was a positive relationship between leaf infestation and blackleg. However, correlation coefficients were too low to be used as a basis for forecasting. On plant residues from the stem base, aggressive isolates were dominant (>80%) on all sites. From higher parts of the stem and from leaves also, non-aggressive isolates were isolated with higher frequencies on some locations, but the proportion of aggressive isolates was not related to the blackleg intensity. Taken all together, the three criteria alone seem to be insufficient for the development of a system of blackleg forecasting and targeted control. Further factors (e.g. climatic factors, seed-and soilborne inoculum, cultural practices) have to be included in models for forecasting the impact of blackleg on oilseed rape. Significant damage on vegetable crops by tospoviruses had occurred sporadically in Argentina in the past but since 1994, severe outbreaks have been recorded every year. The crops that have been most affected, tomato, lettuce, and pepper, were surveyed in the provinces of Mendoza and Buenos Aires in 1994–95 and 1995–96. A few weeds and miscellaneous crops were also collected. A total of 543 samples showing symptoms typical for tospoviruses were analysed by double-antibody sandwich-enzyme-linked immunosorbent assay with polyclonal antibodies to groundnut ringspot tospovirus (GRSV), impatiens necrotic spot tospovirus (INSV) and tomato spotted wilt tospovirus (TSWV). The 339 samples collected in 1995–96 were also assayed for tomato chlorotic spot tospovirus (TCSV). In addition, the incidence of tospoviruses in tomato crops was assessed in 41 farms representing 310 ha and 10 cultivars. GRSV was identified in 222 samples (40.8%), TSWV in 194 samples (32.7%), TCSV in 50 samples (14.7%), INSV was not detected and 77 samples did not react with the antisera used. TSWV was found to prevail in Buenos Aires and GRSV in Mendoza. Mixed infections were not found in this survey. In tomato crops the mean incidence of tospoviruses was 33%. These results show that the disease formerly assigned to TSWV, is caused by at least three tospoviruses.