Visual and PCR assessment of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) cultivars

Abstract

Methods to assess light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape cultivars were compared in laboratory, controlled-environment and field experiments. In controlled-environment experiments with seedling leaves inoculated at GS 1,4, the greatest differences in percentage area affected by P. brassicae sporulation were observed with inoculum concentrations of 4 x 10(3) or 4 x 10(4) spores mL(-1), rather than 4 x 10(2) or 4 x 10(5) spores mL(-1), but older leaves had begun to senesce before assessment, particularly where they were severely affected by P. brassicae. In winter oilseed rape field experiments, a severe light leaf spot epidemic developed in 2002/03 (inoculated, September/October rainfall 127.2 mm) but not in 2003/04 (uninoculated, September/October rainfall 40.7 mm). In-plot assessments discriminated between cultivars best in February/March in 2003 and June in 2004, but sometimes failed to detect plots with many infected plants (e.g. March/April 2004). Ranking of cultivar resistance differed between seedling experiments done under controlled-environment conditions and field experiments. The sensitivity of detection of P. brassicae DNA extracted from culture was greater using the PCR primer pair PbITSF/PbITSR than using primers Pb1/Pb2. P. brassicae was detected by PCR (PbITS primers) in leaves from controlled-environment experiments immediately and up to 14 days after inoculation, and in leaves sampled from field experiments 2 months before detection by visual assessment.