Sporulation of Pyrenopeziza brassicae (light leaf spot) on oilseed rape (Brassica napus) leaves inoculated with ascospores or conidia at different temperatures and wetness durations

Abstract

In controlled environment experiments, sporulation of Pyrenopeziza brassicae was observed on leaves of oilseed rape inoculated with ascospores or conidia at temperatures from 8 to 20degreesC at all leaf wetness durations from 6 to 72 h, except after 6 h leaf wetness duration at 8degreesC. The shortest times from inoculation to first observed sporulation (l (0)), for both ascospore and conidial inoculum, were 11-12 days at 16degreesC after 48 h wetness duration. For both ascospore and conidial inoculum (48 h wetness duration), the number of conidia produced per cm(2) leaf area with sporulation was seven to eight times less at 20degreesC than at 8, 12 or 16degreesC. Values of Gompertz parameters c (maximum percentage leaf area with sporulation), r (maximum rate of increase in percentage leaf area with sporulation) and l (37) (days from inoculation to 37% of maximum sporulation), estimated by fitting the equation to the observed data, were linearly related to values predicted by inserting temperature and wetness duration treatment values into existing equations. The observed data were fitted better by logistic equations than by Gompertz equations (which overestimated at low temperatures). For both ascospore and conidial inoculum, the latent period derived from the logistic equation (days from inoculation to 50% of maximum sporulation, l(50)) of P. brassicae was generally shortest at 16degreesC, and increased as temperature increased to 20degreesC or decreased to 8degreesC. Minimum numbers of spores needed to produce sporulation on leaves were approximate to25 ascospores per leaf and approximate to700 conidia per leaf, at 16degreesC after 48 h leaf wetness duration.