Pseudothecial maturation and ascospore release of Leptosphaeria maculans and L. biglobosa in south-east Poland

Abstract

Stem canker of brassicas is a severe disease of oilseed rape in Australia, Canada and Europe, including Poland. The disease is caused by Leptosphaeria maculans and L. biglobosa - two pathogens belonging to the class Dothideomycetes. The species differ in pathogenicity, but they have identical shape and size of fruiting bodies and spores of the generative and vegetative stages. Both pathogens are often found together in infected tissues of oilseed rape plants. The main goal of the experiments was to measure the rate of pseudothecial maturation and to monitor ascospore concentration of L. maculans and L. biglobosa in air samples. The paper is the first investigation on the generative stage development of these two species in south-east Poland. The studies were done for three consecutive years (2005-2007), for six most important months in pathogen development and plant infection, including 3 months in the spring (March - May) and 3 months in the autumn (September - November). The stage of pseudothecial maturation was assessed visually, based on the development of asci and ascospores. Monitoring of spore concentration in the air was performed using a Hirst-type 7-day volumetric trap. It was proved that differences in pseudothecial maturation rate in south-east Poland, encompassing the climatic regions of the Carpathian Foothills and Cracow, do not exceed two weeks within one season. The first and the highest ascospore concentration dates depended on weather conditions in a particular season. The total number of spores during the studied seasons varied from 9 to 12 spores/m³, which was from 70 to 90 times lower than the average from five other monitoring sites around Poland. The short exposition to spore showers and very small concentrations of L. maculans and L. biglobosa ascospores in air samples were the most probable reasons for relatively small damage of oilseed rape crops by stem canker in the south-east part of Poland.