Two years of spatiotemporal monitoring of Mycosphaerella graminicola airborne inoculum in Belgium

Two types of dissemination propagules play a role in the epidemiology of Septoria tritici blotch (STB). Airborne ascospores are the major source of primary infections in autumn and winter, whereas the development of the epidemic in spring and summer is thought to be driven by splash-dispersed pycnidiospores. Recent observations in various countries, however, have shown that the airborne inoculum of Mycosphaerella graminicola can be produced all year round. In order to understand the role of airborne inoculum in STB epidemics in Belgium, a monitoring technique involving 7-day recording Burkard spore traps and real-time PCR was developed to quantify the total daily airborne inoculum at five locations in the Walloon region in Belgium over 2 years, from April 2009 to April 2011. Four traps were placed in wheat fields and a fifth trap was placed on the roof of a 25 m high building. The reliability of the quantification technique and the homogeneity of the distribution of the STB airborne inoculum at the field scale were controlled in preliminary assays. STB airborne inoculum was detected throughout both years at each of the five locations. Lower but sometimes important quantities were detected in the air above the building, indicating the possibility of long-distance transport. Differences in airborne inoculum quantities in fields were also observed at the network scale and might be linked to the STB level. From the seedling to stem extension stages, there were frequent peaks of detection at each site. The quantities trapped were correlated with the severity of the disease the previous year. Significant quantities of the airborne inoculum were also trapped between the cereal stem elongation and heading stages, suggesting the possible influence of this type of aerial inoculum on STB incidence on the upper leaves in spring and summer. The highest detections, however, generally occurred later, between heading and harvesting, especially in 2009 when disease pressure was high, supporting significant production of pseudothecia during that period. These results illustrate the presence of airborne inoculum all year round, suggesting a possible effect of M. graminicola airborne inoculum on STB epidemics throughout the growing season