Relative importance of different types of inoculum to the establishment of Mycosphaerella graminicola in wheat crops in north‐west Europe

Abstract

The contribution of wheat debris to the early stages of septoria leaf blotch epidemics was assessed in a 3‐year field experiment. First lesions were detected very early (December) in the case of an early sowing (mid‐October), showing that the first contamination could occur as soon as the seedlings emerge. The tested debris management options (chopped debris, removal of debris followed by tillage, or tillage in absence of debris) had a strong effect, although transient, on the epidemic dynamic: the more debris present on the soil surface, the more severe initial disease was. The magnitude of differences between treatments differed substantially between years. The relative production of pycnidiospores and ascospores was measured on the chopped debris. Peaks in pycnidiospore and ascospore production coincided in October–November. Both types of spores can be involved as primary inoculum in north‐west European conditions. The local amount of pycnidiospores available on debris in the field, estimated per square metre, was 1000‐fold the local ascospore production. Moreover, inoculum production was quantified on debris exposed to different environmental conditions. Autumnal conditions, characterized by moderate temperature with alternating wet and dry periods, were favourable for the production of both pycnidiospores and ascospores, as shown by the high inoculum production on debris exposed to field or outdoor conditions. By late autumn, the canopy became the most important source of pycnidiospores, and this period, characterized by the decreasing role of debris as a local source of inoculum compared to distant potential sources, can be considered as the end of the early epidemic stages.