Patterns of development of Septoria nodorum and S. tritici in some winter wheat crops in Western Europe, 1981-83

In data collected at 19 sites in Western Europe during 1981-83. two patterns of development of Septoria nodorum and S. tritici on foliage of winter wheal were distinguished. In sudden outbreaks, lesions appeared simultaneously on the upper leaf layers of crops, usually after the end of stem extension; these outbreaks were ascribed to short, heavy rain storms in which pycnidiospore inoculum in basal leaves was elevated up to 60 cm through the crop canopy. Gradual epidemics were characterized by disease arising on successive leaf layers as they appeared during sustained periods of weather suitable for inoculum transport and infection.
The data indicate incubation periods of 2-4 weeks for S. nodorum and 3-5 weeks for S. tritici. it is suggested that a leaf layer cannot normally sustain more than one pathogen generation and that its infection arises from inoculum borne on leaves older than in the layer situated immediately below it. The potential level of disease in a crop may relate to the amount of inoculum present in spring. The proportions of disease caused by the two Septoria species varied greatly between sites and years, but the data provided no explanation.
It is concluded that a septoria forecast scheme needs to recognise the importance of sudden disease outbreaks and to include not only weather but also host growth and inoculum factors.