No abstract

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, is the most prevalent disease of wheat worldwide. Primary inoculum and the early stages of STB epidemics are still not fully understood and deserve attention for improving management strategies. The inoculum build-up and overseasoning involves various fungal structures (ascospores, pycnidiospores, mycelium) and plant material (wheat seeds, stubble and debris; wheat volunteers; other grasses). Their respective importance is assessed in this review. Among the mechanisms involved in the early stages of epidemics and in the year-to-year disease transmission, infection by ascospores wind-dispersed from either distant or local infected wheat debris is the most significant. Nevertheless, infection by pycnidiospores splash-dispersed either from neighbouring wheat debris or from senescent basal leaves has also been inferred from indirect evidence. Mycosphaerella graminicola has rarely been isolated from seeds so that infected seed, although suspected as a source of primary inoculum for a long time, is considered as an epidemiologically anecdotal source. Mycosphaerella graminicola can infect a few grasses other than wheat but the function of these grasses as alternative hosts in natural conditions remains unclear. Additionally, wheat volunteers are suspected to be sources of STB inoculum for new crops. This body of evidence is summarized in a spatio-temporal representation of a STB epidemic aimed at highlighting the nature, sources and release of inoculum in the early stages of the epidemic.

Septoria tritici blotch, caused by Mycosphaerella graminicola, is a major foliar disease of wheat. The quantitative traits of pathogenicity are not comprehensively described in this pathosystem. The objective of this study was to identify and quantify the most relevant variables to describe traits of aggressiveness. Four wheat cultivars were inoculated in a greenhouse with four isolates. Inoculation was performed on a limited surface of the two uppermost leaves of adult plants. The dynamics of chlorotic, necrotic and sporulating areas were assessed twice a week. Pycnidia were counted at the same time. A Gompertz model was fitted to the resulting curves. Parameter combinations with easily interpreted biological relevance were examined further as descriptors of aggressiveness. Within each category of descriptor, those which were the most pairwise correlated and which explained the largest part of the variance were retained: incubation and latent period, development rate of sporulating area, maximal sporulating area, pycnidial density, and sporulation capacity. Correlations between these variables were discussed, assuming they reflect biological relationships between the corresponding aggressiveness quantitative traits. It is suggested that the selected variables, providing a good measure of M. graminicola fitness, can be used to estimate quantitative resistance of wheat to septoria tritici blotch, to characterize differences among isolates within a pathogen population, and to study quantitative adaptation of the pathogen to its host and to its environment.