Inferring the origin of primary inoculum of Zymoseptoria tritici from differential adaptation of resident and immigrant populations to wheat cultivars

We investigated whether the origin of primary inoculum of the fungal wheat pathogen Zymoseptoria tritici can be inferred from evidence of adaptation to host cultivars. We compared the aggressiveness of two pathogen populations collected locally, the first considered as resident (collected from debris in a wheat cv. Soissons monoculture plot) and the second considered as immigrant (collected from leaf lesions in a 300 m apart wheat cv. Soissons plot containing no debris, exposed to an inoculum pool of distant origin), with the aggressiveness of a third population (collected from early leaf lesions in the same monoculture plot) the origin of which we wanted to determine (local vs. distant). The three populations were sampled twice, in 2009 and 2012, from a 6-year field trial. Latent period and sporulating area of 6 x 12 isolates were assessed in greenhouse on adult plants of cv. Soissons and of Apache, another cultivar commonly grown around the field for several years. Firstly, we detected a differential host adaptation: after several years of monoculture, the resident pathogen populations became less adapted to the other cultivar. Secondly, we showed that when the inoculum pressure was high (2009), the aggressiveness profile of the third pathogen population was more similar to that of the resident populations than of the immigrant. This indicates that early lesions in the monoculture plot were mostly caused by within-field (local) primary inoculum. The comparison of aggressiveness profiles when the inoculum pressure was lower (2012) was less conclusive, suggesting that the tested population could have a mixed origin.