Investigating the agronomical and epidemiological importance of latency in the wheat-Zymoseptoria tritici pathosystem

Septoria tritici blotch (STB) disease (caused by the hemibiotrophic pathogen Zymoseptoria tritici) is the most important yield limiting biotic stressor of wheat in Western Europe. Z. tritici but yet has remained an understudied pathosystem. STB disease has a long biotrophic asymptomatic phase (latent period) during which the pathogen grows slowly, before switching to produce pycnidia coinciding with necrotrophic lesions. Important questions regarding the latent period relate to how does the pathogen grow in planta and what is the impact of biomass levels on STB development during this period. In Chapter 3 of this thesis, 14 genetically distinct isolates of Z. tritici were screened against wheat cultivars Gallant and Stigg, which have contrasting host resistance to STB disease. The results show the presence of clear host specificity in the collected isolates, but this was dependent on the in planta growth phase of the pathogen. Host resistance was identified as the only significant factor for in planta growth rate of Z. tritici, measured by qPCR, between 5 & 11 days post-inoculation (dpi), with Z. tritici isolates not strongly influencing this metric. In contrast, the in planta growth rate between 11-16 dpi showed a significant (P<0.05) effect of cultivar and isolates. Additionally, growth rate during these two phases also demonstrated a significant association with components of pathogenicity e.g. latent period and leaf coverage with pycnidia. This relationship was further supported by linear regression where approx. 33% and 36% of the variation in latent period and leaf coverage, respectively, was explained solely by in planta pathogen growth during 11-16dpi. In Chapter 4, the implications of in planta Z. tritici growth under natural inoculum conditions in the field was investigated. In planta Z. tritici growth rate in flag leaves between week 1 & 2; week 2 & 3 and week 3 & 4 post flag leaf emergence showed a significant (P