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<0.05) association with latent period, development of leaf areas covering with necrosis (AUDPCnecrosis) and bearing pycnidia (AUDPCpycnidia). This relationship was also observed in linear regression of AUDPCpycnidia for growth rate between week 1 & 2 and week 2 & 3, where growth rate had explained approx. 47% of total variation of AUDPCpycnidia. Yield responses due to STB were also correlated with in planta Z. tritici growth rate, indicating the importance of early detection and control of STB on final yield. Although cultivars with moderate STB resistance performed similar to strongly resistant cultivars under moderate disease epidemics, when exposed to higher STB epidemics pressure their performances for latent period, AUDPCnecrosis and AUDPCpycnidia shifted more towards susceptible cultivars. These results signify the importance of in planta Z. tritci growth as key to understand both pathogen aggressiveness and host resistance responses in the field and the need to examine durability to STB across multiple environments before the deployment of novel cultivars.