First report of virulence to the septoria tritici blotch resistance gene Stb16q in the Irish Zymoseptoria tritici population

In June 2019 high levels of septoria tritici blotch (STB), caused by the fungal pathogen Zymoseptoria tritici were observed in Recommended List evaluation plots of winter wheat cv. Cellule (Fig. 1) in the Republic of Ireland. Levels of STB were unexpected as Cellule has the STB resistance gene Stb16q and previous evaluations confirmed its high level of resistance (Table 1). Septoria tritici blotch is the most destructive disease of winter wheat in Western Europe and control is heavily reliant on fungicides (O’Driscoll et al., 2014). Due to increased regulations on the use of fungicides in Europe varietal resistance is regarded as key to managing STB, with 21 resistance genes identified and mapped (Brown et al., 2015). Amongst these, Stb16q is unique, providing high levels of resistance equally at both the seedling and adult plant stages (Ghaffary et al., 2012). As such it is important to determine if the levels of disease observed in 2019 were due to the emergence of virulence to Stb16q.

A collection of Z. tritici isolates were established from infected leaf samples of the Stb16q-resistant cv. Cellule (n=10) and the moderately susceptible cv. Costello (n=10). All isolates were from fungicide-untreated commercial crops or trial plots. To determine if those isolates retrieved from Cellule exhibited virulence on Cellule when compared to those isolated from Costello, glasshouse seedling assays were performed. Two-week old seedlings of both Cellule and the susceptible cv. KWS Lumos were inoculated with spore suspensions (10⁶ spores/ml) of each isolate until runoff; placed in a clear polyethylene bag for 48 hr and subsequently moved into a glasshouse. Levels of disease were assessed 21 days post inoculation. The experiment was conducted twice. A beta generalised mixed-effects model (block and source cultivar as fixed and test cultivar as a random component) was fitted to the data and the estimated marginal means were separated using Tukey's Honest Significant Difference (α=0.05). All isolates infected the susceptible variety, with no differences observed between isolate collections (Fig. 2). Significant differences (P<0.0001) were observed between the collections in their ability to cause infection on Cellule, with almost no disease detected following inoculation with Costello isolates (Figure 2). High levels of disease were observed on Cellule following inoculation with the Cellule isolates, confirming the presence of virulence to Stb16q in the Irish Z. tritici population. Whilst the prevalence of virulence in the wider Z. tritici population is unknown, its presence in the population must be taken into account when considering cultivating varieties reliant on Stb16q as the source of STB resistance.