Interaction Types Produce Distinct Microscopic Phenotypes in the Exserohilum turcicum-Maize/Sorghum Pathosystem

Exserohilum turcicum, the causal agent of northern corn leaf blight and sorghum leaf blight, exhibits host-specific interactions at both the species and genotype levels. Although maize- and sorghum-adapted strains are generally restricted to their respective hosts, further specificity is observed in maize through gene-for-gene interactions with Ht resistance genes. In this study, we characterized the infection process of E. turcicum across three interaction types: unadapted (sorghum-specific strain on maize, or maize-specific strain on sorghum), adapted-susceptible (each strain on its respective host), and adapted-resistant (maize-specific strain on the Ht1 maize differential). We collected quantitative data for the early colonization phenotypes using epifluorescence and confocal fluorescence microscopy. There were significant differences between adapted and unadapted interactions at multiple infection steps. In the unadapted interactions, the pathogen formed more appressoria, and the host had more dead cells under the appressoria, suggesting a stronger hypersensitive response. Adapted-susceptible interactions exhibited higher levels of xylem and mesophyll penetration and colonization than unadapted and adapted-resistant interactions. Moreover, successful xylem penetration without colonization was observed in unadapted interactions, whereas adapted-resistant interactions demonstrated both xylem penetration and colonization without secondary mesophyll colonization. Progeny strains adapted to one or both hosts confirmed these patterns and showed no fitness cost associated with dual host adaptation. These findings provide new insights into the basis of host specificity and resistance to E. turcicum and highlight the potential for discovering novel resistance mechanisms by studying unadapted interactions across host species.