Presence of ice-nucleating Pseudomonas on wheat leaves promotes Septoria tritici blotch disease (Zymoseptoria tritici) via a mutually beneficial interaction

Helen N Fones

doi:10.1038/s41598-020-74615-7

Abstract

Zymoseptoria tritici causes Septoria tritici blotch (STB) of wheat, an economically important disease causing yield losses of up to 10% despite the use of fungicides and resistant cultivars. Z. tritici infection is symptomless for around 10 days, during which time the fungus grows randomly across the leaf surface prior to entry through stomata. Wounded leaves show faster, more extensive STB, suggesting that wounds facilitate fungal entry. Wheat leaves also host epiphytic bacteria; these include ice-nucleating (INA+) bacteria, which induce frost damage at warmer temperatures than it otherwise occurs. Here, STB is shown to be more rapid and severe when wheat is exposed to both INA+ bacteria and sub-zero temperatures. This suggests that ice-nucleation-induced wounding of the wheat leaf provides additional openings for fungal entry. INA+ bacterial populations are shown to benefit from the presence of Z. tritici, indicating that this microbial interaction is mutualistic. Finally, control of INA+ bacteria is shown to reduce STB.

Highlights

Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB), an economically important disease of temperate wheat[1]
At 14 days post inoculation, more advanced STB symptoms were seen in leaves subjected to the more severe treatments; leaves exposed to − 6 °C already showed pycnidia (Supplementary Figure S2)
This work demonstrates that frost damage can increase the speed and severity of STB

Summary

Introduction

Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB), an economically important disease of temperate wheat[1]. Fones et al.[8] demonstrated that wounded leaves are more susceptible to Z. tritici, and show fungal sporulation more rapidly They postulated that wounds provide additional entry points for randomly growing hyphae, accelerating fungal ingress and reducing the symptomless period. Frost damage is relevant for winter wheat because of its growth throughout winter and early s pring[2]. Such damage may be facilitated by the presence of ice-nucleating (INA+) bacteria[14,15,16]. The hypothesis that frost damage induced by INA+ bacteria facilitates leaf entry by Z. tritici is tested, and the possibility of STB control via control of INA+ bacteria is explored. Populations of INA+ and INA− bacteria are compared on frost-treated leaves in the presence or absence of Z. tritici, exploring

Methods

Results

Conclusion