Fig. 2: Pg aeciospore release speed can be accurately predicted using lubrication theory.

a Following moisture ingress and hydration of Pg aeciospores, the interspore gaps become perfused with water, coating aeciospores with a thin lubrication film. The increase in aeciospore volume through changes in turgidity increases the pressure on neighboring aeciospores, leading to violent ejection. b As the mature Pg aecium peridium ruptures at the apex it creates an angle of inclination (\(\alpha\)) that propells aeciospores forward in the direction of the axis of symmetry, i.e., the positive x direction. Aeciospores were modeled as cuboids, with sides of length \(L\times \lambda \times \lambda\). c The edge of the Pg aeciospore of length L is located at \(y=H\left(t\right)\), where \(H\left(t\right)\) is the time-dependent gap. The force arising from the film pressure is given by \({{{{{\boldsymbol{P}}}}}}=P{{{{{\boldsymbol{n}}}}}}\), where \({{{{{\boldsymbol{n}}}}}}\) is the unit vector that is normal to the spore face and pointing away from the liquid film and \({p}_{0}\) reflects the heightened pressure acting at the base of each chain of aeciospores as a result of swelling within the aecial cup. The velocity components are given by \({V}^{\left(y\right)}={{{{{\mathrm{d}}}}}H}/{{{{{\mathrm{d}}}}}t}\) and \({V}^{\left(z\right)}=\left({{{{{\mathrm{d}}}}}X}/{{{{{\mathrm{d}}}}}t}\right){{{{{\rm{cos }}}}}}\,{{\alpha }}\). d Contour plots illustrate that predicted ejection speeds ranged from 0.02 to 0.5 m s⁻¹ for a single aeciospore. Ejection speed was determined by varying the ejection angle \(\alpha\) based on approximate values obtained through image analysis (1–11°, i.e., 0.02–0.2 radians).