Spreading plant spores by splashes upon raindrop impacts*

Raindrop impact can discharge rust fungi from plant leaves. In this study, we aim to characterize splashing mechanisms when a raindrop hits a leaf with spores, where we used a wheat leaf infected with rust fungus, Puccinia triticina, or a superhydrophobic substrate with micro-sized particles for the quantification. Like typical hydrophobic or leaf surfaces, a liquid droplet impacts, spreads, and retracts back. We observed two different splashing mechanisms depending on the phase of the droplet motion. In the spreading phase, the formation of daughter droplets was promoted by the structural corrugation of micro-sized rust spores even at low impact speeds. We found that a splash threshold on leaves infected with P. triticina is lower than that of health leaves. In the retraction phase, the flattened liquid film burst and disintegrated to produce more splashed droplets on diseased leaves. Consequently, these splashed droplets transport the rust fungi in a way that the splashed droplets either scavenge rust spores along, or generate air current to suspend fungal spores in the air. Here we elucidate distinctive splashing dynamics through experiments and theoretical models.