Simply spraying water on a plant’s leaves can stimulate the plant’s immune system. Researchers believe this happens because rain, particularly heavy rain, can create an unhealthy environment for plants in the short term. A greater risk of fungal disease is created by the humid conditions that follow rain, as well as the potential for soil to splash up on leaves and for water to drip off of diseased plants onto healthy plants. By releasing hormones that are part of a plant’s defence system, the plant is better protected from these threats. Touching a plant can trigger similar immune responses, whether it be from a human or plants jostling against each other in the wind. In a greenhouse environment, growers can improve the resistance of plants to pests by stroking plants or allowing breezes to flow through.
However, there can be too much of a good thing. The immune response peaks 10-25 minutes after the stimuli and after an hour, hormone levels are back to normal. Repeated stimulation by wind, rain or humans can create too much stress.
Source: Alex Van Moerkercke, Owen Duncan, Mark Zander, et al. A MYC2/MYC3/MYC4-dependent transcription factor network regulates water spray-responsive gene expression and jasmonate levels. Proceedings of the National Academy of Sciences. 2019. Pages: 1-12.
— Janet Wallace