Downy mildew is the biggest threat to spinach production around the world. While the pathogen has a short life cycle (approximately a week), it can produce millions of spores during the spinach growing season. Overhead sprinkler irrigation systems and dew formation on cool nights leads to more moisture, which enables these spores to infect the spinach.
source https://phys.org/news/2020-11-short-term-moisture-downy-mildew-spinach.html