Leaf wetness
Liquid water sitting on plant tissue, from condensation, guttation or irrigation. It is a different measurement from humidity, and several pathogens require it.
Humidity describes water vapor in the room. Leaf wetness describes liquid water on the plant, and the two diverge: a canopy can be wet at moderate room humidity if surfaces have fallen to the dew point, and dry at high humidity if nothing has cooled that far.
The distinction matters because many fungal spores need free water to germinate rather than merely humid conditions. Botrytis cinerea is the relevant example, taking roughly 8 to 12 hours of continuous leaf wetness rather than a humidity reading alone.
Powdery mildew is the exception that proves the point, since it germinates without free water. That is why it can establish under conditions that leave botrytis unable to start.
Leaf wetness sensors are standard in outdoor agriculture. Indoors the usual proxy is tracking dew point against the coldest surface temperature in the room, because that is what decides whether condensation forms at all.