sir_isOanswered grow question 4 years ago Some things I know about are citric acid, potassium carbonate/bicarbonate, epsom salts, even MPK, plant oils/phenols, lactic acid stuff (like milk) and some bacterias. Lower humidity (though mildew can also often result from drought).
And of course, weaker plants are more susceptible. Especially if they have thin leaves that don't have a decent waxy coating.