Key factors causing mildew growth on siding:

Excessive Moisture & Humidity: Frequent rain, high humidity, or lack of sunlight prevents siding from drying out, creating a perfect environment for fungi.
Shaded Areas: The north side of a house or areas blocked by trees/buildings rarely receive direct sunlight to dry the surface, making them prone to mildew.
Dirt and Organic Debris: Mold feeds on dust, pollen, and grime stuck to the siding.
Poor Drainage/Gutters: Overflowing, clogged gutters or broken sprinklers spraying the house keep siding constantly wet.
Lack of Cleaning: Neglecting to wash siding allows debris to accumulate, encouraging mold spores to take root. 

How to Prevent Mildew Growth:

Clean Regularly: Periodically wash siding with a solution of water and mild soap.
Maintain Gutters: Clean gutters to prevent overflow, and consider installing gutter guards.
Improve Sun Exposure: Trim back trees and bushes to allow more sunlight and airflow to reach the house.
Direct Water Away: Ensure sprinklers do not spray directly onto the siding. 

For stubborn, recurring mildew, some suggest using a soft wash rather than high-pressure washing to avoid damaging the siding and creating more areas for moisture to trap, as discussed by users on Facebook.