A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., and co-author of Finishes for Exterior Wood, responds: Mildew is a fungus found virtually everywhere; all it needs in order to flourish is oxygen, a food source, moisture, and temperatures between 40°F and 90°F.

As a siding stain degrades — due to UV exposure and other environmental factors — the mildewcide it contains breaks down. And if the climate is right (warm and humid is ideal), chalked siding offers the perfect surface for mildew growth, with dirt that's accumulated over the years providing an additional food source.

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