Removing stains from vinyl siding?

I have a rental property with vinyl siding. The siding is white and is about 5 years old. There is a light brown stain forming near the the outside light fixture. Not sure what is causing it.
Any ideas on how to remove this stain and prevent it in the future? I've tried Ajax with no success.
Thanks, Vin - Menotomy Vintage Bicycles http://OldRoads.com
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In this same vein, I recently had my house power washed, and the siding on the porch retained a distinct yellowish cast. It had obviously been a LONG time since the siding was washed (if ever), and the siding on the porch never even gets rained on. Any suggestions for how to whiten this siding?
Jo Ann

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wrote:

Near the bulb or below the fixture?

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The stain is all around the fixture, even above where the fixture would cast light. The bulb is a low-wattage fluorescent bulb.
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says...

Bugs, maybe? Have you had it on at night?
Banty
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The light is certainly on at night a lot. There's a light on the side of the house, but that light does not have the staining. Could it be due to the fluorescent bulb? Any more ideas on how to remove the stain? I have not yet tried bleach.
Vin - Menotomy Vintage Bicycles http://OldRoads.com
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