Aluminum Alloy Wheel Discoloration

[MichMan]

My factory installed aluminum alloy wheels are becoming discolored (black streaks) more and more with each passing year (all 4 tires). I own a 2007 ES350 purchased new in August 2006. I haven't touched the wheels except to wash them from time-to-time with off-the-shelf car soap or at a local car wash. I went to the local Lexus dealer and they stated I must have driven through some type of chemical. They stated that they never seen this before. I also have never experienced this before in all the cars I've owned throughout the years. Has this happened to anyone else or does anyone have any idea what is going on with the wheels?

[SW03ES]

The dealer is probably correct. The only time I have ever seen wheels pitted and stained like that is from contact with a chemical. In Michigan you have tough winters, could be whatever they use on the roads. How often do you wash the car? Do you ever use an automatic carwash?