Few things are more disadvantageous to the leather of a shoe than road salt and rain water runoff. Among other maladies, both tend to stain leather with white lines. I believe the lines are caused by salt. They are much like the sweat lines you will find on the brim of your hat. They are also unsightly on any color shoes and if left in place for too long can cause a problem. They should be treated as soon as possible, luckily the process is quite simple.
1. Wipe off all excess dirt and dust from the shoe.
2. Make a solution of 50% white vinegar and 50% water.
3. Using a towel, gently rub the 50/50 solution on the affected areas. Do not rub to hard or for too long as you could risk damaging the leather.
4. Clean the area that the solution was applied to by wiping with a towel dampened with water.
5. Dry the dampened area.
6. Either have the shoes shined or shine them yourself, but the sooner the better.