Troubleshoot volumetric fog with depth map shadows
If the fog appears with streaks, or the entire
effect disappears from your scene when rendering in mental ray for
Maya, you can use one of the following solutions.
In
the Attribute Editor for the volume
primitive shape node, turn on Volume Samples Override.
Set
the Volume Samples value to 5,
or greater (between 5 - 20 is recommended). (You may need to tune
this setting repeatedly, until you achieve a good result.)