Objects coming in front of other objects?

I used mesh merger script to combine my meshes which uses tree texture. Also, I used "transparent/diffuse" shader. I dont want alpha as it reduces the performance. Now my question is that when i enter the game mode, some trees draws in front of the tree which are nearer. i am attaching an image.kindly help.

or to be more specific can anyone give me some script so that object queue is rendered according to the camera distance so that objects nearer are drawn later and objects far away draw first.!