Unity 5 transparency in custom shader

Hi,

i cannot seem to wrap my head around it since converting the project to unity 5 every transparency shader hasn't worked or got some weird background transparency on the trees.

Here's a pic from my grass billboard :

And here's the shader code for that grass billboard :

 Shader "Partner/Billboard_alpha_to_coverage" {
     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _MainTex ("Albedo (RGB)", 2D) = "white" {}
         _Brightness ("Brightness override", range (0,25.0)) = 0.0
     }
     SubShader 
     {
         Tags {"Queue"="Transparent"}
         LOD 200
         Blend SrcAlpha OneMinusSrcAlpha
         AlphaToMask On
         Cull Off
         
         CGPROGRAM
         // Physically based Standard lighting model, and enable shadows on all light types
         #pragma surface surf BlinnPhong alpha
         //#pragma target 3.0
 
         sampler2D _MainTex;
         float _Brightness;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         fixed4 _Color;
 
         void surf (Input IN, inout SurfaceOutput o) {
             // Albedo comes from a texture tinted by color
             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
             o.Albedo = c.rgb * _Brightness;
             o.Alpha = c.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }