Is that your all code for the instantiation and texture assignment? If it is, you should add the rest of the code for a c++ script:

using UnityEngine;
using System.Collections;

public class grass : MonoBehaviour
{
    public Texture2D grassTexture;

    // Use this for initialization
    void Start () {
        // make new components in a loop
        // for each component in the loop assign texture and x, y position
        GameObject cube;
        for (int i = 1; i <= 65; i++){
            cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
            cube.transform.position = new Vector3(i*3, 0, 0);
            cube.renderer.material.mainTexture = grassTexture;
        }
    }
}

I've you do it it works perfectly, at least for me

Try this:

public Material materialll;


 // Use this for initialization
    void Start () {
        // make new components in a loop
        // for each component in the loop assign texture and x, y position
        GameObject cube;
        for (int i = 1; i <= 65; i++){
            cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
            cube.transform.position = new Vector3(i*3, 0, 0);
            cube.renderer.material = materialll;
        }
    }