Draw textured circle / 2D shape

I have the following code to create a circle Path and a SCNShape from this path. orbit_gradient.png is a horizontal 1px high image that represents a gradient.

let material = SCNMaterial()
material.isDoubleSided = true
material.lightingModel = .constant     
material.diffuse.contents = UIImage(named: "art.scnassets/orbit_gradient.png")

let shapePath = Path.circle(radius: radius, segments: 512)

let orbitShape = SCNShape(shapePath)
orbitShape.materials = [material]  
self.orbitNode.geometry = orbitShape

The problem I have is that when applying the texture to this geometry I'm just left with a white circle. If I set the diffuse to be UIColor.red it displays as red. What I want to accomplish is a stroked circle that appears to gradually fade, creating a rotating trail effect.