Not sure if this is the most efficient but here's how I would approach this:
Layer - New Adjustment Layer - Hue Saturation - Adjust the saturation/desaturation to your liking - Select he LAYER MASK (that blank white layer you see on your adjustment layer) -fill it with BLACK (Everything looks like it is back to normal) - Grab a brush , select the color WHITE - Select the Layer Mask again and brush that white paint over the spot you want desaturated.
You will find that your saturation adjustment will appear on where you brush it.
I hope that makes sense. I use this technique a lot in various settings.
PS- I also noticed your usename. lol made me smile. XD