I would just find another tree.

Update: The smell has disappeared with this past rain, I could not find any garlic mustard plants in the area. I will have to wait and see if it really is sabotage.

Thanks

If you're on loamy soil (ie. not sandy), my money is on leeks, especially if you've ruled out Garlic Mustard. Leeks peak and go dormant in early-to-mid summer, which means there is little visual evidence of them above the ground but the bulbous root is there year round. All you'll see this time of year is a tiny dried up twig sticking up, but if you dig down you can find the onion-looking root. Some times they smell much more strongly than others...I think it has something to do with how moist the soil is. When you can smell them, you can really smell them. They're that strong at times. The deer don't seem to mind them though.