Certified GKH Gardening Expert
It would, likely, not have any effect on it at all. Garlic is already full of its own natural fungicide, sulfur, so something a lot less mild like Neem oil will not suffice.
You will probably have to amend with a copper fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/what-is-copper-fungicide.htm