The yellowing of leaves can result from :
1. Garlic ready for harvest, plants begin to turn yellow or brown and begin to fall over.
2. Fungus (white rot, Rust or Purple blotch)
a. White rot, usually older leaves yellowing; stunted growth; death of all leaves; fluffy white growth on base of bulb which spreads up bulb to storage leaves
b. Purple blotch, small water-soaked lesions on leaves or stalk with white centers; which enlarge to become zonate and brown to purple in color with red or purple margin surrounded by yellow zone
c. Rust, Small white flecks on leaves and stems which develop into circular or elongated orange pustules; severe infestations can cause leaves to yellow and die
d. Downy mildew, leaves turning pale then yellow; leaf tips collapsing
3. Over watering
We can better advise you on treatment methods once you upload a picture for proper diagnosis.