The town council issues the contamination warning after sewers overflowed in January.
People are being urged not to forage wild garlic due to a "serious risk" of E. coli contamination.
Marlborough Town Council in Wiltshire has issued the urgent notice for areas near a tributary of the River Kennet, including St George's Preshute.
Nearby Coopers Meadow flooded with "a significant amount of human waste" after sewers overflowed on 5 January.
The Royal Horticulture Society said it should be safe to eat crops the following year after flooding.
"After flooding, edible crops near to harvest are best not eaten: no assurances can be given that root crops will be safe to eat, so they should be discarded.