Horseradish root contains an oil similar to mustard oil called isothiocyanate. Present in some other plants in lower concentrations it exhibits mild antibacterial and anti-fungal properties.
While the amount present in its natural form only helps to curtail fungal and microbial activity, it doesn't prevent it - it does serve well as a companion plant for susceptible herbs, fruits and vegetables as it improves the odds.
Potatoes - The same properties also make horseradish an insect repellant of many pests. It is documented that horse radish deters potato bugs, potato beetles, aphids, blister beetles, some caterpillars, white flies, and and multiple soft-bodied insects. It also enhances the disease resistance of potato plants.
When planting horse radish with potatoes it should only be planted near the corners of the potato patch - not intermingled. It adversely effects the flavor of potatoes.
The roots of horseradish, even the tiniest fragment can and frequently does produce more horse radish plants - it is highly invasive and can easily commandeer the entire potato patch.