According to www.luv2garden.com...Good candidates to companion plant with potatoes include Onions, garlic and Alliums, Horseradish, beans, corn, cabbage and members of the cabbage family, dead nettle, flax, peas, marigold, comfrey, Sweet alyssum.
Bad candidates to companion plant with potatoes include Apple, Celery, Cherry, Cucumber, Pumpkin, Raspberry, Rosemary, Sunflower, Tomato, eggplant, pepper, radishes, turnip, asparagus, cucumber, kohlrabi, pumpkin - squash family, rutabaga.
Planting Onions with Potatoes
Intercropping of Alliums, in particular Onions and Garlic with Potatoes, has been demonstrated to have substantial impact on pest populations. Aphids and other sap sucking green flies and their maggot offspring, leafhoppers and bean beetles were repelled from the potato crops.