According to my well worn plant guide (as well as personal experience) "...the presence of the tomato plant lowers the potato plant's individual resistance to Phytophthora infestants, commonly known as blight. This disease affects both tomato and potato plants. Once a plant is infected, blight spreads quickly from one plant to another, and even to other gardens close by. It can wipe out entire fields, and was the cause of the Irish potato famine in 1845. Potatoes grown near tomatoes often slow the growth of the tomato plant. When using crop rotation techniques, which means planting a crop in a different location each season, do not plant tomatoes or potatoes where the other one was the previous season."
The same guide also warns against planting potatoes ans cucumbers in close proximity... for the same basic reasons.
Thanks for asking, hammerman!