Gardeners are occasionally surprised to find small, round, green, tomato-like fruit on their potato plants. These fruit are not the result of cross-pollination with tomatoes. They are the true fruit of the potato plant. The edible tubers are enlarged, underground stems.
Tomatoes and potatoes belong to the Solanaceae or Nightshade Family. Plants within a family share certain morphological characteristics. The flowers on tomatoes and potatoes are similar in appearance. Potato fruit are similar in shape (though much smaller in size) to those on tomatoes.
Most flowers on potato plants dry up and drop from the plant and don't develop into fruit. The fruit that do develop are relatively small and inconspicuous and often go unnoticed by most gardeners. The variety 'Yukon Gold' fruits more heavily than most other potato cultivars.
Potato fruit are of no value to gardeners. The small fruit should not be eaten as they contain a poisonous alkaloid (solanine). The fruit are not useful for planting purposes as potatoes don't reproduce true from seed.