Contaminated Roma tomatoes were the cause of three salmonella outbreaks last summer, including one that sickened patrons of a convenience store chain in Western Pennsylvania.

The CDC in Atlanta said 561 people in the United States and Canada were infected by salmonella after eating contaminated tomatoes. The worst of the outbreaks occurred among those who became ill after eating delicatessen sandwiches and salads with Roma tomatoes at Sheetz convenience stores.

Although a single tomato-packing house in Florida was common to all three outbreaks, other growers or packers also might have supplied contaminated Roma tomatoes that resulted in some of the illnesses.

The CDC says that 429 culture-confirmed, outbreak-associated salmonellosis cases were identified among residents of Pennsylvania, Ohio, West Virginia and six other states who ate products with Roma tomatoes prepared at the Sheetz stores in early July.