The need for Hashgacha is based on the following concerns. While vegetables grown outside of Eretz Yisroel would be considered inherently Kosher, they are often processed in equipment that is also used for non-Kosher items. Fresh vegetables are generally harvested at a specific and short period of time. For example, peas, corn, and string beans are available for canning for only several weeks, during which time the entire crop must be processed. Years ago production facilities were designed for certain vegetables, and when the canning season for these vegetables ended the plant was shut down.
Some plants process non-Kosher soups and sauces.
Tomatoes and tomato sauces are often processed in the same equipment used to make meat and cheese flavored pizza sauces. Clearly, products coming form such canneries pose serious Kashrus concerns. Canneries that process such non-Kosher products can compromise the Kosher status of otherwise Kosher items in the following ways. The brine used for vegetables can be cooked in the same kettles used for non-Kosher items. Depending on the system, hot water used to cook non-Kosher products can also be recycled and used to cook the otherwise Kosher items.