LONDON (AP) _ Pillsbury Co., a Minneapolis-based subsidiary of Britain's Grand Metropolitan PLC, signed a 20-year agreement with Seneca Foods Corp. that will make Seneca the main processor of Green Giant canned vegetables.

The deal will result in the closure of Pillsbury processing plants in Le Sueur, Minn.; Ripon and Beaver Dam, Wis.; and Hoopeston, Ill., Pillsbury said Thursday.

Pillsbury will get about $71.8 million over 15 years for six vegetable processing plants it will sell to Pittsford, N.Y.-based Seneca. The plants are largely used for seasonal production.

Pillsbury will close the four plants over the next 18 months. Some of the employees 425 employees affected will go to work for Seneca, Grand Met said.

The closures will cost Pillsbury about $65.5 million, which will be reflected in Grand Met's 1994-95 earnings.

Pillsbury expects to save about $32.8 million a year once the agreement is implemented. It retains brand ownership and marketing and technology rights of Green Giant.