CANANDAIGUA, N.Y. (AP) _ Canandaigua Wine Co., one of the largest wineries in the country, announced plans Friday to buy two wineries, including one in Brooklyn that makes Manischewitz kosher wine.

Marvin Sands, Canandaigua's president, said the company had reached an ''understanding in principle'' with Monarch Wine Co. of Brooklyn to buy its wine division including the Manischewitz brand.

Negotiations had been under way since last month.

Asked about the speculation that Canandaigua would close the winery and shift operations to its Finger Lakes headquarters, Stafford Krause, vice president for finance and administration, said final details will be disclosed later.

''We don't telegraph how we're going to do business,'' he said.

Sands also said an agreement has been signed for Canandaigua to acquire Widmer's Wine Co. of Naples, which is known for its dessert wines, sherries and Lake Niagara brand.

He said the purchase of Monarch should be completed by the end of October and the purchase of Widmer's will be completed at the end of this month. Financial terms for the deals were not announced.

The last purchase by Canandaigua was in the spring of 1984 when it acquired Robin Fie, Ltd., a winery in Batavia, N.Y. It currently operates as Batavia Wine Cellars Inc.

Canandaigua, for the first nine months ending May 31 of its fiscal year, had sales of $118 million and profits of $6 million. For the same period last year, it had sales of $88 million and profit of $5 million.

The increase was largely a result of continuing strong sales of Sun Country Wine Cooler, which has sent profits skyrocketing at the winery.