Washington State, with 59,000 vineyard acres, is the second largest producer of wine in the United States. Wine was made in the state as early as the mid-19th century, but Prohibition and, later, restrictive state laws killed the wine making business in the 20th century until the 1960s, when laws changed and large and small producers started making wines. An influential horticulturalist and agriculture professor name Walter J. Clore studied various grape clones in the 1960s to find the best ones for Washington, and by the 1970s Yakima Valley, Walla Walla and Columbia Valley had all become important grape growing areas. The best vineyards in the state are east of the Cascade Mountain range, where hot dry summers and cold winters are conducive to successful viticulture. Numerous grape varieties are grown, with Cabernet Sauvignon, Merlot, Syrah, Chardonnay, Riesling and Sauvignon Blanc at the head of the list.