DNA evidence has shown Cabernet Sauvignon is the result of the crossing of two other Bordeaux grape varieties— Cabernet franc and Sauvignon blanc— which has led grapevine historians, or "ampelographers", to believe that the grape originated in Bordeaux. Early records indicate that the grape was a popular planting in the Médoc region during the 18th century. The loose berry clusters and thick skins of the grape provided a good resistance to rot in the sometimes wet maritime climate of Bordeaux. The grape continued to grow in popularity until the "Powdery mildew" epidemic of 1852 exposed Cabernet Sauvignon's sensitivity to that grape disease. With vineyards severely ravaged or lost, many Bordeaux wine growers turned to Merlot, increasing its plantings to where it soon became the most widely planted grape in Bordeaux. As the region's winemakers started to better understand the area's "terroir" and how the different grape varieties performed in different region, Cabernet Sauvignon increased in plantings all along the Left Bank region of the Gironde river in the Médoc as well as Graves region, where it became the dominant variety in the wine blends. In the Right bank regions of Saint-Émilion and Pomerol, Cabernet is a distant third in plantings behind Merlot & Cabernet franc.