The French wine industry in the 19th century grew many different cultivars of grapes. Yet it still fell victim to phylloxera. In the end, they had to switch t

The French wine industry in the 19th century grew many different cultivars of grapes. Yet it still fell victim to phylloxera. In the end, they had to switch to American rootstocks that came not just from different cultivars, but from different species of grapes. This was in some ways more drastic of a change than the switch from Gros Michel bananas to Cavendish bananas. The bananas were two different cultivars of the same species. When the phylloxera plague struck, the French were already growing dozens of different cultivars, in a variety of environments, from Champagne to Provence. But it didn't matter. The grapes all died. Genetic diversity can provide resiliency if a crop disease happens to attack the genes that are different. If the disease is indifferent to those genes, then genetic diversity will not save your crop.