Which of the following are varieties considered in cordon pruning?

Cordon pruning is a specific method of grapevine training that involves developing a horizontal vine structure. This system is particularly suited to varieties that benefit from a controlled growth habit and allows for better management of fruit quality and yield.

The key to understanding the correct answer lies in the characteristics of the grape varieties. Cardinal and Emperor are known for their suitability to the cordon system. These varieties thrive under this method due to their growth patterns and fruiting characteristics, making them ideal candidates for cordon pruning. This approach optimizes the exposure of fruit to sunlight while allowing for effective canopy management.

In contrast, Chardonnay, Zinfandel, Merlot, Cabernet Sauvignon, Sauvignon Blanc, and Riesling are typically associated with other pruning techniques or training systems. While they may be successfully grown in various systems, they are not primarily recognized for their compatibility with cordon pruning as a defining characteristic. Thus, understanding these distinctions in varietal behavior and their responsiveness to pruning practices clarifies why the chosen answer is the most accurate.