Quark is a fresh white cheese, with a texture that has kindly been described as pastelike. In our recollection from student days in Germany, it is chalky. Different varieties of quark range from nonfat to about 11% fat, and the higher the fat, of course, the creamier the consistency.
Marcella Hazan uses ricotta cheese in her white pesto variation. If that is more readily available than quark where you are, it would be a fine substitute. It is a little odd that the white pesto recipe doesn’t include any olive oil, which is such a big part of pesto.
Two readers have sent in suggestions for making faux quark. The first is to mix 9 parts of ricotta with 1 part sour cream and mix in a blender. The second option is a little more involved: Take two quarts of buttermilk and pour into an oven-safe covered pan. Bake in the oven at 150°F (65°C) for five hours or overnight. Strain the solids from the liquid and what's left is quark. To make it creamy, says our correspondent, simply blend in small amounts of cream, milk, or half-and-half.