Which bacterium is associated with crown gall in walnuts?

The association of Agrobacterium tumefaciens with crown gall in walnuts is well-documented in plant pathology. This bacterium is known for its ability to induce the formation of tumorlike growths, or galls, on the stems, roots, and sometimes branches of various plants, including walnuts. The mechanism of gall formation is a result of the bacterium transferring a portion of its DNA into the plant cells, which then causes the plant to grow uncontrollably, forming these characteristic galls.

Other options do not relate to crown gall in walnuts. Phytophthora cinnamomi is primarily known for causing root rot in various plants, Colletotrichum gloeosporioides is associated with anthracnose disease in several crops, and Xanthomonas campestris pv. juglandis is responsible for bacterial cankers, but not crown gall specifically. This particular specificity is what distinguishes Agrobacterium tumefaciens as the correct answer regarding crown gall disease in walnuts.