Armillaria root rot is caused by which organism?

Armillaria root rot is specifically caused by the fungus Armillaria mellea. This organism is known for its ability to decimate the roots of various plants and trees, leading to significant plant health issues and sometimes even plant death. Armillaria mellea infects the root systems, causing decay and rot, which can manifest as yellowing, wilting, and overall decline of the affected plants.

Understanding the context of this disease helps clarify why Armillaria mellea is the correct choice. Other organisms listed, such as Agrobacterium tumefaciens, are known for causing crown gall disease, a different type of plant ailment that affects the plant's growth systems rather than directly causing root rot. Phytophthora cinnamomi is a pathogen responsible for root rot but is a separate genus and species that primarily infects other types of plants. Lastly, Xanthomonas campestris pv. juglandis is a bacterial pathogen that affects walnut trees, leading to a different set of symptoms and does not directly relate to root rot caused by fungi.

Thus, the association of Armillaria mellea with root rot distinctly places it as the primary causative agent of this particular disease in plants.