Which of the following pests is NOT considered a plant disease agent?

The correct choice is the Common Greenbriar because it is a type of plant, specifically a climbing shrub, and does not act as a disease agent. Plant disease agents typically include organisms that can cause illnesses in plants, such as fungi, viruses, and bacteria. These agents are responsible for infectious diseases that can lead to plant damage or death. In contrast, Common Greenbriar does not induce disease but is a perennial vine that can sometimes be considered an invasive species or a weed in certain contexts. Its role in the ecosystem differs significantly from that of pathogens like fungi, bacteria, and viruses that specifically target plant health.