What can the sap-sucking action of root-feeding aphids cause?

The sap-sucking action of root-feeding aphids can lead to the formation of galls on the roots of plants. These galls are abnormal plant growths that result from the aphids injecting saliva into the plant tissues. This saliva contains growth-regulating hormones that cause the plant cells to proliferate and form these galls. Galls serve as a protective environment for the aphids, allowing them to thrive.

While root-feeding aphids do impact the plant's overall health, other conditions or factors typically cause symptoms such as yellowing leaves, wilting flowers, or root rot. For instance, yellowing leaves often indicate a nutrient deficiency, while wilting flowers can result from water stress rather than directly from aphid feeding. Root rot is primarily caused by fungal infections in overly wet soil rather than direct feeding damage from aphids. Thus, the specific and direct consequence of root-feeding aphids is the formation of galls, making it the correct answer.