Do aphids spread plant diseases

Viral is the type of disease which aphids transmit.

Specifically, the insect in question (Aphidoideasuperfamily) applies mouthparts known as stylets to break through plant tissue in order to access phloem and xylem sap. In the process, an aphid injects toxic saliva and leaves sheaths around entry-points. An aphid's mouthparts, saliva and sheaths all may transmit viruses to the plant, which is weakened by the loss of vital life-sustaining hormone-, mineral-, sugar-enriched watery solutions.