Answer
Horizontal transmission means that a virus jumps from one plant to another via some vector carrying viral particles, while vertical transmission means that the transmission is during development from parent plant to offspring.
Work Step by Step
The terms horizontal and vertical are referring to movement within a group of unrelated plants or among parent and offspring. To help see this, review pedigree analysis (earlier chapter) and use your imagination to see how these terms would apply to a pedigree.
