Pollen transmission from male flower to female occurs for a few viruses. Such viruses are seed borne. Most viruses are do not get into the gametes and therefore are not seed-borne. Until a virus is detected, its presence is not known. The monkey cells used to grow that vaccine were infected by SV40 but no one knew SV40 existed.
Many plants are carriers of plant viruses but show no disease.
The yield might be reduced but that would not be visbible either. Then the the tray is washed under running water. A special enzyme has been bonded to the antibody. Very little of this antibody-enzyme binds to the well because all the plastic of the tray was covered with protein. Now a colorless dye complex which the enzyme can split.