Big Vein of lettuce is transmitted by

Big Vein of lettuce is a viral disease that affects lettuce crops.

The disease is transmitted by a soil-borne fungus called *Olpidium brassicae*.

Olpidium brassicae does not cause the disease directly but acts as a vector that transmits the Mirafiori lettuce big-vein virus (MLBVV) into the plant.

This fungus infects the roots of the lettuce plant and facilitates the entry of the virus that leads to the characteristic enlarged veins in the leaves.

Nematodes, bacteria, and aphids are known to spread other types of plant diseases, but they are not responsible for transmitting Big Vein disease in lettuce.

Therefore, the correct answer is Fungus.