Yellow streaks, mottling, flecks or mosaic patterns appear on the leaf. The whole plant may be stunted or distorted.
Cause
There are a wide range of different viruses. Many are carried by sap-sucking insects. They can pick up a virus from an infected plant and carry it around with them infecting new plants as they feed.
Harm
Viruses can drastically weakening a plant and cause premature death.
Treatment
There are no cures for viruses. If there are only one or two infected leaves they can be taken off and burnt. If badly affected, the whole plant should be dug up and burnt.