You can't really "immunize" a plant agaist a virus, but there is a
phenomenon known as "cross protection". Infection of a plant with one
strain of a virus will protect it from infection with similar strains.
This can be used as an innoculation if their is a wild strain of a
virus which will protect from a severe strain. It is easy to indroduce
TMV into a plant. Just rub a solution of TMV on a leaf (better if you
use an abrasive to create small wounds, but with TMV it will probably
work anyway). Other viruses can be much more difficult to introduce.
Hope that helps,
St. Mary's College of Maryland
St. Mary's City, MD 20686
eestrauss at osprey.smcm.edu