Types of change in potato virus X infections

In tobacco plants infected with mild strains of virus X , severe strains may arise as mutations which multiply locally.
Several strains of virus X gradually lost infectivity for potato on continued culture in other hosts such as tobacco.
Strains that are poorly infective or invasive for potato may take considerable time to move into the growing shoot from the tuber in detectable amounts.
Pronounced and apparently spontaneous increases in the severity of the strain that dominates in potatoes may occur in field crops.