Experiments on the Spread of Potato Virus X Between Plants in Contact

Experiments on the spread of five strains of potato virus X were made with seven potato varieties and with tomato plants both under glass and in the field. Spread by leaf contact between healthy and infected plants was confirmed, and it was also found that spread could occur between plants whose only contact was below ground.
The rate of spread was much greater in tomato than in potato plants, and virulent strains of the virus, which achieve a high concentration in infected plants, spread more rapidly than avirulent strains. In only one experiment with potatoes did more than 10% of the healthy potato plants exposed to infection become infected during one season.
Datura stramonium and tomato plants became infected when growing in soil containing sap or residues from X -infected plants.
It was common in the field for potato plants whose foliage gave no reaction for virus X at the end of the season to yield a mixed progeny of healthy and infected tubers. Such infections are thought to result from underground spread.
Attempts to transmit virus X from infected to healthy potatoes by means of Rhizoctonia solani failed. No examples of infection were found except when healthy plants came into direct contact with sources of the virus.