Seed transmission rates of 0-100% have been reported for PSTVd
in potato, and similarly variable rates have been reported in tomato
[17-20]. In tomato it was reported that seed harvested when only the
male parent was infected resulted in PSTVd being transmitted to 9%
of the seedlings, and when the female parent was infected PSTVd was
transmitted to 6% of the seedlings. In the same study, it was determined
that the transmission rate increased to 11% when both the parents were
infected [20]. Others have reported a lack of seed to seedling infection;
for example, no viroid was detected in seedlings (tested at 8, 16 and 27
week stage) grown from infected pepper or tomato seeds [21]. Given
the variability in reported transmission rates combined with the high
probability of transmission occurring due to contact it is extremely
important to evaluate and quantify the risk of seed transmission.