Detection, molecular characterisation and aspects involving the transmission of tomato chlorotic dwarf viroid in eggplant.
Tomato chlorotic dwarf viroid (TCDVd) is a pospiviroid that causes severe disease symptoms in tomato. TCDVd is also naturally found in other crops and plants, in most occasions being asymptomatic. Apart from the natural hosts reported up to now, artificial inoculations have revealed that TCDVd can infect other plants, including eggplant (Solanum melongena). In a screening of seedlings of eggplant from a breeding programme we detected a pospiviroid, which we identified as TCDVd, representing the first report of natural infection of eggplant by TCDVd. The new TCDVd isolate of eggplant was detected by reverse transcription polymerase chain reaction (RT-PCR) using primers TG21/CT20, initially designed to detect potato spindle tuber viroid. The new isolate sequence is close to a Brugmansia sanguinea isolate of TCDVd from the Netherlands, and most of the nucleotidic changes with respect to this isolate and to the reference genome sequence of TCDVd are found in the TR region. Naturally infected plants of eggplant with this TCDVd isolate did not display any disease symptoms. We demonstrated that in eggplant TCDVd is mechanically transmitted with low to moderate efficiency with cultivation practices, but not by plant-to-plant contact. Tomato plants artificially inoculated with the eggplant isolate of TCDVd tested positive for the presence of the viroid at 50 days after inoculation, but did not display any disease symptoms. Seed transmission to germinated seedlings of eggplant was variable among progenies from infected plants, ranging from 7.7% to 100.0%. Disinfection of seeds with chemical treatments with sodium hypochlorite and trisodium phosphate solutions plus thermotherapy at 80°C for 24 hr or 90°C for 6 hr was ineffective in reducing the rate of transmission by seed. We did not find evidence of horizontal transmission of TCDVd by pollen, but vertical transmission was highly efficient when healthy eggplant plants were pollinated with infected pollen. Our results indicate that asymptomatic infection of eggplant by TCDVd and high seed and pollen transmission rates may contribute to the spread of this viroid. The information we obtained is useful in order to implement measures for the prevention, control and eradication of TCDVd in eggplant crops, as well as to avoid their transmission to other hosts.