Submitted to: Acta Horticulture Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/1995
Publication Date: N/A
Interpretive Summary: Several Asian pear cultivars in quarantine have been shown to be infected with the exotic apple scar skin viroid. Pear cultivars infected with the viroid cannot be released from quarantine into the U.S. unless the viroid is eliminated from infected tissue. We have developed methods for successful viroid elimination from Chinese pear tissue by in vitro thermotherapy and apical meristem tip culture. To our knowledge, this is the first successful report for eliminating viroid from infected pome fruits. The information developed will be useful to quarantine officials, research plant pathologists and nurserymen.
Technical Abstract: Shoot tips from the Chinese pear cultivar "Liu Yue Shian" infected with Apple Scar Skin Viroid (ASSVd) were established in vitro and multiplied on Cheng's medium containing benzyladenine(1mg/L). Following in vitro multiplication, infection was verified in each sample by reverse transcription-polymerase chain reaction(RT-PCR) using DNA primers specific for ASSVd. All in vitro plants tested positive for ASSVd. Plants were transferred to a medium without hormones in heat-sealed, semi-permeable, plastic bags for treatments. Cultures were either (1) maintained in a growth room at 22øC (control); (2) grown in a chamber at temperatures alternating between 30øC and 38øC every 4 hr; or (3) cold hardened for 7 days at temperatures alternating between 22øC (8 hr) and -1øC (16 hr), and then moved to a cold room and grown at 4øC. After 49-55 days, apical meristems approximately 0.5 mm long were dissected from each culture and grown into plantlets in vitro. RT-PCR tests for ASSVd after 3 months growth revealed that 86% of plantlets from heat treated meristems and 85% from cold treated meristems were viroid negative. Only 17% of control plantlets tested negative for the viroid. Of 26 in vitro plantlets from all treatments, 11 were successfully established in the greenhouse by tip-grafting onto healthy pear rootstocks. RT-PCR assays of 9 grafted trees for ASSVd, 9 weeks after grafting, produced identical results to assays of the corresponding in vitro cultures. In vitro heat or cold therapy combined with apical meristem culture eliminates ASSVd from infected pear trees and reduces the risk of introducing this pathogen from Asia to the U.S.