A Highly Sensitive Method to Detect Avocado Sunblotch Viroid for the Maintenance of Infection-Free Avocado Germplasm Collections

The United States Department of Agriculture (USDA) Agricultural Research Service (ARS) Subtropical Horticulture Research Station (SHRS) in Miami, FL holds a large germplasm collection of avocado (Persea americana). The recent threat of infection by laurel wilt has encouraged the creation of a backup collection at a disease-free site. Creating the backup collection is complicated by infection of some trees in the germplasm collection with avocado sunblotch viroid (ASBVd). Infected trees are frequently asymptomatic, necessitating the use of a molecular diagnostic assay. Although a reverse-transcription based assay already exists and has been used to assay all germplasm at the station, some trees showed inconsistent results. We have developed a more sensitive and specific assay involving pre-amplification of the entire viroid cDNA followed by detection using real-time PCR and a TaqMan assay. A second screening of all germplasm identified additional ASBVd -infected trees and allowed us to confidently remove these trees from the station. This method enables avocado germplasm curators to proceed with the creation of a viroid-free backup collection.