Identification of individual plants carrying viruses or a viroid in Pyrus betulifolia seedlings and virus-tested pear mother plants by group testing

Monitoring the sanitary status of a large population of vegetative propagation materials is a challenge because it requires expensive and time-consuming testing. Here we report a group testing strategy to identify individual plants carrying viruses and/or a viroid in Pyrus betulifolia seedlings and virus-tested pear mother plants. First, the effect of pool size on RT-PCR detection of apple stem grooving virus (ASGV), apple stem pitting virus (ASPV), apple chlorotic leaf spot virus (ACLSV) and apple scar skin viroid (ASSVd) was evaluated; all of them could be sensitively detected in a bulk (pool) of eight samples. The virological condition of 2770 P. betulifolia seedlings and 110 virus-tested pear mother plants were then investigated by RT-PCR using a grouping strategy with 5 samples. We identified 20, 35 and 3 P. betulifolia seedlings as positive for ASGV, ASPV, and ASSVd, respectively, and 4 pear mother plants were identified as carrying ASGV. Our data indicated that RT-PCR testing on samples pooled before RNA extraction is a reliable and resource-efficient strategy to monitor the sanitary status of large numbers of pear vegetative propagation materials. The results presented here also highlight the possibility of pear seedling rootstocks being a source of viral infection.