Incidence of major pome fruit tree viruses and viroids in commercial pear orchards in China and in Pyrus betulifolia seedlings

Using reverse transcription (RT) PCR and quantitative RT-PCR, we showed the presence of apple stem grooving virus (ASGV), apple stem pitting virus (ASPV), apple chlorotic leaf spot virus (ACLSV), and apple scar skin viroid (ASSVd) in all tested parts of pear plants from China. Relative RNA accumulation of ASGV, ASPV, and ASSVd in the annual branch phloem of different pear cultivars displayed different seasonal variations, but all showed relatively high RNA accumulation in October. Based on these results, 821 samples (annual branches) in October 2018 and 2019 were collected from major pear-growing areas to evaluate the phytosanitary status of pear orchards in China by RT-PCR-based testing and sequencing. ASGV was found in 91.2% of the samples, followed by ASPV (74.8%) and ASSVd (46.9%); in contrast, ACLSV, pear blister canker viroid, and apple mosaic virus were rarely detected, if at all. About 79.5% of the samples showed mixed infections, the most common being of ASGV+ASPV+ASSVd. Two batches of Pyrus betulifolia seedlings were tested for the presence of the above viruses and viroids; one batch displayed 97.9% infection rate of ASSVd, including 14.1% of seedlings coinfected with ASGV; the other batch showed 0.9%, 1.3%, and 0.9% infection rates with ASGV, ASPV, and ACLSV, respectively. This is the first extensive report on the incidence of major pome fruit tree viruses and viroids in pear orchards in China, and also provides information on the occurrence of viruses and viroids in pear rootstock seedlings.