Occurrence and Molecular and Biological Characteristics of Apple Rubbery Wood Virus 2 in Some Apple-Producing Areas of Northern China

Apple (Malus domestica) is one of the most economically valuable horticultural crops worldwide, and its production is affected by numerous plant viruses. The occurrence and molecular and biological characteristics of apple rubbery wood virus 2 (ARWV-2) in four apple-producing areas of northern China were analyzed. The average detection rate of ARWV-2 was 39.3%, with an 86.9% rate concurrently infecting apple stem grooving virus, apple stem pitting virus, and apple chlorotic leaf spot virus and an 8.1% mixed infection with apple rubbery wood virus 1. New primers were designed to amplify the nearly complete sequences of the variants a and b of the medium (Ma and Mb) and small (Sa and Sb) fragments in 34 ARWV-2-infected samples. The results showed that the detection rates of Ma and Sa (100%) fragments were higher than those of Mb and Sb (11.8%). The coding regions of Ma and Sa segments had 67.1 to 67.8% and 72.4 to 73.9% nucleotide (nt) sequence identity with that of the corresponding Mb and Sb, respectively. Samples infected with only Ma and Sa (n = 30) shared 96.2 to 99.8% and 95.6 to 100% nt sequence identity with mixed infection samples containing Mb and Sb (n = 4), respectively. In the Ma and Sa phylogenetic trees, all isolates from this study were clustered into Groups I and II, while isolates from other countries were concentrated in Groups III and IV. In the Ma tree, the genetic distances of Group I with Groups II, III, and IV were 0.026, 0.027, and 0.042, respectively. Finally, the ARWV-2 isolate containing only Ma and Sa was inoculated in seven indicator plants with infection rates ranging from 28.6 to 94.4%. Moreover, the infection was systemic, and all inoculated plants were asymptomatic. This study revealed the occurrence of ARWV-2 in apples from northern China, identified the divergent viral segments Mb and Sb at a lower rate, and addressed the herbaceous host indicators. These findings provide important data about virus prevalence and identification and will be helpful to the formulation of efficient viral control strategies.