Occurrence Regularities, Tissue Distribution, and Sequence Diversity of Apple Necrotic Mosaic Virus in Apples from Liaoning Province, China

Apple necrotic mosaic virus (ApNMV) is the main pathogenic factor of apple mosaic disease in China. Liaoning Province is a major apple-producing area in Bohai Bay, Northeast China. In this study, we investigated ApNMV occurrence in 188 apple plants obtained from Liaoning and analyzed its distribution in different tissues and sequence diversity from 2019 to 2023. ApNMV was detected in 83 samples, among which 32 samples showed mosaic symptoms over 5 years; only five apple trees showed symptoms annually. All ApNMV-positive samples were grouped by the annual onset of symptoms: symptoms annually, asymptomatic annually, and interannual symptom instability. Viral diversity and variant composition did not differ significantly among the three types. The mosaic symptom onset period lasted from late April to mid-July. Regarding dynamic symptom development, lesion scope and size increased slightly from April to May but did not change significantly after May. Titer testing was used to detect ApNMV distribution in different sample tissues, and periods of apple trees that did and did not exhibit mosaic symptoms were quantified. The results showed that ApNMV titer distribution among different parts of the flowers, leaves, and 1-year-old shoots of the two phenotypes did not differ significantly. A high ApNMV titer was observed in the upper leaf and lower shoot of 1-year-old shoots, petals, and petioles. The highest ApNMV titers in apple plants with and without mosaic symptoms were observed in July and August, respectively. Our results have important implications for mosaic disease management in apples.