Apple cultivars and rootstocks assay for the identification of diverse viruses and healthy genotypes for breeding

The prevalence of harmful viruses, viz., apple stem grooving virus (ASGV), apple stem pitting virus (ASPV), apple chlorotic leaf spot virus (ACLSV), apple mosaic virus (ApMV), and tomato ringspot virus (ToRSV) in apple tree plantations in the Ryazan, Yaroslavl, and Moscow regions, Russian Federation, based on genotype features, planting type (industrial, collection, and repository), and tree age was studied during 2018–2021. The prevalence of harmful and latent viruses ranged from 49.6% to 53.8% in apple cultivars and from 8.3% to 100% in apple clonal rootstocks. The most common virus in the Moscow and Yaroslavl regions was ACLSV (34.7% and 53.8%) and that in the Ryazan region was ApMV (33.3%). Relative to that in young apple trees, the virus influence in older tree tissues was higher by 20% to 43% depending on virus type. Monoviral infection prevailed (59% of all trees were infected with one virus) in the studied apple tree cultivars. Of the cultivars, 25% were infected by a complex of two viruses (ASPV + ACLSV), 10% were infected by three viruses, and 6% were infected by four viruses. The highest prevalence of latent viruses was observed in old Russian cultivars (53.5%) and selections from old foreign cultivars (57.2%). Virus occurrence was slightly lower (51.6%) in new Russian cultivars than in other cultivars. Columnar apple tree cultivars had the lowest virus occurrence (30%) and were found to be more tolerant than other genotypes. The highest virus incidence was recorded in industrial orchards (63.4%), followed by that recorded in collection (20.8%) and repository (18.3%) plantations. By using ELISA, the apple genotypes that were free from harmful viruses were identified as source plants, i.e., 183 plants from 18 apple tree cultivars and 131 plants from clonal rootstocks. Results indicated that virus-free apple germplasm is highly effective for the successful implementation of breeding and genetic improvement.