Seed Transmission of Apple Stem Grooving Virus and Citrus Concave Gum-Associated Virus in Apple ( <i>Malus</i> × <i>domestica</i> )

Viruses of cultivated apple (<i>Malus</i> × <i>domestica</i>) are believed to be transmitted nearly exclusively via vegetative propagation, with few known biotic vectors and limited evidence of vertical transmission. To evaluate the seed transmission capabilities of six viruses and one viroid of apple, a large-scale seedling grow-out experiment was conducted using seeds harvested from 51 trees infected by several combinations of six viruses and one viroid. Virus detection via multiplex PCR-based amplicon sequencing followed by RT-qPCR validation demonstrated that citrus concave gum-associated virus and apple stem grooving virus were transmitted to seedlings at rates of 4.0% (32/792) and 0.3% (3/908), respectively. No evidence of seed transmission was obtained for apple chlorotic leaf spot virus, apple green crinkle-associated virus, apple hammerhead viroid, apple rubbery wood virus 2, or apple stem pitting virus. These findings document a previously unknown mode of transmission for two widely distributed apple viruses with direct implications for breeding programs, the selection of virus-free trees, and the exchange of germplasm.