Plant viruses can be passed on by insects, nematodes and pollen, but mechanical means (such as budding and grafting) are the most common methods of transmission. Viruses can adversely affect the bud take, tree vigor and cropping of all genus of tree fruits. The effects can range from weak trees to death of the tree. See Common Viruses. When propagating their stock growers and nursery owners are encouraged to use the certified virus indexed Budwood Orchard operated by PICO.