The organism responsible for this disease of apples, stone fruits, and other perennial fruit trees is a phytoplasma: a very small bacterium lacking a cell wall and enclosed in a single membrane. Inhabiting primarily the phloem tissue of plants, it spreads from infected trees to healthy ones by tissue-feeding insects such as leafhoppers and psyllids. Of importance to the tree fruit industry, the disease can also spread via the use of infected plant material during propagation of scion wood and rootstocks used for grafting. Apple proliferation phytoplasma occurs in Europe and, if it spreads into North America, could have a significant negative impact on tree fruit production.