Apple proliferation (AP) disease is a bacterial affliction affecting fruit size, quality and tree vigor. The disease is caused by Candidatus Phytoplasma mali (Ca. P. mali), a bacterial parasite that lives within the host tree. It is of significant concern in Europe due to its widespread prevalence and impact on apple orchards. Attributing specific economic losses to AP disease is difficult due to the disease’s subtle symptoms. One outbreak generated an estimated €125 million in combined losses among orchards in Germany and Italy in 2001. As of 2022, AP disease has not been reported in the United States and the USDA Animal and Plant Health Inspection Service (APHIS) lists Ca. P. mali as an invasive quarantine pest.