Virome analysis reveals apple mosaic virus in the monumental tree Castagno dei cento cavalli in Sicily

Summary. The Castagno dei cento cavalli (One hundred horse chestnut) is a monumental Castanea sativa (sweet chestnut) located on the eastern slope of Mount Etna in Sicily (Italy), and is the oldest (>2000 years) known sweet chestnut in the world. In the spring of 2022, symptoms indicating virus infection were observed on this tree. Attempts to visualize virus particles from extracts using transmission electron microscopy were unsuccessful, so high-throughput RNA sequencing was carried out. This identified a tripartite genome (RNA1, RNA2, and RNA3) corresponding to apple mosaic virus (species Ilarvirus ApMV). Serological analyses using polyclonal antiserum confirmed infection in symptomatic but not in asymptomatic leaf samples. This is the first report of ApMV infecting C. sativa, and this virus possibly threatens monumental trees elsewhere internationally. Although efficient ApMV vectors are lacking, limiting the risk of spread, this knowledge highlights the need for monitoring monumental trees as potential reservoirs of novel host-virus associations, and the importance of careful management of this Sicilian botanical monument.