Alfalfa mosaic virus (AMV) was first reported in California and has also been described in Italy and Central Europe.
One of the reactions associated with AMV is the spectacular “calico symptom”, which consists of a bright yellow and green aucuba mosaic. Dark green “islands” and green bands can be seen on leaflets around the primary and secondary veins in their proximal part (photo 1). Leaf necroses and deformations are frequent; the leaves are wrinkled and folded around the primary vein. Necrosis may extend to the petioles; in which case the leaves hang down along the stems.
On tubers, necrosis begins under the skin at the stolon end, then extends to the entire tuber in the form of corky patches similar to those induced by TRV or PMTV. These internal necroses may be observed at harvest time.
AMV is transmitted by aphids in non-persistent mode (like PVY). It is transmissible by seed tubers but appears to self-eliminate from one generation to another. It can usually be detected by the Elisa test.
The potato is not a common host for this virus but it can be infected by susceptible crops such as alfalfa and clover, in neighbouring fields.