Pepino mosaic virus: a successful pathogen that rapidly evolved from emerging to endemic in tomato crops

Taxonomy: Pepino mosaic virus (PepMV) belongs to the Potexvirus genus of the Flexiviridae family. Physical properties: PepMV virions are nonenveloped flexuous rods that contain a monopartite, positive-sense, single-stranded RNA genome of 6.4 kb with a 3' poly-A tail. The genome contains five major open reading frames (ORFs) encoding a 164-kDa RNA-dependent RNA polymerase (RdRp), three triple gene block proteins of 26, 14 and 9 kDa, and a 25-kDa coat protein. Genome diversity: Four PepMV genotypes, with an intergenotype RNA sequence identity ranging from 78% to 95%, can be distinguished: the original Peruvian genotype (LP); the European (tomato) genotype (EU); the American genotype US1; and the Chilean genotype CH2. Transmission: PepMV is very efficiently transmitted mechanically, and a low seed transmission rate has been demonstrated. In addition, bumblebees have been associated with viral transmission. Host range: Similar to other Potexviruses, PepMV has a rather narrow host range that is thought to be largely restricted to species of the Solanaceae family. After originally being isolated from pepino (Solanum muricatum), PepMV has been identified in natural infections of the wild tomato species S. chilense, S. chmielewskii, S. parviflorum and S. peruvianum. PepMV is causing significant problems in the cultivation of the glasshouse tomato Solanum lycopersicum, and has been identified in weeds belonging to various plant families in the vicinity of tomato glasshouses. Symptomatology: PepMV symptoms can be very diverse. Fruit marbling is the most typical and economically devastating symptom. In addition, fruit discoloration, open fruit, nettle-heads, leaf blistering or bubbling, leaf chlorosis and yellow angular leaf spots, leaf mosaic and leaf or stem necrosis have been associated with PepMV. The severity of PepMV symptoms is thought to be dependent on environmental conditions, as well as on the properties of the viral isolate. Minor nucleotide sequence differences between isolates from the same genotype have been shown to lead to enhanced aggressiveness and symptomatology. Control: Prevention of infection through strict hygiene measures is currently the major strategy for the control of PepMV in tomato production. Cross-protection can be effective, but only under well-defined and well-controlled conditions, and the effectiveness depends strongly on the PepMV genotype.