Emerging and Reemerging Virus Diseases of Plants

Abstract

A number of RNA plant viruses constitute new threats to tomato, cucurbits, grapevine, and citrus. These are referred to as ‘emerging or re-emerging’ as they are new viruses or virus strains causing new and severe diseases. The most relevant examples include viruses belonging to a number of different genera with filamentous particles, namely pepino mosaic virus (PepMV) (genus Potexvirus family Flexiviridae ), criniviruses (genus Crinivirus , family Closteroviridae ), cucumber vein yellowing virus (CVYV) (genus Ipomovirus , family Potyviridae ), variants of grapevine leafroll-associated virus 2 (GLRaV-2) (genus Closterovirus , family Closteroviridae ), and citrus sudden death-associated virus (CSDaV) (genus Marafivirus , family Tymoviridae ). Viral infections induce symptoms on the leaves that resemble herbicide damage (PepMV) or yellowing of the veins (CVYV) or the whole leaf blade (criniviruses). Economic losses may be relevant in glasshouse-grown crops. In the so-called graft incompatibility of grapevine, symptoms consist in a generalized decline that develops dramatically in certain scion–rootstock combinations. Affected vines may show prominent swelling at the scion/rooststock junction (Kober 5BB imcompatibility, young vine decline) sometimes accompanied by necrosis at the graft union (necrotic union), and variously extended necrotic lesions that develop on the roostock stem (roostock stem lesion, stem necrosis). An ordinary strain of grapevine leafroll-associated virus 2 (GLRaV-2) is consistently associated with Kober 5BB incompatibility, while molecular and biological variants of GLRaV-2 (e.g., GLRaV-2 RG) are involved in the other disease patterns. Symptoms of citrus sudden death (CSD) disease resemble very much those elicited by citrus tristeza, although they develop on trees grafted on tristeza-resistant rootstocks. CSD-affected plants show a generalized yellowing of the leaves that precedes intense defoliation and root system decay. Infected plants host a strain of citrus tristeza virus (CTV) and, with nearly 100% association, a spherical virus denoted CSDaV. Recommended control strategies focus on sanitation, use of certified propagation material, and control of the known vectors.