A Review of Viruses Infecting Yam (Dioscorea spp.)

Abstract

Yam is an important food staple for millions of people globally, particularly those in the developing countries of West Africa and the Pacific Islands. To sustain the growing population, yam production must be increased amidst the many biotic and abiotic stresses. Plant viruses are among the most detrimental of plant pathogens and have caused great losses of crop yield and quality, including those of yam. Knowledge and understanding of virus biology and ecology are important for the development of diagnostic tools and disease management strategies to combat the spread of yam-infecting viruses. This review aims to highlight current knowledge on key yam-infecting viruses by examining their characteristics, genetic diversity, disease symptoms, diagnostics, and elimination to provide a synopsis for consideration in developing diagnostic strategy and disease management for yam.

Keywords: Dioscorea spp.; badnavirus; macluravirus; potyvirus; yam mild mosaic virus; yam mosaic virus; yam viruses.

Figure 1

Typical virus symptoms on yam. (A) D. rotundata TDr 99/02674 showing mosaic symptoms; (B) D. rotundata cv. Adaka showing chlorotic leaf discoloration, and (C) mottling; (D) D. rotundata TDr 00/00168 showing leaf deformation ((A–D) source G.R.E. Silva (NRI) [20]); (E) D. rotundata leaf discoloration (Source: Stephen Winter, DSMZ); (F) D. alata showing diffused mottling.

Figure 2

Schematic genome representation of species Yam mosaic virus (YMV), genus Potyvirus, family Potyviridae. The 11 functional proteins represented are P1: first protein (translation, modulator of replication), HC-Pro: helper component protease (silencing suppression and aphid transmission), P3: third protein (virus replication and movement), 6K1 (formation of replication vesicles), CI: cylindrical inclusion protein (virus movement and replication), 6K2 (formation of replication vesicles), VPg: viral genome-linked protein (translation, virus movement and replication), NIa: major protease of small nuclear inclusion protein (polyprotein processing), NIb: large nuclear inclusion protein (RNA-dependent RNA polymerase), CP: coat protein (virus movement, virion formation, and aphid transmission) and PIPO: pretty-interesting-potyvirus-ORF (virus movement).

Figure 3

Schematic representation of species Chinese yam necrotic mosaic virus (CYNMV), genus Macluravirus genome, family Potyviridae. The 10 functional proteins represented are HC-Pro: helper component protease (gene silencing and vector transmission), P3: third protein (virus replication), 7K, CI: cylindrical inclusion protein (virus movement and replication), 9K, VPg: viral genome-linked protein (viral replication), NIa: major protease of small nuclear inclusion protein (polyprotein processing), NIb: large nuclear inclusion protein (RNA-dependent RNA polymerase), CP: coat protein (virus movement and virion formation) and PIPO: pretty-interesting-potyvirus-ORF (virus movement).

Figure 4

Schematic representation for the linearised genome organization of members of the genus Badnavirus, family Caulimoviridae. ORF1 encodes P1 protein (function unknown), ORF2 encodes P2 virion-associated capsid protein (nucleic acid binding activity), ORF3 encodes polyprotein P3 (VMP, CP, AP, RT, RNase H1). tRNA^met binding site designated as the origin of the circular genome.

Figure 5

Schematic representation for the linearised genome organization of species Dioscorea nummularia-associated virus (DNUaV) genus Dioscovirus, family Caulimoviridae. ORF1 encodes P1 protein (function unknown), ORF2 encodes P2 (function unknown), ORF3 encodes polyprotein P3 (VMP, CP, AP, RT, RNase H1) and ORF4 encodes P4 (TAV). tRNA^met binding site designated as the origin of the circular genome.

Figure 6

Schematic representation of the genome of species Cucumber mosaic virus (CMV), genus Cucumovirus, family Bromoviridae. The proteins encoded by each RNA are 1a: replication protein, 2a: RNA-dependent RNA polymerase, 2b: silencing suppressor, 3a: movement protein, and 3b: capsid protein.