Sweet potato feathery mottle virus is the casual agent of Sweet Potato Virus Disease (SPVD) in Italy

G. Parrella¹*, A. De Stradis² and M. Giorgini¹

¹ Istituto per la Protezione delle Piante del CNR, Via Università, 133, 80055, Portici (Na), Italy
² Istituto di Virologia Vegetale del CNR, Via Amendola, 165/A, 70126, Bari, Italy

Accepted for publication 07/03/06

Sweet potato (Ipomoea batatas) plants with virus-like symptoms were observed in many fields in Latina province (Lazio, Central Italy) during early spring, 2005. The percentage of diseased plants ranged from 30 to 60%, depending on the inspected field. Initial symptoms consisted of vein clearing and leaf chlorotic spots (Fig. 1), followed by general leaf yellowing, leaf area reduction, plant stunting, with a significant reduction of tuber size. 

Figure 1: Symptoms of vein clearing and chlorotic spots observed
on the leaves of SPFMV-infected sweet potato, grown in Central Italy

Observations of leaf-dip preparations by electron microscopy revealed elongated virus particles with a modal length of about 840 nm; whereas in ultrathin sections potyvirus type-IV cylindrical inclusions bodies (CIBs) were observed (Fig. 2; Edwardson & Chrisitie, 1996). Extracts from symptomatic leaf samples were tested by immuno-electron microscopy (IEM) and enzyme-linked immunosorbent assay (ELISA), using a panel of antibodies to Sweet potato feathery mottle virus, Sweet potato chlorotic stunt virus, Sweet potato mild mottle virus, Sweet potato chlorotic fleck virus and Sweet potato chlorotic stunt virus. A weak reaction was observed only with Sweet potato feathery mottle virus (SPFMV) polyclonal antiserum (obtained from the International Potato Center, Peru), in both tests. In ELISA, the mean optical density of samples from symptomatic plants was between two and three times the values from healthy controls.

Figure 2: A. Virus particle observed in a negatively-stained leaf-dip preparation, made from a symptomatic sweet potato leaf (bar correspond to 200 nm). B. Type-4 cylindrical inclusions bodies observed in an ultrathin section taken from the same material (48,600 times magnification). Labels: Bd=bundles; La=short curved laminated aggregates; Pw=pinwheels; Sc=scrolls; Cw=cell wall; N=nucleus; C=chloroplast

The presence of SPFMV was confirmed using RT-PCR amplification on total RNA extracted from symptomatic leaves by using a universal Potyviridae forward primer (5’-GG(G/C/T)AA(C/T)AA (C/T)AG(C/T)GG(A/G/T)CA(A/G)CC-3’; Gibbs & Mackenzie, 1997) and a specific SPFMV reverse primer (5’-TTGCACACCCCTCATTCC(C/T)AAG-3’). An amplification product of approximately 1570 bp was produced only from samples with symptoms but not from healthy controls. The amplicon was cloned and sequenced in both orientations (EMBL Acc. No. AM076411). BLAST analysis of the sequenced amplicon revealed the highest percentage of identity (94%) with the Sor isolate of SPFMV from Uganda (EMBL Acc. No. AJ539129). Further analysis of the Italian isolate (designated SPFMV-Ita1), based on both the predicted size of the putative coat protein-encoding region (939 nucleotides) and the phylogenetic relationship between it and other SPFMV isolates (Fig. 3), indicates that SPFMV-Ital belongs to SPFMV subgroup C (Tairo et al., 2005).

Figure 3: Phylogenetic tree obtained from ClustalW alignments of the partial nucleotide sequence of the coat protein genes of selected SPFMV with the sequence of the Italian isolate (SPFMV-Ita1). Sweet potato mild mottle virus (Acc. No. AJ783452) was used as sequence outgroup. The number at nodes indicate bootstrap confidence values (1000 replicates); only values ≥ 75% were reported. The position of SPFMV-Ita1, inside the cluster of isolates belonging to subgroup C, is indicated by an arrow.

SPFMV is the most widespread virus infecting sweet potato and possibly occurs wherever sweet potato is grown (Brunt et al., 1996). However, to our knowledge this is the first report of SPFMV from Italy.

Acknowledgements

The authors would like to thank Dr. H. Josef Vetten (Dept. Plant Virology, Microbiology and Biosafety, BBA, Braunschweig, Germany) for his valuable help with the serological assays.

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Edwardson JR, Christie RG, 1996. Cylindrical inclusions. Gainesville, Florida, USA: University of Florida Agricultural Experiment Station: Bulletin No. 894.

Gibbs A, Mackenzie A, 1997. A primer pair for amplifying part of the genome of all potyvirids by RT-PCR. Journal of Virological Methods 63, 9-16

Tairo F, Musaka SB, Jones RAC, Kullaia A, Rubaihayo PR, Valkonen JPT, 2005. Unravelling the genetic diversity of the three main viruses involved in Sweet Potato Virus Disease (SPVD), and its practical implications. Molecular Plant Pathology 6, 199-211

The British Society for Plant Pathology