Species and genetic variability of sweet potato viruses in China

China is the world’s largest producer of sweet potato (Ipomoea batatas (L.) Lam.). Considering that there are numerous sweet potato-producing regions in China and sweet potato is a vegetatively propagated crop, the genetic diversity of sweet potato viruses could be high in the country. However, studies on species and genetic variabilities of sweet potato viruses in China are limited, making it difficult to prevent and control viral diseases in this crop. During 2014–2019, sweet potato samples with viral disease-like symptoms were randomly collected from sweet potato fields in 25 provinces in China. Twenty-one virus species, including 12 DNA and 9 RNA viruses, were identified in the samples using next-generation sequencing, polymerase chain reaction and rolling-circle amplification methods. One novel sweepovirus species, Sweet potato leaf curl Hubei virus (SPLCHbV), was identified. Two species, Sweet potato collusive virus and Tobacco mosaic virus, were identified for the first time in sweet potato in China. Full-length or nearly full-length genomic sequences of 111 isolates belonging to 18 viral species were obtained. Genome sequence comparisons of potyvirus isolates obtained in this study indicate that the genome of sweet potato virus 2 is highly conserved, whereas the other four potyviruses, sweet potato feathery mottle virus, sweet potato virus G, sweet potato latent virus and sweet potato virus C, exhibited a high genetic variability. The similarities among the 40 sweepovirus genomic sequences obtained from eight sweepovirus species are 67.0–99.8%. The eight sweepoviruses include 14 strains, of which 4 novel strains were identified from SPLCHbV and 1 from sweet potato leaf curl Guangxi virus. Five sweet potato chlorotic stunt virus (SPCSV) isolates obtained belong to the WA strain, and the genome sequences of SPCSV are highly conserved. Together, this study for the first time comprehensively reports the variability of sweet potato viruses in China.

First report of sweet potato chlorotic stunt virus infecting sweet potatoes in Hungary

Sweet potato chlorotic stunt virus (SPCSV), a crinivirus in the family Closteroviridae, is a quarantine pest in Europe and one of the most economically important viruses of sweet potato (Ipomoea batatas (L.) Lam) crops globally. It forms synergies with other viruses in sweet potato, leading to yield loss of 30-100% (Qin et al., 2014). In summer 2020, 62 symptomatic and 38 symptomless sweet potato vines were randomly collected in farmers’ fields in the south (Ásotthalom, Szeged) and central (Galgahévíz) parts of Hungary and transplanted in an insect-proof greenhouse. Six of the plants expressed SPCSV-like symptoms, including stunting, vein clearing and leaf purpling (Suppl1). To check for common viruses of sweet potato (Suppl2), total RNA and DNA were extracted from leaves of each of the 100 plants using Trizolate reagent (UD-GenoMed, Debrecen, Hungary) and Zenogene kit (Zenon Bio, Szeged, Hungary), respectively. Primer pair Ch2N (Suppl2) was designed using Primer3 (v. 0.4.0) to amplify a 194 bp fragment of SPCSV RNA1. Presence of the RNA viruses was checked by qPCR using qPCRBIO SyGreen 1-step qPCR kit (PCR Biosystems, London, UK), while DNA viruses were checked by PCR using DreamTaq DNA Polymerase (Thermo Scientific, Vilnius, Lithuania), followed by 1% agarose gel electrophoresis. Four samples (labelled A5.1, A6.1, A6V9-1, A6V9-2) out of the 100 tested positive for SPCSV. Two of them (A6V9-1 and A6V9-2) were co-infected with SPCSV, a badnavirus sweet potato pakakuy virus (SPPV) and a potyvirus sweet potato virus 2 (SPV2), while the other two (A5.1 and A6.1) lacked SPV2. Plants infected with SPCSV, SPV2 and SPPV displayed more severe symptoms. To confirm the results, cDNA synthesized from the four SPCSV positive samples using RevertAid first strand cDNA synthesis kit (Thermo Scientific, Vilnius, Lithuania) underwent PCR (94oC 4 min, 94oC 1 min, 53oC 30 s, 72oC 70 s and 72oC 10 min for a total of 30 cycles) using primers CL43U and CL43L for the viral heat shock protein 70 gene (Maliogka et al., 2020). An expected band size of 486 bp was obtained in all cases. The amplicon from sample A6.1 was sequenced and found to be identical to SPCSV Guatemalan isolate GT:B3:08 (acc. JF699628). RNA1 and RNA2 complete sequences from sample A6.1 were obtained via PCR amplifications of cDNA using primers (Suppl2) designed (from acc. KC888966 for RNA1 and acc. KC888963 for RNA2) to amplify overlapping fragments of West African strain of SPCSV. QIAquick gel extraction kit (QIAGEN, Hilden, Germany) was used to purify the PCR fragments, which were then cloned into pGEM-T Easy Vector (Promega, Madison, USA) and sequenced using Sanger sequencing technique (Biomi, Gödöllő, Hungary). BLASTn search revealed that RNA1 of our isolate Hun_01 (acc. MW892835) had 99.63% sequence identity to SPCSV isolate su-17-10 (acc. MK802073), while RNA2 of Hun_01 (acc. MW892836) was 99.68% similar to SPCSV isolate min-17-1 (acc. MK802078) and isolate 24-1 (acc. MK802080). Phylogenetic analysis using MegAlign (v. 7.1.0, 44.1) showed a close relationship between our isolate and those isolated in China, suggesting that they may have a common origin (Suppl1). Severe stunting and leaf yellowing symptoms developed in I. setosa indicator plants grafted with SPCSV infected sweet potato scions. qPCR test for the virus confirmed its presence in the I. setosa leaves. To the best of our knowledge, this is the first report on the occurrence of SPCSV in Hungary and the third in Europe (Valverde et al. 2004; EPPO 2021).

Molecular Identification of Sweet potato virus C on Sweetpotato in Bali, Indonesia

A survey was conducted in several sweet potato cultivations in Bali Province. Survey found that many plants exhibited potyvirus symptom, such as chlorosis blotches. This study was to determine disease incidence, detection and identification of the virus causing these symptoms on sweet potato plants in Bali. Samples were collected by purposive sampling of 10 plants from each location in Bali (Denpasar, Gianyar, Badung, Buleleng, Tabanan, Klungkung, Karangasem, Jembrana, Bangli). Disease insidence was observed based on viral symptoms in the field. Identification of nucleic acids was done using Potyvirus universal primer and DNA sequencing. Disease incidence in Bangli, Buleleng, and Denpasar Regencies was > 50%. RT-PCR and CiFor/CiRev Potyvirus universal primers successfully amplified ± 700 bp of CI genes from all samples from Bangli, while samples from 8 other districts were not amplified using the same primers. The SPVC isolate of sweet potato showed nucleotide and amino acid homology similarities with the sweet potato isolate from East Timor (MF572066), 96.8% and 97.4%, respectively and these were referred to the "Asian" strain. This indicates that SPVC has spread in East Java and Bali.

Incidence and distribution of Sweetpotato viruses and their implication on sweetpotato seed system in Malawi

A survey was carried out in 19 districts to investigate the prevalence and distribution of sweetpotato virus disease (SPVD) and its implication on the sustainability of clean seed system in Malawi. A total of 166 leaf samples were collected and tested for the presence of 8 viruses using nitrocellulose membrane enzyme-linked immunosorbent assay (NCM-ELISA). SPVD foliar symptoms were observed in 68.42% of the surveyed districts. There were significant variations in disease incidence and severity (p < 0.001) among districts, with the highest incidence in Mulanje (28.34%). Average SPVD severity score was 3.05. NCM-ELISA detected sweet potato feathery mottle virus (SPFMV, 30.54%), sweet potato mild mottle virus (SPMMV, 31.14%), sweet potato mild speckling virus (SPMSV, 16.17%), sweet potato C-6 virus (SPC6V, 13.77%), sweet potato chlorotic stunt virus (SPCSV, 22.16%), sweet potato collusive virus (SPCV, 30.54%), sweet potato virus G (SPVG, 11.38%), cucumber mosaic virus (CMV, 7.78%) either in single or mixed infections. Data from this study indicate a significant SPVD occurrence in the country, and the consequence implications towards national sweetpotato seed system.

Global Plant Virus Disease Pandemics and Epidemics

The world’s staple food crops, and other food crops that optimize human nutrition, suffer from global virus disease pandemics and epidemics that greatly diminish their yields and/or produce quality. This situation is becoming increasingly serious because of the human population’s growing food requirements and increasing difficulties in managing virus diseases effectively arising from global warming. This review provides historical and recent information about virus disease pandemics and major epidemics that originated within different world regions, spread to other continents, and now have very wide distributions. Because they threaten food security, all are cause for considerable concern for humanity. The pandemic disease examples described are six (maize lethal necrosis, rice tungro, sweet potato virus, banana bunchy top, citrus tristeza, plum pox). The major epidemic disease examples described are seven (wheat yellow dwarf, wheat streak mosaic, potato tuber necrotic ringspot, faba bean necrotic yellows, pepino mosaic, tomato brown rugose fruit, and cucumber green mottle mosaic). Most examples involve long-distance virus dispersal, albeit inadvertent, by international trade in seed or planting material. With every example, the factors responsible for its development, geographical distribution and global importance are explained. Finally, an overall explanation is given of how to manage global virus disease pandemics and epidemics effectively.

Evaluation of Selected Zambian Popular Sweet Potato Genotypes for Response to Sweet Potato Virus Disease

Aim: The objective of this study was to determine the effect of sweet potato virus disease (SPVD) on the beta carotene content, tuber weight and vine weight of selected popular sweet potato genotypes. Study Design: The experiment was laid as a randomized complete block design (RCBD) with three replications. Place and Duration of Study: The experiment was conducted for two cropping seasons (2015/16 and 2016/17) at the Zambia Agriculture Research Institute in Chilanga district of Zambia. Methodology: The uninfected (control) genotypes of Kanga, Chiwoko and Chingovwa were evaluated alongside their SPVD infected genotypes. Genotypic infection was confirmed using molecular approaches, and data was collected at harvest on beta carotene content, tuber weight and vine weight. Results: The results showed that SPVD affects the yield and beta carotene content of sweet potato. Significant differences (P< .001) for yield performance and beta carotene were observed. The yield reduction in percentage across seasons for all genotypes between the uninfected and infected genotypes ranged from 77% to 79% and 67% to 76% for tuber weight and vine weight respectively. Only Chiwoko exhibited higher levels of beta carotene among the genotypes. However, the SPVD infected Chiwoko genotype compared to the uninfected treatment produced mean beta carotene content of 39.1 µg/g and 91.5 µg/g respectively. Conclusion: SPVD reduces the tuber weight, vine weight and beta carotene content in infected sweet potato genotypes.