Cassava common mosaic disease (CCMD) has been reported in all regions where cassava is grown in the Americas and the causal agent, Cassava common mosaic virus (CsCMV), has been identified as a mechanically transmitted potexvirus (Alphaflexiviridae). In Argentina, cassava is grown mainly in the northeast (NEA) region that shares borders with Brazil and Paraguay. Increasing incidences of CCMD were observed during the years 2014 to 2016 associated with severe leaf mosaic symptoms and yield reductions where the occurrence of CsCMV was confirmed by RT‐PCR and sequencing. In this work, the virus has been successfully purified and a double‐antibody sandwich (DAS‐) ELISA test has been developed from an Argentinean isolate of CsCMV to extend the diagnostics of the disease. A collection of 726 samples was screened and CsCMV was detected with 100% prevalence in the NEA region. Additional co‐infecting viruses were detected in some plants (64.4%); in these, CCMD symptoms correlated with CsCMV only, although more severe symptoms could be observed in mixed infected plants. Sequence analysis of the conserved RdRp domain showed a wider diversity of CsCMV isolates. Interestingly, a separate phylogenetic cluster was formed by isolates from the NEA region that only shared 77.1% to 80.3% nucleotide identity with the other clusters. These results indicate the presence of mixed strains occurring in the NEA region and suggest the presence of geographically distinct strains of CsCMV in South America.

Cassava common mosaic virus (CsCMV) is a potexvirus that causes systemic infections in cassava plants, leading to chlorotic mosaic and producing significant yield losses. To date, the physiological alterations and the mechanism underlying biotic stress during the cassava-CsCMV compatible interaction remains unknown. In this study, we found that CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts in source leaves during the course of viral infection. Extrusion of the chloroplast membrane with amoeboid-shaped appearance and disorganized grana stacks were observed in infected mesophyll cells. These alterations were associated with up to 35% reduction of relative chlorophyll content, and a decline of CO 2 fixation (13.5% and 24.2% at 90 and 210 days after planting, respectively). The effects of CsCMV infection on the performance index on absorption basis dropped up to 37%. The analysis of chlorophyll a fluorescence showed a progressive loss of both oxygen evolving complex activity and "connectivity" within the tripartite system (core antenna-LHCII-reaction centre). Here, we report the latter phenomenon for the first time in a viral infection. The oxidative stress process was observed in CsCMV-infected plants (20.8% reduction of antioxidant capacity with respect to noninfected plants). Other effects of the pathogen included reduction of starch and maltose content in source leaves, and a significant increase (24.7%) of the sucrose:starch ratio, which indicates an altered pattern of carbon allocation. Our results suggest that CsCMV induces chloroplast distortion associated with progressive chloroplast function loss and diversion of carbon flux in source leaf tissue, leading to the loss of cassava tuber yield.

Cassava is an important crop with great economic and social significance in many countries. Most of its biomass is usable: storage roots can be destined for industry or fresh consumption, leaves are a source of protein, vitamins, and minerals, and stems can be used as propagation material. Due to its vegetative propagation, cassava yield is greatly constrained by viral diseases. Cassava common mosaic virus (CsCMV), one of the most widely spread viruses in Latin America, is the only cassava‐infecting potexvirus able to cause disease in a single infection: cassava common mosaic disease (CCMD). Here, we evaluated the effect of an Argentinian strain of CsCMV on several agronomic traits. Field trials were established in north‐eastern Argentina for three successive cropping seasons, 2016/17, 2017/18, and 2018/19. CsCMV presence was confirmed by PCR or ELISA tests in mechanically inoculated cassava plants. The evaluated traits were plant height (PH), total fresh weight (TFW), shoot fresh weight (SFW), diameter and FW of tuberous root per plant (TRD and FWTR), FW of each individual tuberous root (FWiTR), length and number of tuberous roots (TRL and TRN), harvest index (HI), and estimated yield of tuberous root (EY) in both infected and noninfected plants. Significant yield losses were recorded in infected plants. The most affected traits were EY and FWTR (both showing 44.3% of losses), followed by TFW, SFW, TRN, and FWiTR (37.8%, 33.5%, 29.9%, and 24.8% of losses, respectively). These losses are extremely high for Argentina, where this crop provides food security and demand for the raw material is unsatisfied.

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454-pyrosequencing was applied as a tool to identify etiological agents involved in the symptoms observed in a sweet potato sample from Argentina. RNA was purified from symptomatic material and sequenced using a Roche 454 GS-FLX+. BLAST analysis of the viral reads identified the presence of Sweet potato feathery mottle virus (SPFMV)-O and SPFMV-RC strains and Sweet potato virus C (SPVC). For SPFMV-O and SPFMV-RC, 10 878 and 10 812 nucleotides, respectively, were sequenced, whereas a sequence of 10 793 nucleotides was obtained for SPVC. Pairwise comparison of polyprotein nucleotide sequences of O and RC Argentinian isolates (Arg) showed 99% and 98.4% sequence identities with SPFMV-O and SPFMV-RC-M2-41, respectively, whereas SPVC-Arg showed 94.2%, 92.9% and 94.6% sequence identities with SPVC-Il, SPVC-C1 and SPVCBungo, respectively. These results allowed us to develop a one-step multiplex reverse transcription-polymerase chain reaction assay (mRT-PCR) for the simultaneous detection and differentiation of SPFMV-O-Arg and SPFMV-RC-Arg and SPVC-Arg. Three specific forward primers unique to each virus and one reverse primer based on a region conserved in all three viruses were designed and used in the assay. These primers were further evaluated using field samples collected from four provinces of Argentina. The results showed that SPVC was the most common virus in samples analysed. This study shows the usefulness of deep sequencing not only to rapidly identify mixed infections between SPFMV-O and SPFMV-RC and SPVC, but also to develop a reliable mRT-PCR assay for detection of these sweet potato pathogens, which cannot be discriminated by serological techniques.

1 changes in chloroplast ultrastructure and carbohydrate metabolism of cassava 2 plants 3 4

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