Identification of the Begomoviruses Squash Leaf Curl Virus and Watermelon Chlorotic Stunt Virus in Various Plant Samples in North America

Geminiviruses are a group of plant-infecting viruses with single-stranded DNA genomes. Within this family, viruses in the genus Begomovirus are known to have a worldwide distribution causing a range of severe diseases in a multitude of dicotyledonous plant species. Begomoviruses are transmitted by the whitefly Bemisia tabaci, and their ssDNA genomes can be either monopartite or bipartite. As part of a viral survey, various plants including those in the families Alliaceae, Amaranthaceae, Apiaceae, Asteraceae, Brassicaceae, Cactaceae, Cucurbitaceae, Lamiaceae, Lauraceae, Malvaceae, Oleaceae and Solanaceae were sampled and screened for begomoviruses using both a high-throughput sequencing and a begomovirus-specific primer pair approach. Based on the sequences derived using these approaches, the full-length genome of various begomoviruses were amplified from plants using abutting primers. Squash leaf curl virus (SLCV) and watermelon chlorotic stunt virus (WCSV) were identified in Cactaceae (n = 25), Solanaceae (n = 7), Cucurbitaceae (n = 2) and Lamiaceae (n = 1) samples. WCSV is an Old World bipartite begomovirus that has only recently been discovered infecting watermelons in the Americas. Our discovery of WCSV in the USA is the first indication that it has reached this country and indicates that this virus might be widespread throughout North America. Phylogenetic analysis suggests WCSV was introduced to the New World twice. The detection of begomoviruses in cactus plants suggests possible spillover events from agricultural areas into native vegetation. Since WCSV and SLCV have previously been found in mixed infections, pseudo-recombination infection experiments were conducted. We demonstrate that WCSV DNA-B is successfully trans-replicated by SLCV DNA-A despite very low degree of similarity between the replication-associated iterative sequences present in their common region, an essential feature for binding of the replication associated protein. This study highlights the importance of viral surveys for the detection of spillover events into native vegetation, but also suggests the need for more surveillance of WCSV in the USA, as this virus is a serious threat to watermelon cultivation in the Middle East.

Watermelon Chlorotic Stunt Virus is Associated with Cucumber Yellow Mosaic Symptoms in Oman

Cucumber (Cucumis sativus; family Cucurbitaceae) plants exhibiting begomovirus-like symptoms such as yellowing, mosaics and stunting were studied using cloning, sequencing, Species Demarcation Tool followed by phylogenetic clustering. The complete genome of DNA-A showed maximum sequence identity of 98.7% with the corresponding DNA-A of an isolate from “Iran” strain of Watermelon chlorotic stunt virus (WmCSV). The DNA-B displayed 97.5% nt identity with the component of DNA-B of WmCSV from Iran, too. Our results confirmed that yellowing and mosaic symptoms of cucumber are associated with a bipartite begomovirus (WmCSV). This study is the first characterization of WmCSV in association with described symptoms in cucumber from Oman. © 2021 Friends Science Publishers

Molecular Detection of Begomoviruses Infecting Some Vegetable Crops in Saudi Arabia

Begomoviruses have a huge impact on crop production worldwide. Investigations of begomviruses in four vegetable crops namely cucumber, squash, tomato, and watermelon were conducted by visual symptoms and molecular analysis using PCR with universal primers for begomoviruses. Obtained sequencing results of the core Coat Protein confirmed the presence of three already reported begomoviral species. TYLCV was detected on tomato samples exhibiting leaf curling and yellowing. Squash and greenhouse-grown cucumber plants were infected with the Oman strain TYLCV-OM. Watermelon chlorotic stunt virus (WmCSV) isolates were detected in watermelon and greenhouse cucumbers. The begomoviral species ToLCPMV shared 99% identity with the Iranian isolates that are known to be very destructive. Integrated pest management should be implemented against whitefly (Bemisia tabaci) to prevent the spread of begomoviruses on vegetable crops.

Insights Into a Watermelon Virome Contribute to Monitoring Distribution of Whitefly-Borne Viruses

The composition of a plant virome may represent spatiotemporal patterns of plant virus abundance. Using next generation sequencing, we investigated the viromes of watermelon fruits grown in two adjacent open fields located in Eastern Israel: Kalia and Mitzpe-Shalem. The two viromes were comprised of distinct virus species and genera. Studying spatial and temporal effects on virus occurrence, we detected the crinivirus Cucurbit yellow stunting disorder virus in Kalia and not in Mitzpe-Shalem, irrespective of collection time. A spatial effect was also observed regarding the occurrence of the begomovirus Watermelon chlorotic stunt virus, which was not detected in watermelons from Kalia, but rather in watermelon fruit in Northern Israel in 2018. A temporal effect was observed on the appearance of the ipomovirus Cucumber vein yellowing virus, which was detected in watermelons from Kalia in 2017 and was absent in the 2016 virome analysis. Importantly, regardless of temporal and spatial effects, the crinivirus Cucurbit chlorotic yellows virus, which was new to Israeli landscape, was detected in all the tested watermelon plants. These changes in viral abundance were associated with an early whitefly (Bemisia tabaci) occurrence, which might be the cause for the severe disease spread in watermelons.