Detection of cucurbit yellow stunting disorder virus (CYSDV) infecting watermelon in South Carolina

Authors: Kousik, Chandrasekar - Shaker; Adkins, Scott

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2020
Publication Date: 4/24/2020
Citation: Kousik, C.S., Adkins, S.T. 2020. Detection of cucurbit yellow stunting disorder virus (CYSDV) infecting watermelon in South Carolina. Plant Health Progress. https://doi.org/10.1094/php-03-20-0016-br.
DOI: https://doi.org/10.1094/php-03-20-0016-br

Interpretive Summary: Watermelon is an important vegetable crop grown in 44 states in the U.S.A. Many diseases and pests attack watermelon seedlings and plants and reduce their yield resulting in monetary loss for growers. Diseases caused by viruses have become a major problem in the southeastern USA. Cucurbit yellow stunting disorder virus (CYSDV) is one such virus that has been found infecting watermelon and other cucurbits in Florida and Georgia. However, this virus was not found before in South Carolina. In 2019, ARS researchers for the first time identified the presence of the virus CYSDV in South Carolina which is transmitted by whiteflies. Results of the study suggest that watermelon growers in the state need to be aware of the presence of this virus and take appropriate management actions when necessary including managing whiteflies when growing transplants in greenhouses and later in the field. The results of this study will be useful to watermelon growers, extension workers, seed industry, regulatory agencies and University researchers to be aware of the presence of the virus in South Carolina and help take appropriate action when necessary.

Technical Abstract: Watermelon (Citrullus lanatus) is an important cucurbit crop grown in 44 states in the United States of America (USA). South Carolina had the seventh largest watermelon production area in the U.S. with 4,500 acres in 2018 valued at approximately $17 million (www.nass.usda.gov/Surveys/Guide_to_NASS_Surveys/Vegetables/index.php). In the past few years, whitefly-transmitted viruses including cucurbit yellow stunting disorder virus (CYSDV), squash vein yellowing virus (SqVYV) and cucurbit leaf crumple virus (CuLCrV) have become an economic problem in watermelon production in Georgia and Florida in the southeastern U.S. (Gadhave et al. 2018; Polston et al. 2008; Turechek et al. 2010). In late 2017, CuLCrV was identified in South Carolina for the first time (Keinath et al. 2018). In June 2019, four watermelon plants displaying symptoms of virus infection including stunting, leaf crumpling, interveinal chlorosis, and necrosis of the leaf margins were observed in a research trial at the USDA, ARS, U.S. Vegetable Laboratory farm in Charleston, SC. Whiteflies were also observed on the abaxial surface of the leaves. One plant was tested for the presence of CuLCrV, CYSDV, SqVYV and aphid-transmitted papaya ringspot virus (PRSV) by reverse transcription-polymerase chain reaction (RT-PCR) using previously described primers (Keinath et al. 2018; Rubio et al. 2001; Turechek et al. 2010). Amplicons of the expected size were only observed for CuLCrV (~1 kb) and CYSDV (~0.7 kb). Since CYSDV was not known to be present in South Carolina, we continued to monitor the field regularly for virus symptoms during the summer season. In mid-August 2019, 15 additional plants (1.9% incidence) in the field were observed with similar symptoms of virus infection. Total RNA was extracted from all 15 samples using a Direct-zol RNA isolation kit (Zymo Research, Irvine, CA) per manufacturer’s instructions and tested for CYSDV, SqVYV and PRSV by RT-PCR as described above. Total DNA was also extracted using a DNeasy Plant Mini kit (Qiagen, Germantown, MD) and tested for CuLCrV by PCR as described above. Amplicons of the expected size were again observed for CYSDV (~0.7 kb) and CuLCrV (~1 kb) from all 15 samples. No amplicons were obtained for SqVYV or PRSV. To confirm the presence of CYSDV, a second set of primers specific for the Hsp70h gene (Kou et al. 2007) was also used for RT-PCR. Amplicons of the expected size (~0.15 kb) were observed from all 15 samples. All three amplicons (two CYSDV and one CuLCrV) were cloned from six representative samples, sequenced and analyzed. Amplicon sequences from three samples were deposited in GenBank. Partial CYSDV coat protein sequences for these samples (GenBank accession nos. MN807937-MN807939) were 100% identical to each other and to CYSDV isolates from Georgia, Florida, Arizona and California (GenBank accession nos. MF960766, EU596528, FJ492808 and AF312799, respectively). Similarly, partial CYSDV Hsp70h gene sequences for these samples (GeneBank accession nos. MN807940- MN807942) were 100% identical to each other, and 97 to 100% identical to CYSDV isolates from Arizona, California, Lebanon and Spain (GenBank accession nos. FJ492808, EU596530, KC633820 and AY242078, respectively). Partial CuLCrV DNA A sequences (GenBank accession nos. MN807943- MN807945) for these samples were 99-100% identical to each other, and 99% identical to prior South Carolina CuLCrV sequences (GenBank accession nos. MG920141 and MH013228). The geographic range of CYSDV and other whitefly-transmitted viruses continues to expand into and within major cucurbit producing regions with each production season, especially in the southeastern U.S. This is the first report of CYSDV infecting watermelon or any other plant in South Carolina, although the virus has been described from other state in USA including Florida and Georgia in the southeast, and Arizona