First Report of Watermelon Crinkle Leaf-Associated Virus-2 (WCLaV-2) in Watermelon (Citrullus lanatus) in Georgia, USA.

Abstract

Watermelon (Citrullus lanatus) is a key horticultural crop in Georgia with farmgate value of 142 million USD (2022 Farm Gate Value Report), yet faces challenges from whitefly-transmitted viruses, especially during periods of elevated whitefly populations in the fall. Foliar symptoms on watermelon plants including yellow mottling and chlorosis, wrinkling, bunching, and upward curling, were observed in experimental fields at UGA Tifton and commercial fields in Colquitt County in the fall of 2023. These were similar to those described for watermelon crinkle leaf-associated viruses (WCLaV-1 and WCLaV-2) from Florida (Hendrick et al, 2021) and Texas (Hernandez et al., 2021). The disease incidence reached 100% in both locations. WCLaV-1 was previously identified in Georgia (Adeleke et al., 2022a); however, WCLaV-2 remained undetected in further surveys (Adeleke et al., 2022b). Total nucleic acid was extracted from symptomatic leaf tissues with the MagMAXTM 96 Viral RNA isolation kit (ThermoFisher Scientific, USA), following the manufacturer's guidelines, with the omission of DNAse treatment. The presence of WCLaV-1 was identified through reverse transcription-polymerase chain reaction (RT-PCR) (Hernandez et al., 2021) in 17 out of 24 samples in Tift County, and 12 out of 15 samples from Colquitt County. Within the same set of samples, WCLaV-2 was identified in three samples from Tift County and four samples from Colquitt County by RT-PCR directed at the RdRp gene (Hernandez et al., 2021). WCLaV-1 was not detected in the three samples from Tift County that had WCLaV-2, while mixed infections of WCLaV-2 and WCLaV-1 were observed in the samples form Colquitt County. Two whitefly transmitted viruses, previously reported in Georgia were also identified as mixed infection in these samples (Table S1). The presence of WCLaV-2 was further confirmed by amplifying the movement protein (MP) gene of WCLaV-2 by RT-PCR assays (Hernandez et al., 2021). The amplicons, with expected sizes of 968bp for the RdRp gene and 562bp for the MP gene of WCLaV-2, located on RNA 1 and RNA 2 segments respectively, were directly sequenced from both directions (Genewiz, USA) from a sample collected in Tift County. The resulting data were analyzed via BLASTn search. The MP gene fragment (PP178543) shared 100% identity with isolates from Brazil (LC636074.1), Texas (MW559086.1), and Florida (MZ325858.1). RdRp gene (PP178542) shared >99.7% identity with isolates from Brazil (LC636073.1), Texas (MW559083.1) and Florida (MZ325855.1). WCLaV-1 and WCLaV-2, initially discovered in Asia (Xin et al., 2017), have been assigned to the genus Coguvirus, in the family Phenuiviridae (Walker et al., 2022). Subsequent reports from the USA (Hendrick et al., 2021; Hernandez et al., 2021), Australia (Mulholland et al., 2023), and Brazil (Maeda et al., 2022) indicate the global spread of these viruses. Watermelon is the primary host of WCLaV-2. Despite these findings, biological information, including vector relations, for both viruses and other members of the genus Coguvirus remains elusive. The impact of these viruses on watermelon production and yield in the identified regions remains largely unknown, underscoring the need for further investigations.