First Report of Cocksfoot Mottle Virus Infecting Dactylis glomerata in Forage Production Fields in California.

Orchardgrass (Dactylis glomerata L.) is a cool season perennial grass commonly used in pastures and also grown for hay and silage. A Sobemovirus infecting orchardgrass, cocksfoot mottle virus (CfMV), was originally described in 1963 in England (Serjeant 1964). The symptoms of CfMV are visible in spring with yellow streaking in mild cases, but these symptoms can progress into whitening of the leaves, followed by necrosis and plant death in severe cases (Serjeant 1964). Although beetles (Oulemma spp.) are thought to introduce this virus into pastures, mechanical transmission is believed to be responsible for the spread of this virus once it has been introduced into an area (Serjeant 1964). This virus is associated with stand decline in orchardgrass fields in Europe, Japan, New Zealand (Mahy and Van Regenmortel 2010), Canada (Bittman et al. 2006), and recently in U.S. seed production fields in Oregon, where 79% of the orchardgrass fields surveyed in 2016 were infected with CfMV (Gilmore et al. 2017). Subsequently, in early April of 2017, several forage growers in northern California that were experiencing extensive die-off in their fields requested testing for CfMV and other pathogens. Orchardgrass samples were tested for CfMV using double antibody sandwich ELISA, with antiserum developed to an isolate of CfMV from British Columbia, Canada (R. R. Martin). Ten samples, collected (into individual plastic bags) from areas of the field with die-off, were ELISA positive for CfMV, and one was also positive for cereal yellow dwarf virus (CYDV). Five healthy samples collected from this same field tested negative for CYDV and CfMV. Total RNA was extracted from four of the CfMV-ELISA-positive samples following the nucleic acid extraction protocol of Tzanetakis et al. (2007) and used for cDNA synthesis using the SuperScriptIII First-Strand Synthesis SuperMix kit (Invitrogen by Life Technologies, CA). PrimeSTAR HS DNA Polymerase (Takara Clonetech, Japan) was used for polymerase chain reaction with CfMV-specific primers CfCP-F1 (GATGGAGCCAGTCTCTCG) and CfCP-R2 (ATCCGTCAATCTTCAAGC) (Gilmore et al. 2017) using a BioRad thermal cycler with the following program: 98°C for 2 min; 40 cycles of 98°C for 15 s, 55°C for 5 s, and 72°C for 45 s; and 72°C for 7 min. The predominant band at ≈750 bp was reamplified and sequenced using CfCP-F1 and CfCP-R2 primers. A BLAST search indicated one sample with 99% nucleotide sequence identity to CfMV reported in the Willamette Valley, OR, and three samples that were most closely related (≈95 to 96% identity) to CfMV reported in Ohio. Sequences have been submitted to GenBank (accession nos. MG765457 to MG765460). To our knowledge, this is the first report of CfMV in orchardgrass forage production fields in California. Although infection with CfMV can be lethal (Serjeant 1964), it is also possible that the presence of CfMV in combination with the drier and hotter summers experienced in the last few years may have weakened the plants and increased susceptibility to other pathogens, leading to the devastation observed in orchardgrass fields in northern California. Growers are encouraged to look for symptoms of CfMV in early spring and prevent mechanical transmission of CfMV by harvesting clean fields first, or by cleaning machinery thoroughly between fields. Additionally, growers are encouraged to grow CfMV-resistant cultivars, if possible. Cultivars Cheam-VR, Haida-VR, and Chilliwack-VR are listed as resistant to CfMV.