Oat (Avena sativa, Poaceae) is one of the world's most important cereal crops, following maize, rice, wheat, barley, and sorghum (Hackett, 2018), and even though oat contains much higher unsaturated fatty acid, mineral vitamin, and phytochemical contents than other cereal species (Rasane et al., 2015), the evaluation of its quality has mainly focused on its value as a feed crop. Interestingly, recent studies that have confirmed the nutritional values and advantages of oat for human health have led to increases in oat consumption in Korea (Banaś et al., 2007; Gorash et al., 2017), and oat can also grow well in poor environments (e.g., cold and humid climates) and exhibits a lower demand for chemical fertilizers than other grain crops (Lee et al., 2019).

Barley yellow dwarf virus (BYDV, Luteoviridae), a group of related single-stranded RNA viruses (Luteovirus, Polerovirus, or unassigned), are the most economically important and widespread viral pathogen of graminaceous crops, affecting over 150 species in the Poaceae (Bekele et al., 2001). A previous study separated BYDV into two subgroups that cor-responded to differences in the region coding for an RNA-dependent RNA polymerase (ORF2) (Malmstrom and Shu, 2004), with BYDV subgroup I including BYDV-PAV, BYDV-MAV, BYDV-SGV, and BYDV-PAS and subgroup II containing BYDV-RPV and BYDV-RMV. Of the various strains, BYDV-PAV is assumed to be the most widespread and is the most well studied at the molecular level (Jarošová et al., 2013). As with other luteoviruses, BYDV can be transmitted by various aphid species, and there is no evidence of mechanical or vertical transmission. The aphids Rhopalosiphum padi and Sitobian avanae are the primary vectors of BYDV (Thackray et al., 2009), and increases in aphid populations have been associated with both increases in BYDV incidence and sub-sequent reductions in crop production. Even though BYDV infections can be asymptomatic (McKirdy et al., 2002; Perry et al., 2000), BYDV-infected oat plants generally exhibit stunting, reduced root growth, and leaf discoloration (reddening), and BYDV has been reported to cause yield losses of up to 25% in barley, 46% in wheat, and 15% in oat, with a global mean of 30% (Adhikari et al., 2020; Edwards et al., 2001; Ordon et al., 2009). Most of the BYDV-PAV genome is ∼5,660 bp in length, with six open reading frames and four untranslated regions (Liu et al., 2007). In particular, ORF3 encodes the main components of the coat protein (CP) required for aphid transmission and systemic infection (Filichkin et al., 1994; Gildow, 1993; Khine et al., 2020; Mohan et al., 1995).

Because oat is generally considered less important than wheat, rice, and barley, in terms of both quantity and profit-ability, the crop has attracted considerably less agricultural and disease research. The present study aimed to evaluate the incidence and distribution of BYDV in the oat-growing areas of Korea and to perform a phylogenetic analysis of BYDV isolates from Korea.

Even though oat is one of the world's most widely cultivated cereal crops, its production has, until recently, focused more on forage than on food products (Capstaff and Miller, 2018). However, increase in public awareness of the crop's health benefits has resulted in rapid increases in oat consumption around the world (Rasane et al., 2015). In Korea, both the importation and domestic production of oats have increased. Oat is grown in several areas in Korea, including Gangjin (Jeollanam-do) and Jeongeup (Jeollabuk-do). BYDV represents a significant threat to cereal crop production, and although BYDV outbreaks in barley and wheat in Korea have been reported, current research regarding BYDV in oats is insufficient (Jo et al., 2018; Woo et al., 2001). Increasing global temperatures will likely promote increases in aphid populations (Ju et al., 2015), thereby increasing the occurrence of BYDV outbreaks and, thus, the resulting damage to affected crops. Therefore, investigation of the distribution of BYDV in Korea's domestic oat industry is critical.

In the present study, samples of symptomatic oat plants were collected from eight commercial oat fields across Korea, with most samples collected from Gangjin and Jeongeup, and multiplex RT-PCR was used to detect and identify BYDV strains in the samples. The results of the study suggest that sampling based on visual symptoms is ineffective. Indeed, symptoms that are similar to those of BYDV infection can also be caused by other diseases and physiological disorders, including nutrient stress, drought, and cold weather. In addition, the incidence of BYDV varied among the districts, from 0% (Pocheon) to 83% (Gimje), and BYDV-PAV was the most frequently detected strain, regardless of region, being detected in 33% of the 322 samples. Other BYDV strains (BYDV-SGV, BYDV-PAS, and BYDV-RPV) were also detected but were relatively rare, whereas others (BYDV-MAV and BYDV-RMV) were not detected in any region. Previous research has indicated that BYDV-PAV causes more severe symptoms and reductions in plant height and yield than other strains (Jarošová et al., 2013). Several strategies, including rapid detection, eradication, and breeding, have been developed for the control of viral crop pathogens. However, because BYDV incidence and damage will likely be promoted by climate change, due to the effects of increasing temperature on insect vectors, it will be increasingly important to continuously monitor BYDV and aphid outbreaks around the world.

In addition, BYDV-PAV isolates from different regions of Korea yielded divergent CP sequences. More specifically, the BYDV-PAV isolates were separated into two groups, which each contained closely related isolates. However, the overall sequence similarity of the 23 Korean BYDV CP sequences ranged from 79.5% to 99.7%. Interestingly, the Korean isolates were also more closely related to isolates from China and Australia than to those from other countries. This characterization and analysis provide insight into BYDV population dynamics and evolution.

The present study examined the status of BYDV in eight major major oat-producing areas in Korea and detected BYDV in 38.7% of collected samples using multiplex RT-PCR. However, the absence of BYDV in many symptomatic samples suggests that further research into other viral cereal crop pathogens is needed. The findings of the present study are expected to provide a basis for BYDV prevention, and continuous investigation is needed into BYDV transmission and interactions of the virus with vector insects.