A muscadine virus survey was conducted in 2024 to assess the incidence of grapevine viruses across North Carolina (NC). This was a continuation of prior research in 2018-2020 on bunch grapes which confirmed the presence of grapevine viruses within NC vineyards. For this survey, muscadine vines were tested for grapevine leafroll associated virus complex (GLRaV-2, 3, 4, 7), grapevine red blotch virus (GRBV), grapevine virus A and B (GVA and GVB), grapevine rupetsris stem pitting-associated virus (GRSPaV), tomato ringspot virus (TRSV), and grapevine Syrah virus-1 (GSyV-1). Leaf-petiole samples were collected from 270 asymptomatic vines sampled across 8 vineyards. Samples were collected using a sterile technique with gloved hands and ethanol solution and evaluated at the NC State Micropropagation Unit. RNA was extracted from samples and then evaluated for viruses using Rt qPCR assays. The results show 37.1% incidence of GLRaV-2, 25.7% of GVB, 2.4% incidence of Xylella fastidiosa, and 0.5% incidence of GSyV-1. The total virus incidence was a 42.3% (89 vines) with co-infection incidence of 22.9% (48 vines). Additional muscadine testing will continue to understand the implication of muscadines viruses to both muscadines and bunch grapes crops.

Muscadine (Vitis rotundifolia; Muscadinia rotundifolia) wine makes up about 50% of all wine sales in North Carolina (NC). Muscadine fresh grapes are a profitable small fruit, grown mostly in the lower Eastern regions of NC. Few people know that muscadines are a small fruit crop in NC with a rich history dating back to the 16^th century. The plant itself is native to the Southern United states and muscadines were the first cultivated grapes in North America. The oldest currently existing muscadine vine stands at over 400 years old at the Eastern shore of NC.

The economic significance of muscadines to the NC small fruit market was marked through the bronze cultivar Scuppernong being named the state fruit in 2001. Against public believe, Scuppernong does not describe all bronze cultivars, but rather one specific cultivar of muscadines. Currently there are more than 30 cultivars of muscadines (bronze, purple and dark), grown on about 1,200 acres in NC. Their markets are centered on fresh eating as well as processing for juice and wine production. Muscadines are naturally tolerant to many diseases found in the southeast region of the US. However, despite the disease tolerance found in muscadines, neither the plant physiology itself nor the growing systems are well researched.

A recent study of NC bunch grape (not muscadine) vineyards has revealed a great abundance of a range of grapevine viruses, some of which can cause damages to bunch grapes. We wanted to know whether or not commercial NC muscadine vineyards are also hosts for grapevine viruses.

Plant diseases are caused by plant pathogens, typically microbial organisms (fungi, bacteria and viruses) that can infect plant tissue and cause illness. Plant diseases can be detrimental and have large economic impact for many horticulture crops. For this survey, we focused on grapevine viruses which are known to be hosted by muscadines and that are relevant for the NC grape and wine industries.

Information on grapevine virus incidence in NC is limited. Muscadines are known to harbor a grapevine leafroll associated viruses (GLRaV), grapevine virus B (GVB), as well as the Xylella fastidiosa (Xf), the causal agent of Pierce’s Disease. Muscadine vines usually are not affected by those infections and often do not show foliar disease symptoms. However, no previous data has been collected for NC. Bunch grapes were confirmed to have widespread grapevine viruses in NC vineyards through a statewide survey conducted from 2018 to 2020 (Hoffmann et al. 2021). This survey was the first of its kind for NC and generated the first data on state bunch grape virus incidence. To explore whether grapevine viruses are also present in NC muscadines, a second virus survey was conducted in 2024. The objective of this survey was to assess the presence of common grapevine viruses in muscadines plantings across the state.

We tested muscadines for grapevine leafroll associated virus complex (GLRaV-2, 3, 4, 7), grapevine red blotch virus (GRBV), grapevine virus A and B (GVA and GVB), grapevine rupetsris stem pitting-associated virus (GRSPaV), tomato ringspot virus (TRSV), and grapevine Syrah virus-1 (GSyV-1).These viruses were selected since they were the same viruses tested during the grape survey in 2018-2020 with the addition of GRSPaV and GSyV-1. These viruses are known to cause damage to bunch grapes through a variety of ways, but generally they reduce vine vigor, fruit yield, and wine quality. Additionally, we tested for the bacterium Xf, the causal agent of Pierce’s Disease, due to its major impact on the NC grape and wine industry.

Samples were collected from 8 vineyards across the state, totaling 210 vines tested. Vines were sampled in summer, June through August, and again after harvest, October and November. Due to the impact of Hurricane Helene, three vineyards (70 vines) could not be resampled in the fall. The cultivars sampled were Noble, Carlos, Regale, Supreme, Late Fry, Triumph, and Paulk. One to three blocks were sampled at each vineyard location. A block was considered a group of 10 to 40 rows consisting of the same cultivar. Ten vines per block were selected for sampling following a zig-zag schematic (Figure 1). All vines sampled were asymptomatic for viral diseases.

Six leaf petioles were removed from the selected vine canopy, placed in a plastic baggie, and immediately placed on ice in a cooler (Figure 2). All sampling took place using gloves and an 85% ethanol solution to ensure samples were collected in a sterile manner. Further sampling procedures are listed in Table 1. Samples were delivered on the same day to the NC State Micropropagation Unit. Samples were stored in -80 C until total RNA was extracted. Rt qPCR assays were performed on each sample to evaluate for the presence/absence for abovementioned viruses.

Figure 1. Muscadines were sampled by cultivar blocks, 10 samples per block. Asymptomatic vines were sampled in a zig-zag schematic to maximize vineyard coverage, as shown by this example. Blue dots indicate vines selected for sampling.

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Figure 1. Muscadines were sampled by cultivar blocks, 10 samples per block. Asymptomatic vines were sampled in a zig-zag schematic to maximize vineyard coverage, as shown by this example. Blue dots indicate vines selected for sampling.

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Figure 2. Left: Muscadine leaf petiole sample kit used in research described in this article. Right: An example for a typical asymptomatic vine that was sampled in this study.

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Figure 2. Left: Muscadine leaf petiole sample kit used in research described in this article. Right: An example for a typical asymptomatic vine that was sampled in this study.

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Across the state, there was a 37.1% (78 vines) incidence for GLRaV-2, 25.7% (54) incidence for GVB, 2.4% incidence (5 vines) for Xylella fastidiosa, and 0.5 % incidence (1 vine) for GSyV-1 (Figure 3). There were no positive results for GLRaV-3,4,7, GRBV, GVA, GRSPaV, and TRSV. There were 44 vines co-infected with GLRaV-2 and GVB, 1 vine co-infected with GLRaV-2 and Xylella fastidiosa, 1 vine co-infected with GLRaV-2 and GSyV-1, 1 vine co-infected with GVB and Xylella fastidiosa, and 1 vine infected with GLRaV-2, GVB, and Xylella fastidiosa. In total 89 vines out of 210 vines sampled tested positive for at least one virus (42.3% incidence). These results indicate there is a high prevalence of GLRaV-2 and GVB in NC muscadines and a few cases of Xylella fastidiosa, and GSyV-1.

Please keep in mind that this is a pre-liminary study. Currently, we can only report the incidence, but we do not know whether or not muscadine vines are affected by viruses, nor do we know how those viruses are transmitted in muscadines. However, there are a few initial take-aways. For muscadines, the vines tested in this survey did not show any disease symptoms, while infected bunch grapes regularly express moderate to severe discoloration of leaves (see Hoffmann et al. 2021). This might indicate that grapevine viruses do not to impact the yield or health of a muscadines grapevine. Secondly, we now know that there is a significant overlap in the viruses and diseases present in muscadines and bunch grapes (Table 2). However, the relationship between crop incidences, transmission of viruses or Pierce’s Disease, not the pathology of those organisms in muscadines is far from being understood.

More research is needed to understand the incidence and impact of grapevine viruses harbored by NC muscadine vineyards. However, this study underscores the importance of clean plant material for muscadine planting. This fact sheet should rise awareness among growers and extension agents, introducing the fact that grapevine viruses are harbored in many NC muscadine vineyards.

Further research will need to answer questions if muscadines serve as an active virus reservoir for other more susceptible plants and if muscadines themselves are harmed by virus infections. This initial virus survey is only a snapshot and more information is needed to understand the full picture of grapevine viruses. As a next step, an additional round of muscadine virus testing will happen in 2025.

NC State operates the Plant Disease and Insect Clinic (PDIC) where the public can submit vines for virus and disease testing. At this point of time, we do not recommend to submit muscadine samples for virus testing.

Figure 3. Counts of positive muscadine vines by each virus type. Results are grouped by vineyard, listed here as Vineyard A – G.

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Figure 3. Counts of positive muscadine vines by each virus type. Results are grouped by vineyard, listed here as Vineyard A – G.

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The viruses GLRaV-2 and GVB are widespread in muscadines in NC with incidence rates of 39.5% and 22.9%. Additionally, there were a few cases of GSyV-1 and two for Xylella fastidiosa. There were 47 vines which had multiple infections. However, all muscadines vines tested were asymptomatic (without symptoms), and the infections did not seem to harm production or growth of the vines. While muscadines could potentially serve as virus reservoirs, additional research is needed to test this hypothesis. Continuous Muscadine virus research, coordinated by NC State, will continue in 2025.

Hoffmann, M., Volk, E., Talton, W., AlRwahnih, M., Almeyda, C., Burrack, H., Blaauw, B. and Bertone, M. (2021): Grapevine Virus Distribution, Identification, and Management in North Carolina. AG-911.