Pollen-borne ilarviruses of peach: biology, ecology, and disease management

Abstract

Prunus necrotic ringspot virus (PNRSV) and prune dwarf virus (PDV) are pollen-transmitted ilarviruses that affect stone fruits. Both viruses are widespread in peach orchards in the southeastern United States. Although symptoms may not always be present, typical symptoms of PNRSV infection in peaches include stunting, chlorosis, and decline of the tree over time, while PDV infection is associated with stunted growth and shortened internodes. Co-infection with PNRSV and PDV can lead to peach stunt disease, causing yield losses of up to 55%. Here we provide information on the diagnostic methods currently available with considerations for selecting the best method based on plant tissue, sampling season, sample number, and cost. The ecology of PNRSV and PDV, including their movement within and between plants and the role of wild hosts as reservoirs, is also discussed. Both PNRSV and PDV are primarily transmitted through vegetative propagation and pollen; the latter could be horizontal between co-occurring blooming trees and/or vertical from parent to progeny. Certain insect groups play a role in spreading PNRSV and PDV in greenhouse and field settings. Management strategies to prevent virus spread, including clean plant programs which provide virus-negative propagation materials, and cultural management practices, such as removing infected trees, are emphasized to prevent secondary spread. While practical challenges persist in managing infection sources and wild hosts, proactive measures are essential to mitigate the spread of PNRSV and PDV and safeguard the peach industry.

Introduction

Prunus necrotic ringspot virus (PNRSV) and prune dwarf virus (PDV) are both members of the Ilarvirus genus in the family Bromoviridae (Mayo and Van Regenmortel 2000, Pallas et al. 2012, Bujarski et al. 2019). Ilarviruses have tri-partite genomes comprised of single-stranded, positive-sense RNA and quasi-isometric particles, so the term Ilarvirus was also coined based on the virus properties, isometric, labile particles associated with ringspot symptoms (Pallas et al. 2013). Most ilarviruses spread naturally in the field through pollen (Mink 1993, Card et al. 2007). Ilarvirus spread through pollen can be vertical from parent to progeny or horizontal from plant to plant (Card et al. 2007, Hamelin et al. 2016, Fetters and Ashman 2023). Since ilarviruses are considered latent pathogens, their economic importance has been underestimated in fruit trees (Pallas et al. 2012). Both PNRSV and PDV are endemic in the southeastern United States, where peaches are one of the predominant fruit crops (Scott et al. 1989, 2001, Rodriguez-Bonilla and Cieniewicz 2022, Tayal et al. 2023).

Peach (Prunus persica [L.] Batsch) belongs to the Rosaceae family and is an important fruit commodity with numerous health benefits (Bento et al. 2022). It is grown in numerous regions globally, with production concentrated in China (which is also the region of origin), the European Union, Turkey, the United States, Chile, and Russia. Production has been declining in the United States over the last decade due to climatic conditions and market trends (USDA-FAS 2023). In the United States, California is the leading producer of peaches, where nearly all of the clingstone peaches are grown for processing. Production in the southeastern United States in South Carolina and Georgia, the 2nd and 3rd leading peach states, respectively, is primarily for fresh market consumption. Although the economic impact of ilarviruses is likely underestimated (Pallas et al. 2012), the economic yield losses caused by PNRSV and PDV are significant for peach growers.

Both PDV and PNRSV continue to threaten peach crop production worldwide (Pallas et al. 2012, Pavliuk et al. 2021). Initially, PNRSV was identified and characterized in apricot (Prunus armeniaca L.) and plum (Prunus domestica L.), followed by sweet (Prunus avium L.) and sour cherry (Prunus cerasus L.) (Cochran and Hutchins 1941, Milbrath and Zeller 1945, Moore et al. 1948). In peach, PNRSV was initially described as a “peach ringspot virus” based on symptoms (Cochran and Hutchins 1941). Prune dwarf disease (caused by PDV) was first characterized by “Fellenberg prune” (P. domestica) with stunted and abnormal growth (Thomas and Hildebrand 1936). These viruses are also ubiquitous in major fruit-growing regions due to transmission through grafting and vegetative propagation (Uyemoto 1992, Uyemoto and Scott 1992, Uyemoto et al. 2003, Pallas et al. 2012), pollen and seed-mediated transmission in orchards (Greber et al. 1991, 1992, Milne and Walter 2003, Card et al. 2007, Amari et al. 2009).

Virus Genome and Virion Structure

Both PNRSV and PDV have single-stranded, positive-sense, segmented RNA (RNA1, RNA2, and RNA3) genomes without a poly-A tail (Fig. 1) (King et al. 2012, Pallas et al. 2013). The RNA1, RNA2, and RNA3 of PNRSV and PDV consist of approximately 3.2 kb, 2.5 kb, and 2 kb, respectively, and are encapsidated in separate virions (Fig. 1). The RNA1 and RNA2 encode proteins P1 and P2, respectively, while RNA3 encodes both the movement protein (MP) and coat protein (CP) (Guo et al. 1995, Rampitsch and Eastwell 1997, Pallas et al. 2013). The P1 and P2 proteins are involved in the viral replicase complex, whereas MPs are implicated in intercellular movement (Fig. 1) (Bol 2005, Herranz et al. 2005, Sánchez-Navarro et al. 2006). The CP subunits are important for forming the virion to protect the viral genome, but ilarviruses require the presence of CP monomers in association with genomic RNAs to initiate infection, a phenomenon known as genome activation (Sánchez-Navarro and Pallas 1997). The PNRSV and PDV virions exhibit a quasi-spherical particle shape (26–36 nm in diameter) with icosahedral symmetry (T = 3) (Fig. 1). However, bacilliform particles (18–26 nm by 30–85 nm) have also been observed in plants infected by PNRSV and PDV (Bujarski et al. 2019).

Diversity and Phylogenetic Relationships

The International Committee on the Taxonomy of Viruses (ICTV) distinguishes between viruses (i.e., the biological agents) and virus species (i.e., the taxonomic constructs), and has recently established a binomial nomenclature system for virus species names (Gorbalenya and Siddell 2021, Walker et al. 2022). Previous species names were Prune dwarf virus and Prunus necrotic ringspot virus but at the time of writing, the species name revision for the Bromoviridae is pending approval by the ICTV. The species names suggested are Ilarvirus PDV and Ilarvirus PNRSV. It is important to note that virus common names are not changing, and thus the common names associated with PNRSV and PDV will remain unchanged. As with all virus species, the taxonomic organization of the Ilarvirus genus has been revised over time based on biological characteristics, serology, and molecular characteristics of the associated viruses. As of 2024, there are 22 confirmed Ilarvirus species (family Bromoviridae) (ICTV 2022) subdivided into four subgroups (1–4) (Pallas et al. 2013, Bujarski et al. 2019). Although the viruses have similar genome organization, PDV and PNRSV belong to different subgroups of 3 and 4 respectively based on their serological, host range, and sequence similarities (Fauquet et al. 2005, Tzanetakis and Martin 2005, Pallas et al. 2013, Bujarski et al. 2019).

Phylogenetic studies have been performed to assess ilarvirus diversity, but resolving phylogenetic relationships of these viruses in stone fruits (Table 1) is complicated by the global movement of planting material, which has resulted in the dissemination of these viruses worldwide. Earlier phylogenetic analysis clustered PNRSV variants into three groups named after representative PNRSV isolates: PV32 (isolated from apple in Spain: Sanchez-Navarro and Pallas 1997), PV96 (isolated from the cherry in Germany: Guo et al. 1995), and PE5 (isolated from peach in the United States: Hammond and Crosslin 1995) (Aparicio et al. 1999, Vašková et al. 2000). Phylogenetic studies have revealed the complex distribution of PNRSV isolates across geographic regions and host species, highlighting the dynamic nature of viral dissemination in the context of global movement and local adaptation.

Similar to PNRSV, many PDV CP sequences have been characterized from different countries, including Canada, Brazil, Argentina, Russia, the United States, and various hosts including sweet cherry, sour cherry, wild cherry, peach, and plum (Pallas et al. 2012). Phylogenetic analysis of PDV CP gene sequences indicates the existence of four groups related to the host: (i) cherry I (containing only cherry variants), (ii) cherry II (containing one apricot variant and cherry variants), (iii) mixed (including variants from peach, cherry, plum, and one almond tree), (iv) almond (containing only almond variants) (Ulubaş Serçe et al. 2009). Phylogenetic analysis of PDV CP sequences highlights distinct groupings related to the host species.

Symptoms and Diagnostics

Disease Symptoms

These viruses can infect hosts individually or in mixed infections, potentially leading to ringspot and stunting diseases in peach (Uyemoto and Scott 1992, Scott et al. 2001, Caglayan et al. 2011). Both viruses do not always elicit symptoms depending on the cultivar and virus strain, which can exacerbate their spread in orchards. Infection with PNRSV may be associated with “shock” symptoms in young trees (Fig. 2), characterized by stunting and sometimes necrotic leaf spots and “shot holes” in young leaves, which may disappear in later years. In California and southeastern U.S. peach production, typical disease symptoms of PNRSV infection in peaches include leaf chlorosis, necrosis, leaf deformation, and blind wood symptoms resulting in open tree canopy with fewer leaves and vegetative buds (Fig. 3A) (Uyemoto and Scott 1992, Reighard 1997, Scott et al. 2001). In other stone fruits (Table 1), PNRSV could lead to rugose mosaic, bud death, and delayed fruit maturity in cherries, discolored ringspots in apricot fruits and smaller leaves, and tattered appearance in infected sweet cherry trees (Nyland et al. 1976, Wells and Kirkpatrick 1986). Symptoms of PNRSV in peach orchards may include reduced vigor and smaller trunk circumference (Pusey and Yadava 1991) (Fig. 3B).

Infection with PDV can also be economically damaging, its infection may cause chlorotic, mottle, line pattern, and stunted vegetation (Martelli and Savino 1997). Typical symptoms of PDV infection in stone fruits include leaf chlorosis and distortion, stunting of internodes, and rosette formation in developing shoots (Fig. 4) (Uyemoto and Scott 1992, Scott et al. 2001, Caglayan et al. 2011). The severity of PDV infection depends on the host cultivar, virus strain, and environmental conditions, especially temperature (Caglayan et al. 2011). In cherries, PDV infection was found to cause up to 50% yield losses (Caglayan et al. 2011). The other economically important crops infected by PDV include peaches, plums, and sweet cherries, which cause significant drops in bud-take and reduced fruit production (Pallas et al. 2012). However, sometimes fully infected orchards do not exhibit any obvious symptoms. According to the California pest rating proposal, the introduction of PNRSV and PDV into California poses high risks, with a rating of “C” indicating high consequences for the state (CDFA-CPRP 2023a, 2023b)

Peach Stunt Disease

Peach stunt disease (PSD) is a potentially severe disorder associated with mixed infections of PNRSV and PDV, causing synergistic effects on growth reduction and yield losses (Asai and Uyemoto 1991). In Australia, PSD was described as “peach rosette and decline” (Smith and Challen 1977). Typical symptoms of PSD include stunting, shortening of internodes, reduction in fruit yield and vegetative growth, defoliation of leaves, rosetting, leaf chlorosis, gummosis, bark splitting, delay in bud break, as well as an increase in water sprouts production (Asai and Uyemoto 1991, Scott 2001, Scott et al. 2001). Mixed infection could be more harmful than individual infections as PSD was associated with an 80% mortality in stone fruits in Morocco (Srhiri 1998, Caglayan et al. 2011). Co-infected peach trees showed significant defoliation, reduced trunk circumference, and increased production of water sprouts compared to trees infected with either virus alone (Scott et al. 2001). Overall, the impacts of co-infection of PNRSV and PDV appear to be dependent on cultivar, virus strain, and experimental conditions.

Detection and Diagnosis

Diagnosis is a critical and essential step in selecting and executing an appropriate disease management program. Enzyme-linked immunosorbent assay (ELISA) and reverse transcription polymerase chain reaction (RT-PCR) are the most common diagnostic methods used for PNRSV and PDV detection (Mekuria et al. 2003, Sánchez-Navarro et al. 2005, Matic et al. 2008). While ELISA detects the interaction between antibodies and antigens (the viral proteins) (Clark and Adams 1977), PCR is a genome-based test which relies on the amplification of specific nucleic acid sequences (Bhat and Rao 2022). Different PCR methods have been designed for PNRSV and PDV detection either individually (simplex), simultaneously (duplex), or with other stone fruit viruses such as PPV (multiplex) with similar sensitivity and specificity (Kölber et al. 1998, Youssef et al. 2002, Sánchez-Navarro et al. 2005, Massart et al. 2008, Jarošová and Kundu 2010, Kapoor and Handa 2018). The selection of a specific diagnostic method depends on the time of year and type of plant materials to be tested, the number of samples to be tested, and the cost. For example, ELISA is a cost-effective and high-throughput diagnostic assay but cannot detect viruses in low concentrations and can only detect a single virus at a time. On the other hand, PCR is a more sensitive test that provides accurate information on quantitative estimation (RT-qPCR); however, it requires more refined training and equipment. Testing can be performed by private companies, certain plant diagnostic clinics, and some university labs.

The reliability and accuracy of PNRSV and PDV detection depend on different factors such as the selection of diagnostic method, type of tissue sampled, time of sampling, and phenological age of sampled tissue. In sampling trees for PNRSV and PDV, the tissue type and phenological stage of the trees should be considered due to seasonal fluctuations in viral titers. Sampling strategies must account for these viral fluctuations. For example, the seasonal fluctuation of PNRSV in peach trees, with low titers in leaves during summer, can result in false negatives with ELISA (Uyemoto et al. 1989) whereas RT-PCR can typically still detect PNRSV in leaves in the summer. In spring, PNRSV and PDV can be reliably detected in young peach leaves by double antibody sandwich enzyme linked immunosorbent assay (DAS-ELISA) while buds are more reliable for testing in dormancy (Virscerk and Mavric 2005). Cultivar differences may also affect symptom expression and seasonal fluctuations of viral titers (Zotto et al. 1999). While ELISA is reliable and high throughput for detecting PNRSV and PDV in young leaves in the spring, RT-PCR can be conducted at most times of the year because of its higher sensitivity (Mekuria et al. 2003). If sampling for virus detection in spring, it is optimal to collect the newest growth (shoot tips with 4–5 leaves) from each scaffold or major branch of the tree. The leaves can then be combined in the laboratory, but this sampling strategy helps to minimize false negative results. It is important to understand the biology of PNRSV and PDV so that the optimal tissue type can be sampled at the point of highest viral titer to provide a reliable and accurate diagnosis.

Ecology of PNRSV and PDV

Virus Movement Within and Among Plants

Both PNRSV and PDV are transmitted horizontally from tree to tree through pollen, and vertically from parent to progeny via two mechanisms—vegetatively through grafting and through infected seed (Greber et al. 1991, 1992, Uyemoto and Scott 1992, Card et al. 2007). The primary mechanism of seed and pollen transmission involves transferring and fertilizing virus-infected pollen to healthy plants during pollination (Card et al. 2007). The transmission by pollen could be either vertical from parent to progeny (Fig. 5) or horizontal from tree to tree (Fig. 6A) (Card et al. 2007, Hamelin et al. 2016). During fertilization, the infected pollen may infect the embryo within the developing seed, and consequently, infected seedlings germinate (Fig. 5). In horizontal transmission, the virus must pass through the callose layer separating the embryo from the somatic tissues of the tree despite the lack of vascular connection (Fig. 6B) (Mink 1993, Card et al. 2007). However, the lack of any plasmodesmatal, vascular, or direct connection restricts cell-to-cell movement by the viruses (Roberts and Oparka 2003). Mechanical wounds created by insects likely overcome the connection barriers and facilitate virus entry to the mother plant, known as assisted horizontal virus transmission (Fig. 6C) (Sdoodee and Teakle 1987, Greber et al. 1991). In apricot, PNRSV was detected within and on the surface of infected pollen grains (Amari et al. 2007). Interestingly, the PNRSV distribution follows the same pattern as the cellular components required for pollen tube germination and also localizes inside growth zones for pollen tubes (Amari et al. 2007). In sweet cherry and apricot, PNRSV and PDV can be found in the cytoplasm of pollen grains, cotyledons, hypocotyl-radicle tissues, and testa-nucellus-endosperm tissues facilitating pollen and seed-mediated vertical transmission from gametes to seedlings (Kelley and Cameron 1986, Amari et al. 2009).

Role of Insects in Virus Transmission

Due to the pollen-borne nature of PNRSV and PDV, transmission can occur during peach bloom in orchards (Davidson and George 1964, Brunt et al. 1996, Scott et al. 2001, Uyemoto et al. 2003, Virscerk and Mavric 2005). Although peach is a self-fertile crop, a diverse group of flower visitors/pollinators has been recorded visiting peach flowers (Dar and Raza 2016, Dar et al. 2020, Robertson et al. 2020). The dominant groups are (i) Hymenoptera with families viz., Apidae (honeybee, bumblebee, long-horned bee, southeastern blueberry bee), Andrenidae (mining bees), Halictidae (sweet bees), Megachilidae (leafcutter bees); (ii) Diptera with families viz., Syrphidae (hoverflies), Sarcophagidae (flesh flies), Calliphoridae (blow flies), Muscidae (house flies) and Tachinidae (tachinid flies); (iii) Lepidoptera with families viz., Nymphilidae (brush-footed butterflies), and Noctuidae (owlet moths) (Dar and Raza 2016, Robertson et al. 2020). A recent study examining the role of nocturnal moths in peach pollination found that two moth species, the true armyworm Mythimna unipunctata (Haworth) (Lepidoptera: Noctuidae) and the variegated cutworm Peridroma saucia (Hübner) (Lepidoptera: Noctuidae) primarily visited peach flowers but did not increase fruit set of self-fertile peaches (Robertson et al. 2020). On the other hand, honeybee Apis mellifera (Linnaeus) (Hymenoptera: Apidae) and bumblebees Bombus spp. (Hymenoptera: Apidae) are known as major pollinators of peach trees (Dong et al. 2011, Zhang et al. 2015) and therefore, these species have been predominantly studied for their potential role in transmission of pollen-borne viruses (Mink 1983, Bristow and Martin 1999, Okada et al. 2000, Lacasa et al. 2003, Shipp et al. 2008, Darzi et al. 2018). In some studies, PNRSV has been detected on the outer surface of pollen collected by honeybees in almond and sweet cherry orchards (Cole et al. 1982, Hamilton et al. 1984). Pollen samples removed from dominant bee genera: Apis, Bombus, Andrena, Eucera, and Habropoda spp. during peach bloom in South Carolina tested positive for PNRSV and PDV, providing additional evidence of the role of bees in facilitating PNRSV and PDV spread (Tayal et al. 2023). Native bee populations are important as pollinators, but honeybee hives are also used to bolster pollination in some stone fruit crops. These viruses can be carried in pollen collected by honeybees A. mellifera L. and transferred within the hive, which may account for their long-distance spread (Mink 1983, Boylan-Pett et al. 1991).

While bees are known as major pollinators, some thrips also aid in flower pollination (García-Fayos and Goldarazena 2008, Ghosh et al. 2017). Thrips are small (1–2 mm in length), slender, and soft-bodied insects having piercing and sucking mouthparts (Ghosh et al. 2017, Bhat and Rao 2020) which facilitate virus transmission by creating mechanical wounds and transferring virus-infected pollen. Previous studies demonstrated that onion thrips Thrips tabaci (Lindeman) (Thysanoptera: Thripidae) were able to transmit tobacco streak virus, another ilarvirus, to Chenopodium amaranticolor seedlings mechanically by creating wounds for virus infection when mixed with virus-infected pollen from tomato (Lycopersicon esculentum) (Sdoodee and Teakle 1987, 1993). Further, western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) also facilitate the transmission of PNRSV and PDV to cucumber seedlings sourced from infected plum pollen and cherry pollen respectively (Greber et al. 1991, 1992). Recently, Tayal et al. (2023) detected PDV in the thrips samples collected during peach bloom in South Carolina, however, thrips-mediated PDV transmission in peach orchards should be investigated. A mixture of thrips species (T. tabaci, Thrips Australis (Bagnall), Microcephalothrips abdominalis (Crawford), and Thrips imaginis (Bagnall), and others) showed a high PNRSV transmission rate (66%) as compared to only T. tabaci (56%) (Greber et al. 1991). Also, cherry pollen inoculum sources with dual infection of PNRSV and PDV resulted in high transmission rates (25% to 75%) as compared to inoculum sources infected with either PNRSV or PDV alone (less than 10%) (Greber et al. 1992). Thrips-mediated transmission efficiency varies depending upon (i) the virus strain, (ii) the availability and the rate of virus release from the pollen, (iii) the insect vector species, (iv) the differential susceptibility of test plants to virus infection, and (v) the effect of environmental conditions (Klose et al. 1996).

Virus Movement Between Wild Hosts and Cultivated Peaches

Wild Prunus spp. in close proximity to peach orchards can act as virus reservoirs for secondary transmission. Various fruit viruses, including the ilarviruses, can infect wild Prunus spp. (Borisova 2012, Rodriguez-Bonilla and Cieniewicz 2022, Rodriguez-Bonilla et al. 2023). For instance, in the Niagara Peninsula, wild sweet cherry (P. avium), wild black cherry (Prunus serotina Ehrh), and choke-cherry (Prunus virginiana L.) were found infected with PNRSV (Davidson and Rundans 1972). In the southeastern United States, PNRSV was also detected near peach orchards in wild black cherry and Carolina cherry laurel (Prunus caroliniana) (Rodriguez-Bonilla and Cieniewicz 2022). Additionally, PDV has been confirmed in P. avium, P. serotine, and Prunus spinosa (Davidson and Rundans 1972, Borisova 2012). The prevalence of viruses in the wild hosts on peach orchard borders could serve as a source of virus inoculum to cultivated peaches (Oliver et al. 2009, Rodriguez-Bonilla and Cieniewicz 2022). A recent study on understanding the directional movement of PNRSV between wild black cherry (P. serotina) and peach (Prunus persica) found a temporal separation of the flowering period between two species and hypothesized the potential movement of PNRSV from peach to wild black cherry (Rodriguez-Bonilla et al. 2023). However, the role of pollinators, which can transfer virus-infected pollen, should not be overlooked in PNRSV and PDV movement at the agroecological interface. Bees can facilitate the virus transmission by carrying infected pollen from diseased plants to healthy plants. For example, honeybees transmitted the blueberry shock virus in blueberry bushes by carrying virus-infected pollen to healthy bushes (Bristow and Martin 1999). Under protected cropping conditions, bumblebees Bombus terrestris (Linnaeus) and Bombus impatiens (Cresson) spread tobacco mosaic virus, and pepino mosaic virus whereas honeybees were found spreading cucumber green mottle mosaic virus (Okada et al. 2000, Lacasa et al. 2003, Shipp et al. 2008, Darzi et al. 2018). The role of insects in the movement of virus-infected pollen should be further investigated (Rodriguez-Bonilla et al. 2023, Tayal et al. 2023).

Generally, Prunus spp. tend to be self-incompatible, requiring an external pollination source for fertilization (Yamane and Tao 2019). Due to its long domestication history, the peach is a self-compatible crop, however, pollination is encouraged to increase fruit set and crop yield (Weinbaum et al. 1986, Benedek and Nyeki 1996, Zhang et al. 2015). Self-compatibility and self-incompatibility in different Prunus spp. could play an important role in understanding pollen-mediated virus transmission (Eastham and Sweet 2002, Rodriguez-Bonilla et al. 2023). Identification and characterization of virus inoculum source, genomic diversity of virus isolates found in bee pollen samples, and determining the involvement of plant species in virus transmission will enhance our understanding of virus movement at the agroecological interface. Since PNRSV is prevalent in wild cherries and is abundant on peach orchard edges, we suspect the role of bees as a bridge in virus movement at the wild cherry-cultivated peaches interface.

Management

Clean Plant Programs

Once a tree is infected with a virus, it is infected for the duration of its life. Therefore, management actions in perennial crops like fruit trees are nearly always aimed at preventing viral infection. One of the most effective methods of prevention is to plant trees derived from virus-negative source materials known as mother trees. Clean plant programs like those supported by the USDA National Clean Plant Network (NCPN) and specific state certification programs can help to ensure that trees derived from virus-negative sources are available to the fruit tree industry.

Virus diseases in fruit trees can be incredibly costly and therefore warrant prevention efforts. For example, the PPV eradication program in Pennsylvania cost $59 million between 1999 and 2009 (Welliver et al. 2014). The economic value, or return on investment, of clean plants has been demonstrated across several specialty crops, including fruit trees (Fuchs et al. 2021). Ilarviruses tend to spread relatively slowly compared to some other vector-borne viruses of fruit trees, thus planting clean trees can be an especially effective strategy. Some states (e.g., Washington, California, Oregon) have virus certification programs to ensure tree nurseries propagate trees from virus-negative sources. The California Department of Food and Agriculture (CDFA) administers a Deciduous Fruit and Nut Tree Registration and Certification program to provide rootstock and scion sources for propagation of certified nursery stock to regulate PNRSV and PDV (CDFA 2023). However, many states including those in the Southeast lack certification programs, underscoring the importance of grower vigilance and awareness of virus problems. Initially termed the Southeastern Budwood Program in 2001, the NCPN and the Peach Councils of South Carolina and Georgia have supported testing for PNRSV, PDV, and PPV to minimize the spread of these pathogens through nursery propagation. So, despite not operating under the auspices of a certification program administered by the states, the growers in this region have been vigilant and aware of the importance of viruses in this region for decades.

Whether there is a certification program or not, nurseries are encouraged to work with clean plant centers associated with the NCPN to access Foundation material, negative for targeted graft-transmissible pathogens, to propagate trees for sale to commercial fruit growers. NCPN Fruit Tree centers are located at the Clean Plant Center Northwest in Prosser, Washington, Foundation Plant Services at the University of California-Davis, and the Clemson Clean Plant Center at Clemson University in South Carolina. The clean plant centers conduct diagnostics of graft-transmissible pathogens, virus elimination therapies using thermal treatments and meristem tip tissue culture, and maintain clean fruit tree accessions in Foundation collections to minimize the potential of reinfection. The Foundation collections serve as a nuclear source of virus-negative trees, which can be provided to the fruit tree industry for propagation.

Cultural Management

Cultural strategies for virus control rely on consistent preventive practices. Two primary approaches in cultural management involve reducing infection reservoirs and minimizing the rate of secondary spread (, Sastry and Zitter 2014, Barba et al. 2015). Given that PNRSV and PDV are transmitted through vegetative propagation and infected seeds, using seed and propagation material from virus-negative sources is the best starting point to prevent disease establishment (Barba et al. 2015, Fuchs et al. 2021). While this preventive approach effectively limits disease establishment, addressing virus inoculum sources like infected orchard trees and wild hosts presents challenges in managing the secondary spread of viral diseases. The prevalence of PNRSV and PDV in wild hosts and orchard trees necessitates the removal of infected trees to minimize secondary spread (Alexander et al. 2014, Rodriguez-Bonilla and Cieniewicz 2022, Rodriguez-Bonilla et al. 2023). In theory, eliminating most of these infection sources is plausible, but practical difficulties arise due to cropping system restrictions and potential economic losses for growers. Moreover, the removal of wild-infected trees is also challenging because of their importance to the ecosystem and oftentimes because of the lack of access for removal. This proof-of-concept has been demonstrated at the Clemson University Musser Fruit Research Center where they take an aggressive approach to virus management by promptly removing infected trees upon detection of PNRSV and PDV, effectively minimizing the risk of secondary spread. However, despite an aggressive rogueing approach, these viruses are detected each year at this research farm, likely due to latency in the infection process and because it is not practical to test every tree (Rodriguez-Bonilla et al. 2023). In California cherry production, it is also recommended to manage orchard weeds and ground covers from flowering until after trees have bloomed to prevent insect-mediated spread of viruses through infected pollen (Greber et al. 1992, Caprile et al. 2009). Nonetheless, as cost allows, routine diagnosis for viruses is recommended to identify virus-positive trees to inform rogueing practices.

Conclusions

The prevalence of PNRSV and PDV in peach orchards and their complex vegetative and pollen-borne transmission pathways present significant challenges in their effective management. Despite their slow spread and latency in some cultivars, the economic impact of PNRSV and PDV on the peach industry has been underestimated. Recommending routine virus diagnosis and removal of virus-infected trees is crucial to minimize the secondary spread. Current management measures involve using clean (derived from virus-negative material) trees for establishing new orchards and removing infection reservoirs to prevent viral transmission. The detection of PNRSV and PDV in bee pollen samples and their potential role in aiding the spread of PNRSV and PDV in peach orchards underscores the need to restrict beehive movement to minimize the long-distance spread of these viruses.

We can improve the success of virus management strategies by gaining a better understanding of virus ecology at the interface of cultivated and wild hosts. Understanding how viral infections affect pollinator behavior and preferences may provide insights into the secondary spread of PNRSV and PDV in peach orchards. As pollinators are integral to the ecosystem and cannot be removed, efforts should focus on identifying and eradicating infected sources to minimize virus inoculum loads. The effective management of PNRSV and PDV in peach trees involves utilizing clean plant programs, avoiding the transfer of beehives between orchards, and eradicating infection sources as practical. Efforts to communicate findings with growers and stakeholders will also increase the likelihood of adopting virus-disease management strategies.

Acknowledgments

The authors wish to thank members past and present of the Cieniewicz lab for their contributions on advancing knowledge on these viruses. We would like to thank anonymous reviewers for their valuable feedback to improve this manuscript draft. Thanks to the Clemson University Entomology Program for awarding Carl and Ruby Nettles Memorial Fellowship, Joel A. Berly Research Fellowship, and the Clemson College of Agriculture, Forestry, and Life Sciences Wade Stackhouse Fellowship to Mandeep Tayal to support this research. Funding for the preparation of this article was also provided by the federal Hatch project 1700581.

Author contributions

Mandeep Tayal (Conceptualization [equal], Data curation [equal], Funding acquisition [equal], Investigation [equal], Visualization [equal], Writing – original draft [equal], Writing – review & editing [equal], Fabian Rodriguez Bonilla (Conceptualization [equal], Writing – original draft [equal], Writing – review & editing [equal], Garner Powell (Visualization [equal], Writing – review & editing [equal], and Elizabeth Cieniewicz (Conceptualization [equal], Funding acquisition [equal], Investigation [equal], Resources [equal], Supervision [equal], Visualization [equal], Writing – original draft [equal], Writing – review & editing [equal])

References