Taxonomy: Virgaviridae, Tobamovirus, Cucumber green mottle mosaic virus
The most up-to-date and complete virus taxonomy is available on the International Committee on Taxonomy of Viruses webpage.
Cucumber green mottle mosaic virus (CGMMV) is an important emerging virus recently causing serious disease outbreaks on cucumber, watermelon, melon, squash (Cucurbita moschata), and bottle gourd (Lagenaria siceraria) (Dombrovsky et al., 2017, Ann. Rev. Phytopathol. 55). The damage caused by the virus can be severe and result in significant yield and quality losses, depending on host and stage of infection (Ling et al., 2014, Plant Dis. 98; Reingold et al., 2015, Plant Pathol. 64). The virus was first described infecting cucumber in England in 1935 (Ainsworth, 1935, Ann. Appl. Biol. 22). The first detection of CGMMV in North America was on greenhouse cucumber in Alberta, Canada, in 2013 (Ling et al., 2014), and subsequently on melon and watermelon in California in the same year (Tian et al., 2014, Plant Dis. 98). CGMMV is a quarantine pathogen and under official control in the United States (El-Lissy, 2014, USDA PPQ). It is a seed-borne virus. The virus can be transmitted from contaminated seeds to seedlings resulting in a new infection (Sui et al., 2019, Plant Dis. 103). Consequently, the use of contaminated seed may result in disease outbreaks. Planting cucurbit seeds certified free of CGMMV is a mitigation strategy recommended to growers. Thus, standard seed health testing methods (USDA NSHS, 2021) are used by the seed industry and governmental agencies to ensure seed lots are free of CGMMV. Resistance to CGMMV in commercial cucurbit crops is limited. Resistance is available in some cucumber varieties (ASTA, 2018; Ellouze et al., 2020, Agron. 10). Additionally, some sources of potential resistance have also been identified in muskmelon (Ruiz et al., 2021, Plants 10). To breed for disease resistance, one needs to identify genetic sources of resistance, evaluate selected lines and generate F2 and backcross populations for genetic and genomic studies. Currently, CGMMV infections are limited to some cucurbit seed producing fields or other specialty cucurbit crop farms in California, USA (Pitman et al., 2022, Plant Dis. 106) and are under official control. On the other hand, Australia has now experienced some serious disease outbreaks caused by CGMMV on cucurbit crops nationwide (Mackie et al., 2023, Viruses 15).
Based on the genomic sequences of CGMMV isolates around the world, two major genotypes (groups) of CGMMV are recognized, the Asian and European genotypes.
Symptoms of CGMMV infection in cucurbits vary depending on the cucurbit host and variety. On cucumber, two types of symptoms develop, depending on the variety: green mottle mosaic and necrotic spots. Mottling is a type of mosaic with light and dark green or yellow patches or streaks. In early infections, young leaves of infected plants develop a light green mottle and blisters (Figures 1 and 2). Infected plants may be stunted. Mature leaves exhibit darker green blisters and green mottle mosaic. Mature plant collapse occurs upon co-infection of CGMMV with the soilborne pathogen Pythium spinosum (Philosoph et al., 2018, Plant Dis. 102). Cucumber fruits in infected plants may develop mottling and discoloration, making fruits unmarketable.
On watermelon, symptoms can be observed, including mottle, mosaic, stunting, necrosis, and fruit deformation. Typical mottle and mosaic symptoms first appear on new leaves, which may lead to leaf deformation (Figure 3). Once plants are infected, symptoms can also extend to fruits, with fruit abortion, uneven ripening, and internal flesh breakdown and discoloration. Fruit deformation can result in substantial yield loss.
On bottle gourd, typical symptoms of mottle mosaic, with dark green blisters develop on young leaves of infected plants (Figure 4). Symptoms on the mature leaves are milder, showing mosaic and mild chlorosis (yellowing).
On muskmelon (Cucumis melo), mottle and mosaic symptoms initially develop on young leaves of infected plants, but the symptoms may dissipate in mature foliage. Fruits in infected plants may develop different degrees of malformation, mottling, and surface netting.
Squashes and pumpkins (Cucurbita spp.) are more tolerant to CGMMV infection. Most CGMMV-infected squash plants did not generate visible mottle or mosaic symptoms (Sui et al., 2019).
Symptoms of CGMMV infection in cucurbits may be confused with symptoms caused by other viruses that cause mottle and mosaic, including the cucumovirus cucumber mosaic virus (CMV); the potyviruses papaya ringspot virus (PRSV), watermelon mosaic virus (WMV), and zucchini yellow mosaic virus (ZYMV); and the closely related cucurbit-infecting tobamoviruses cucumber fruit mottle mosaic virus (CFMMV), cucumber mottle virus (CuMoV), kyuri green mottle mosaic virus (KGMMV), watermelon green mottle mosaic virus (WGMMV), and zucchini green mottle mosaic virus (ZGMMV).. CMV and some potyviruses are common in cucurbit-producing regions of the United States and world. Currently, of the cucurbit-infecting tobamoviruses, CGMMV is the most widely distributed and damaging globally, but WGMMV has also been identified infecting various cucurbit crops in the United States (Pitman et al., 2019, Plant Dis. 103).
Like other tobamoviruses, CGMMV is seed-borne (contamination on seed coat and endosperm but not in embryos, Reingold et al., 2015) and can be easily transmitted mechanically resulting in new infections on susceptible hosts. Thus, a new infection can be initiated from contaminated seeds to seedlings (Sui et al., 2019). CGMMV can also be transmitted through infected cucumber pollen (Liu et al., 2014 Plant Pathol., 63) and by the movement of contaminated soil and transplants (Dombrovsky et al., 2017).
The mechanism of seed transmission (virus transferred from contaminated seeds to cause a new infection on seedlings through natural germination) in CGMMV is not fully understood. Through seedling grow-out, only low to no seed transmission to seedlings was observed when low number of seeds (hundreds) were tested (Sui et al., 2019). Assuming a low transmission rate (0.1%) from seeds to seedlings (Dombrovsky and Smith, 2017, Advances in Seed Biology), a high number of seeds (thousands) should be used for a seedling grow-out experiment to assess seed transmission. On the other hand, using seed extract processed from contaminated seeds as inoculum through mechanical inoculation, a high rate of virus transmission was obtained, demonstrating that CGMMV transmission can occur from contaminated seed to seedlings through mechanical means during germination (Sui et al., 2019).
All cucurbit species tested, thus far, are susceptible to CGMMV, including cucumber, muskmelon, watermelon, squash, bottle gourd, luffa (Luffa aegyptiaca), bitter gourd (Momordica charantia), snake gourd (Trichosanthes cucumerina), and winter melon (Benincasa hispida). In addition, some ornamentals and weeds are also hosts for CGMMV, including Amaranth (Amaranthus spp.), squirting cucumber (Ecballium elaterium), black nightshade (Solanum Americanum), wild gooseberry (Ribes uva-crispa), and benthi (Nicotiana bethamiana). These non-cucurbit hosts may serve as reservoirs for CGMMV during the off-season (Dombrovsky et al., 2017). However, in California, CGMMV was not detected in weeds in cucurbit seed production areas (Falk et al., 2019, National Plant Disease Recovery System).
Several factors may have contributed to accelerating the global distribution of CGMMV in recent years, including the production of seeds in countries where CGMMV is widespread (Pitman et al., 2022) and increased trade of seed internationally. The production and handling of cucurbit crops using practices that lead to mechanical virus transmission may also contribute to spread. CGMMV can spread quicky through the grafting process if a contaminated seedlot or an infected plant is used for scions or rootstock.
The geographic distribution of CGMMV has expanded from its centers in Europe and Asia to new territories, including North America, Australia, and Africa.
Note: Years in parentheses indicate when CGMMV was first identified in that location.
For the most up-to-date information on geographical incidence, please visit this website (www.eCucurbitviruses.org) or or CAB International’s Invasive Species Compendium.
Symptoms alone cannot be reliably used to diagnose CGMMV infection. Diagnosis requires molecular or serological protocols that are commonly used in many research and diagnostic laboratories.
Serological tests including ELISA (enzyme-linked immunosorbent assay) and lateral flow devices (Immunostrips) are commercially available. Immunnostrips can be used for detection of CGMMV on-site in grower’s fields. ELISA could be used for general detection of CGMMV on seed and plant samples.
Molecular assays, including RT-PCR (reverse transcription polymerase chain reaction; Sui et al., 2019), real-time RT-PCR Hongyun et al., 2008, J. Virol. Meth. 149; Pitman et al., 2022), and LAMP (loop-mediated isothermal amplification; Sui et al., 2019), have been designed to detect CGMMV in plant samples. LAMP assays, generally, have been reported to provide a higher level of sensitivity to that of conventional RT-PCR-based tests for CGMMV (Kwon et al., 2021, Mol. Cell. Probes 60; Li et al., 2013, J. Virol. Meth. 193; Hasiów-Jaroszewska et al., 2019, Plant Protect. Sci. 55; Sui et al., 2019). Also, LAMP can be used for on-site testing without the need for a sophisticated analytical instrumentation (Sui et al., 2019).
Contact your local plant diagnostic laboratory to determine testing capabilities.
Visit the diagnostic laboratories page for information on diagnostic laboratories in your state, protocol publications, and/or references describing detection methods for WCLaV-1 and WCLaV-2.
CGMMV is a quarantine and controlled pathogen in the United States. Because of the easy transmissibility of CGMMV and limited availability of resistant varieties, management methods should heavily focus on prevention. An integrated pest management strategy is needed for CGMMV.
One important measure to prevent virus introduction is to use clean virus-tested seed. Many companies now require testing of seed lots for CGMMV as well as other seed-borne viruses, including squash mosaic virus (SqMV) and melon necrotic spot virus (MNSV). Growers should purchase certified cucurbit seeds that have tested negative for these viruses. However, new CGMMV infection or outbreaks are still being reported, particularly in Australia (Mackie et al., 2023).
When possible, seed should not be saved from cucurbit fruits showing symptoms of disease or virus infection. Current seed treatment methods are incapable of achieving complete deactivation of CGMMV infectivity on seeds without affecting seed germination, although some can reduce the virus titer. The reason is that cucurbit seed is protected by a unique perisperm endosperm envelope underneath the seed coat, chemical disinfectants might have limited access to the right tissue for disinfection, resulting in incomplete seed treatment (Reingold et al., 2015).
Since CGMMV is stable in the environment and easily transmitted, the use of effective hygiene practices (e.g. wearing clean clothing, cleaning shoes, and washing hands) and disinfectants are important measures to prevent virus transmission in cucurbit crop production. The most effective disinfectants for CGMMV are Clorox bleach, Virkon S, and Virocid, which could be used for tool dipping, surface disinfection and facility cleaning (Chanda et al., 2021, Virol. J. 18).
Since the virus can persist in field for years on root tissue, volunteer plants, or weed hosts, cucurbit crops planted into CGMMV-contaminated fields will likely become infected (Dombrovsky et al., 2017). As a result, contaminated fields should be removed from cucurbit production by fallowing or planting to non-cucurbit crops. Soil treatment has been shown to be effective in reducing virus inoculum (Dombrovsky et al., 2022, Horticulturae 8).
Resistance to CGMMV in commercial cucurbit crops is limited. Some cucumber varieties with CGMMV resistance are commercially available (ASTA, 2018, Ellouze et al., 2020). Sources of potential resistance have been identified in muskmelon (Ruiz et al., 2021). Cucumber and watermelon germplasm are currently being screened for CGMMV resistance. When available, cucurbit varieties with resistance to CGMMV should be used in areas with a known history of CGMMV..
Producers and growers with suspected CGMMV should contact and work with their local extension agents and state diagnostic laboratories. As a regulated pathogen, the first reports of CGMMV should be to the state plant regulatory official (SPRO) and the state plant health director (SPHD), who will know the procedures to collect and send official samples for confirmation to the Plant Pathogen Confirmatory Diagnostics Laboratory (PPCDL)–USDA-APHIS-PPQ, Laurel, MD.
Tian, T. 2017. Cucumber green mottle mosaic. In Keinath, A.P., Wintermantel, W.M., and Zitter, T.A. (eds.), Compendium of Cucurbit Diseases and Pests, 2nd edition (pp. 128-130). American Phytopathological Society Press.
Visit the References page for a full reference list of cited articles.
This work is a publication of the Emerging Viruses in Cucurbits Working Group (EVCWG). It is supported by funding from the USDA NIFA (Agreement 2018-70006-28884) through the Southern IPM Center’s grant program project S22-026. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and the EVCWG and do not necessarily reflect the views of the USDA NIFA or the Southern IPM Center.
This publication may be distributed without alteration for nonprofit educational purposes provided that appropriate credit is given to the authors and the Emerging Viruses in Cucurbits Working Group. Permission for any other uses should be requested from the Emerging Viruses in Cucurbits Working Group.
Authors: Kai-Shu Ling, USDA-ARS, Bright Agindotan, USDA-APHIS-PPQ-PPCDL, and Samantha Thomas, Bayer Crop Science.
Senior editors: Rebecca A. Melanson, Mississippi State University, and William M. Wintermantel, USDA-ARS.
Reviewers: Aviv Dombrovsky, The Volcani Center, and Tom Turini, University of California.
The Emerging Viruses in Cucurbits Working Group and its outreach activities are supported by funding from the Southern IPM Center’s Southern IPM Grants 2022. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and the Emerging Viruses in Cucurbits Working Group and do not necessarily reflect the view of the Southern IPM Center.
Last updated August 2022 / Privacy
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