Infective CGMMV particles are rigid rods ca 300 x 18 nm with a helical structure of pitch 2.3 nm and a central canal of radius 2.0 nm. The central canal is usually clearly visible in the electron microscope in negatively stained preparations. The RNA lies at ca 4.0 nm radius from the centre of the core and the helix comprises 49 subunits/3 turns (Hollings et al., 1975).
Related invasive species
- Cucumber green mottle mosaic virus
Related Farm Practice
- Control
- Scions
- Introduction
- Rootstocks
- Work
- Grafting
- Breeding
- Soil
Related location
- Israel
CGMMV is a species of virus in the genus Tobamovirus, which was first described in 1935 in England. Between 1935 and 1985 it spread slowly to other countries, but faster between 1986 and 2006, and rapidly between 2007 and 2018. It now occurs on all continents except South America. In cucurbits, it causes a damaging disease that reduces fruit yields and quality and spreads efficiently by plant-to-plant contact transmission. Outbreaks occur in many cucurbit crops including vegetables and fruits (e.g. squash and melons). CGMMV seed transmission occurs in at least nine different cucurbit crop species and this is the main way the virus has spread worldwide. Importation of contaminated seeds constitutes a considerable biosecurity concern for counties still without CGMMV. Its high stability and its persistence in contaminated plant material and soil allow it to survive between growing seasons, making eradication difficult.
The virus becomes systemic in most infected plants reaching all plant parts, including roots and fruit, and remains able to infect other plants even when symptoms are absent.
In infected foliage and fruit, CGMMV symptoms vary between different cucurbit crop species and cultivars of the same species. In cucumber, green mottling occurs on young leaves and fruit surfaces, and infected plants may collapse. In watermelon, leaf mottling and mosaic develop in young plants, and their stems and peduncles develop brown necrotic lesions. Their foliage may develop a bleached appearance and wilt, and their runners, or even whole plants, may die prematurely. However, foliage symptoms sometimes fade in mature plants, especially in open-field situations. The fruits of infected plants often develop symptoms that render fruit unmarketable, including malformation and internal flesh symptoms of sponginess, rotting and yellowing or dirty red discoloration. In melon, young leaves develop initial mottle and mosaic symptoms that often disappear from mature foliage. Their fruits develop different degrees of malformation, mottling and surface netting. In pumpkin, squash, and zucchini, infected foliage is asymptomatic or leaf mottling and mosaic occur. Pumpkin fruits are always asymptomatic, but squash and zucchini fruits are sometimes externally symptomless and internally discoloured and necrotic. CGMMV symptoms are often indistinct in cucurbit seedlings, except when cotyledons turn yellow. In addition to causing marketable yield losses from poor fruit quality, CGMMV also causes gross yield losses, e.g. 15% and 50% in cucumber and watermelon respectively (Hollings et al., 1975;Dombrovsky et al., 2017 and references therein). Symptomless infection can also induce losses (Kooistra, 1968).
Different CGMMV strains differ in the symptoms they cause. For example, in cucumber, the type strain causes leaf mottling, blistering and distortion and stunted growth. Fruits are usually unmarked, but some strains cause severe fruit mottling and distortion. Some Asian cucumber cultivars show no leaf symptoms but they do suffer yield losses. The aucuba mosaic strain induces bright yellow leaf mottling in cucumbers, with only slight leaf distortion and stunting. Fruits may show yellow or silver-coloured streaks and flecks, especially at higher temperatures. In cucumber, symptoms appear 7-14 days after infection. At low temperatures, when the plants grow more slowly, leaf distortion is more severe (Smith, 1972).
In the field, the virus causes mosaic and occasionally wrinkling, green vein-banding and stunting of muskmelons. In the greenhouse it causes mild chlorosis, mosaic, vein-banding, and at the later stages deformed leaves with blisters (strain CGMMV-M) (Raychaudhuri and Varma, 1978). In Lagenaria siceraria the virus causes mosaic symptoms (VIDE, 1996). In Ecballium elaterium (Antignus, 1990) and other weed hosts (Shargil et al., 2017) the virus causes a symptomless infection.
The majority of plant species infected by CGMMV are in the family Cucurbitaceae. These include both major and minor vegetable and fruit cucurbit species such as rockmelon, cantaloupe and honeydew melon (Cucumis melo), cucumber (Cucumis sativus), watermelon (Citrullus lanatus), zucchini, squash and marrow (Cucurbita pepo), pumpkin (Cucurbita moschata and Cucurbita maxima) and several gourd species (e.g. Benincasa hispida, Lagenaria siceraria, Luffa acutangula, Momordica charantia), which are grown either as crops in their own right or as rootstocks for grafted watermelon (Dombrovsky et al., 2017 and references therein). At least 15 weed species from different continents have been identified as potential natural CGMMV hosts. These belong to nine different plant families (Dombrovsky et al., 2017 and references therein).
Several cucurbitaceous weeds likely act as reservoir hosts of the virus. Symptomless infection by CGMMV occurs in the weed squirting cucumber (Ecballium elaterium) in Israel, where it acts as an important alternative host of the virus (Antignus et al., 1990). Recently Shargil et al. (2017) reported that infected E. elaterium plants could transmit the virus to melon, cucumber and Nicotiana benthamiana via a bioassay. Horvath (1985b) found the same species to be susceptible following artificial mechanical inoculation, and also reported additional systemically infected cucurbit hosts including: Cyclanthera brachystachya, C. pedata, Melothria pendula (locally), M. scabra, Momordica balsamina, Sicyos angulatus (locally) and Zehneria japonica. Horvath (1985a) subsequently found two further systemically infected hosts, Lagenaria siceraria and Luffa aegyptiaca. However, as Horvath’s studies involved artificial inoculation they do not necessarily reflect true natural hosts. Sandeep and Joshi (1989) reported Momordica charantia as a natural host of the virus. Outside Cucurbitaceae, several natural hosts are known, however they are mostly weeds. For instance, Shargil et al. (2017) found additional asymptomatic hosts including: pigweed (Amaranthus graecizans), A. muricatus, dyer’s cotton (Chrozophora tinctoria), dwarf heliotrope (Moluccella laevis) and ashwagandha (Withania somnifera). Shargil et al. (2017) also demonstrated infection of the seed for pigweed and dwarf heliotrope. Prunus armeniaca was reported as a host of the virus (Cech et al., 1980), but this requires confirmation. Silverleaf nightshade (Solanum elaeagnifolium) and black nightshade (S. nigrum) were reported as hosts but this could not be confirmed. Hovárth (1986) reported systemic infection in Emex australis and E. spinosa following artificial inoculation.
For further information on the natural and experimental host ranges of CGMMV, see Hollings et al. (1975), Shargil et al. (2017) and Dombrovsky et al. (2017).