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.