Strain diversity and resistance to cucumber green mottle mosaic virus (CGMMV) in cucumber

Cucumber green mottle mosaic virus (CGMMV) is a tobamovirus that causes a disease affecting cucumber crops worldwide, leading to significant economic losses. To study the variability of CGMMV in southeastern Spain, partial genome sequences were obtained from isolates collected in 2017 and 2020 from cucumber crops. Phylogenetic analyses revealed that isolates clustered into two major groups, Asian (AS)-like and European (EU)-like CGMMV isolates. These two groups coexisted in the same area, crops, and even individual plants, although the AS type predominated. The accumulation and symptom expression of molecularly cloned isolates from these two groups were assessed in two cucumber cultivars (resistant and susceptible) under both summer and winter conditions. An in planta antagonistic interaction was detected between the AS and EU isolates, in which the accumulation of CGMMV-EU was suppressed during mixed infections. A multivariate analysis did not identify statistically significant differences due to variations in environmental conditions. Unlike CGMMV-EU, CGMMV-AS did not show significant differences in accumulation based on the plant genotype. To further investigate this, the response to CGMMV-AS infection was analyzed in additional susceptible and resistant cultivars. All cultivars appeared to be similarly susceptible to CGMMV-AS, in contrast to CGMMV-EU, which accumulated to a much lower extent in resistant compared to susceptible plants. These results reinforce the need to continue epidemiological surveillance, identify new sources of resistance, and implement strict control of infected seed trade-given the growing threat that CGMMV-AS isolates pose to cucumber cultivation.