Resistance to the cucumovirus Cucumber mosaic virus (CMV) in cucumber cv. Delila was manifested as a very low level of accumulation of viral RNA and capsid protein, and an absence of CMV-induced symptoms. In addition, resistance was observed at the single cell level, with a reduction in accumulation of CMV RNAs, compared to accumulation in cells of the susceptible cucumber cv. Bet Alpha. Resistance to CMV in cv. Delila was broken by co-infection with the potyvirus Zucchini yellow mosaic virus (ZYMV). Resistance breakage in cv. Delila plants was manifested by an increase in the accumulation of (+) and (-) CMV RNA as well as CMV capsid protein, with no increase in the level of accumulation of ZYMV. Resistance breakage in the resistant cultivar by ZYMV also occurred at the single cell level. Thus, synergistic interactions known to occur between a potyvirus and a cucumovirus led to resistance breakage during a double infection. However, resistance breakage was not accompanied by an increase in disease symptoms beyond those induced by ZYMV itself. On co-inoculation with an asymptomatic variant of ZYMV-AG an enhancement of CMV infection occurred without disease manifestation. Consequently, intensification of viral RNA and capsid protein accumulation can occur without a corresponding increase in disease development, suggesting that different host genes regulate viral accumulation and disease development in the CMV-resistant cucumber plants.