Virus infections cause economic losses in crops worldwide and their management and control present major challenges. In the field, double infections of two or more viruses are the rule, not the exception. The presence of several viruses in a plant makes it difficult to interpret virus-insect vector-plant interactions. Mixed infections can alter plant symptoms compared with single infections and may also impact their vectors. We describe plant-mediated indirect effects of virus double-infection on feeding behavior of an aphid vector (Aphis gossypii Glover) and virus transmission in melon (Cucumis melo L.). The viruses we used were a circulative cucurbit aphid-borne yellows virus (CABYV, Solemoviridae:Polerovirus) and a non-circulative cucumber mosaic virus (CMV, Bromoviridae:Cucumovirus). When melon plants were CMV-infected, indirect plant-mediated effects on A. gossypii feeding were like those reported on other plant species; specifically, intracellular punctures (pd) were more frequent and longer than on mock-inoculated plants, which enhanced CMV transmission. Similarly, when plants were CABYV-infected, we observed a statistically non-significant trend for increases in extended salivation (E1) and ingestion (E2) activities in phloem compared with mock-inoculated plants, which also enhanced CABYV transmission. When aphids fed on CMV-CABYV double-infected plants feeding behavior activities related to the transmission of both viruses were enhanced even more than when feeding on single-infected plants. Nevertheless, the virus transmission rate was the same on single-infected or double-infected plants. Thus, our results suggest that double infections do not modify viral dispersion compared with single infections since the indirect effect of CMV and CABYV in single infections is already optimized to favor virus transmission.
