Biostimulants are substances that promote plant growth and efficiency of nutrient uptake by improving plant and/or rhizosphere characteristics. The effect of a biostimulant containing alfalfa, brown algae, and molasses extracts on the interaction among zucchini plants (Cucurbita pepo L.), zucchini yellow mosaic virus (ZYMV) and aphid vectors was evaluated. ZYMV is an aphid-transmissible potyvirus that affects several crops and weeds worldwide. Several crop traits were compared between healthy and ZYMV-infected plants which were treated or untreated with biostimulant. Biostimulant-treated plants had increased growth with higher values of vegetative parameters (number of leaves and dry biomass weight) in healthy conditions and of reproductive parameters (number of flowers and fruits) in infected conditions. After treatment, the ZYMV titre decreased over time even if no differences in symptom severity was observed compared to the untreated control plants. Real-time PCR was used to quantify the expression of several plant defence genes, and the pathogenesis related gene 1 (PR1), involved in systemic acquired resistance, and the peroxidase gene (POD), belonging to oxidative stress pathway, were upregulated in ZYMV-infected plants treated with the biostimulant. The effect of biostimulant on the settling preference and life traits of the aphid vector Myzus persicae was also investigated. Aphid choice test experiments using detached zucchini leaves showed that fewer specimens settled on biostimulant-treated plants, with and without ZYMV. Biostimulant treatment led to reduced aphid survival and decreased offspring production, regardless of the infection condition of the plants. The potential role on aphid choice of volatile organic compounds (VOCs) emitted by treated and untreated plants was also assessed in olfactometer experiments. Taken together, these findings suggest that biostimulant treatment can reduce disease risk in C. pepo crops by potentially contributing to the concurrent control of ZYMV and its vector.