Potential effects of an organic biostimulant on the tri-trophic interaction zucchini-ZYMV-aphid vector

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. Several crop traits were compared between healthy and ZYMV-infected plants, which were treated or untreated with biostimulant. Treated plants had increased growth with higher values of reproductive parameters (number of flowers and fruits) even in infected conditions. After treatment, a decrease of ZYMV titre and a progressive slowdown of symptom severity were observed in plant, together with the up-regulation of some defence genes. 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. When the aphids were caged on both infected and healthy potted plants, the biostimulant treatment led to reduce aphid survival and decrease offspring production, regardless of the infection conditions. Volatile organic compounds (VOCs) were collected and analysed by GC-MS, and then used in olfactometer experiments comparing different combinations of treatment and infection conditions. When allowed to choose freely between treated and untreated plants, aphids preferred untreated plants independently of their healthy status, confirming a potential role of biostimulants in repelling the vector. 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.