The project focuses on the phenomenon of mature plant resistance (MPR) to control the spread and impact of yellowing viruses of sugar beet. MPR is a general defence mechanism whereby plants develop increasing resistance towards pests and pathogens over the course of a growing season. Yellowing viruses of sugar beet form a diverse group, but they are all transmitted by various species of aphids. With growing age sugar beet plants produce a substance, which is apparently toxic to the aphids thus limiting aphid colony growth and subsequent virus transmission. Interestingly, yellowing viruses are reported to interfere with the production of this substance and thus promote their own transmission. The project aims to identify both the nature of the aphid toxicity factor as well as the pathway through which this factor is being produced. In addition, we aim to unravel how the viruses are able to interfere with the production of this factor.