Manipulation of light spectral quality disrupts host location and attachment by parasitic plants in the genus Cuscuta

Parasitic plants in the genus Cuscuta (dodders) make their living by extracting resources from other plants. While relatively few dodder species are agricultural pests, those that are can be challenging to control, in part due to their intimate physical and physiological association with host plants. Consequently, dodders remain pervasive and economically damaging pests in a variety of crop systems. The development of improved management strategies would be facilitated by greater understanding of the ecological and environmental factors that influence the establishment and perpetuation of dodder infestations. Light cues play an important role in dodder host location and attachment. To better understand the influence of light conditions on parasite ecology, and potential implications for management, we examined how manipulating the ratio of red to far-red wavelengths (R:FR), via both passive filtering of natural sunlight and active spectral manipulation using LEDs, affects host location and host attachment by two dodder species (C. campestris on tomato hosts and C. gronovii on jewelweed). For both host–parasite combinations, host location and subsequent attachment by dodder parasites was dramatically reduced in high-R:FR environments compared to control conditions (with R:FR characteristic of sunlight) and low-R:FR conditions. Circumnutation by dodder seedlings was also significantly faster under high R:FR. We observed short-term effects of high R:FR on the height and dry mass of tomato host plants (immediately following 7-day exposure), as well as changes in tomato volatile emissions. However, preliminary investigation of long-term effects on host plants suggests that short-term exposure to high R:FR (i.e. during the critical period when dodder seedlings emerge and attach to hosts) has little or no effect on host plant size or fruit yield at the time of harvest. Synthesis and applications. Our findings suggest that spectral manipulation during the early stages of crop plant growth (e.g. via light-filtering row covers) may have significant potential to augment existing methods for managing or preventing dodder infestations in agricultural crops. We discuss potential obstacles to the realization of its potential, as well as next steps towards the development and optimization of spectral manipulation methods for use in agroecosystems.