Holoparasitic plants are nonphotosynthetic and therefore extract resources from hosts. The holoparasite Cuscuta gronovii (dodder) is native to much of the US and has a broad host range that includes Verbesina alternifolia (wingstem), an understory perennial. This dodder species is commonly found in mesic to wet-mesic conditions and often occurs in disturbed habitats, yet is also an agricultural pest of several crop species. Cuscuta gronovii and V. alternifolia can be found in areas that may experience prolonged or repeated episodes of drought. While holoparasites can significantly reduce host growth it is not clear how pattern of drought stress affects tolerance to dodder. We conducted a greenhouse experiment that examined the effects of pattern of drought stress on tolerance of V. alternifolia to parasitism by dodder. Hosts were assigned one of six treatments that were combinations of dodder (no dodder or addition of a seedling 34 days after host germination) and drought stress (well-watered >85% saturation, maintained at 40-45% of saturation, or reduced to 35% of saturation then watered and stressed again). Host shoots and roots were harvested and dried 32 days after water stress began.
Results/Conclusions
MANCOVA using host leaf length as a measure of size before treatment yielded a significant interaction between water and dodder treatments. Response of host shoot mass to drought stress was similar regardless of parasitism by dodder. Well-watered plants had significantly larger shoot mass than either drought-stress treatments. Continuously-stressed and pulsed-stressed plants were about half the size of well-watered plants but did not differ from each other. Response of root mass to water stress depended on dodder. In the absence of dodder, well-watered plants produced the largest root mass and continuously-stressed plants produced the smallest root mass. Differences between water treatments in root mass disappeared when plants were parasitized, and average root mass of parasitized plants was less than half that of parasite-free hosts. These results confirm that parasitism can strongly depress host growth but the impact of parasitism depends on water availability. Understanding how patterns of drought stress affect host-parasite interactions can lend insight into the impact of climate change on natural communities. This information may also guide managers seeking to control the parasite in areas where it is an agricultural pest.