Every year, I spend the first few months of summer waiting for my favorite plant freakazoid to stage an uprising. Cuscuta, or dodder vine, is a native parasitic plant lacking chlorophyll, resulting in a ghostly, pale yellow tinge. Besides the unnatural hue, the plant lacks any vestige that would make it easily identifiable as such, like true roots or leaves. It more closely resembles spaghetti al dente haphazardly draped over a plant, slowly sapping its resources. During my time at Old Dominion University, it was dodder that opened my eyes to the secret world of parasitic plants.
Of nearly 300,000 described species of plants on earth, parasites are only a small fraction — around 1%. However, they represent a multitude of strategies for obtaining nutrients from a host. They can be partially or completely dependent on their host, and parasitize underground or above it. The physical form of parasitic plants is just as varied as their strategy. Among such plants: Indian pipe (Monotropa uniflora,) with its eerie white flower feasts off the mycorrhizal fungi of trees. The beautiful broomrape (Orobanche) wreaks billions of damage to the agricultural sector each year. And mistletoe with its inconspicuous green flowers has made its way into Christmas tradition. It is green, like most plants with chlorophyll for food production, but it also drains nutrients from the trees on which it hosts.
During my undergraduate time at ODU, I had the pleasure of assisting in the germination of Cuscuta seeds for study. After the scarified seeds were germinated, the seedlings’ hooklike growth was placed on a coleus in the greenhouse. After the rootlike structures penetrated and began to siphon nutrients from the coleus, the Cuscuta plants hosted happily there. Unlike many other parasites, Cuscuta’s host selection is not well-understood; it does not always seem to have a preference. In regard to the broad selection of hosts, Lytton Musselman, a botanist at ODU, explained in his journal publication The Genus Cuscuta in Virginia that “each species may be characterized by what it does not attack.” To an untrained eye, and lacking an electron scanning microscope, the nine species of Cuscuta of Virginia look nearly identical. But Musselman goes on to explain that Cuscuta indecora is the only species found on high tide bush (Iva frutscens), and so I know the dodder I wait for each year is Cuscuta indecora.
It is only a matter of weeks before a shrub line will be completely overtaken by the tightly twining dodder, but the hosts are fighting back. According to a study by Justin Runyon et al. in which Cuscuta seeds were germinated on nearby tomato plants, hosts developed defenses against Cuscuta like spiky trichomes, which blocked 30% of the parasitic seedlings from attaching. And tomato plants that did have Cuscuta attached had a spike in their defense chemicals, like salicylic and jasmonic acid, the same way they would during insect herbivory or pathogen infection.
More studies are underway to understand how defense chemicals may slow the growth of parasitic plants.
It seems the war between plants and parasitic plants is raging all around us, each side constantly evolving new strategies. I will continue to enjoy watching the spaghetti-like Cuscuta slowly suck the life out of unwitting hosts.
In Full Bloom is a weekly feature from Allissa Bunner that focuses on sustainable gardening, environmental stewardship and related community news and initiatives. Bunner is a Norfolk resident who is passionate about plants — especially natives — and enjoys growing things from seed. She can be reached at allissa.x@gmail.com.
