I had never seen a plant like it before, a true parasite. I’ve seen my share of mistletoe, but only at Christmas when sprigs of the dying plant are bundled into cellophane bags and sold as a holiday gimmick. The first time I saw Dodder (Cuscuta sandwichiana) was on poor old Lanai’s dry piebald slopes, draped over trees, other plants, boulders, and bare soil, looking like so much faded yellow Silly String, the stuff kids squirt out of an aerosol can as a harmless bit of vandalism. It was everywhere on the bare slopes of the island and so ridiculous looking that, from a distance, I mistook it for some kind of unidentifiable trash like Excelsior packing material. I finally got out of my Jeep to examine the stuff up close; low and behold, it turned to be alive, a viney living mustard-colored blight on the landscape. Untainted by even a milliliter of chlorophyll, of course I loved it immediately. There’s nothing as admirable as a truly perverse plant species, especially if it’s a parasite.
The vines of a Dodder (Cuscuta sandwichiana) plant.
The aggressively parasitic plants belonging to the genus Cuscuta sp. have earned many justifiably derogatory common names, including “Strangleweed”, “Stranglevine”, “Devil Guts”, and “Witches’ Shoelaces”. The more polite Hawaiian name for Cuscuta sandwichiana is Kaunaʻoa. Swarthmore College biology professor Colin Purrington says, “It’s a horrible existence for the host plant. If plants could scream, they’d have the loudest screams when they had Dodder attached.”
A close-up of the vines, flowers, and foliage of Dodder
DESCRIPTION
A mature Kaunaʻoa plant resembles a tangled pile of yellowish-orange straw wrapped tightly around its host plant. It is mostly stem and lacks normal roots and recognizable leaves. The absence of any green coloration is indicative of the relatively low levels of chlorophyll within the plant’s tissues. The stems of Cuscuta sandwichiana entwine themselves around host plants. Its stems are thin, yellow-to-yellowish green thread-like vines, about 0.8 mm in diameter. Its flowers are white, growing in groups of 3-8 in spreading inflorescences spaced irregularly along the vines. The vines appear to be leafless, but the numerous tiny scales on the surface are in fact reduced scale-like leaves. An individual flower (or “corolla”) is about 2.5 mm long and bell-shaped. The light-brown fruit is a boll containing two to four seeds. The tiny brownish seeds are ovoid, 1.25 to 2.5 mm long, and 1 to 1.5 mm wide.
HABITAT
Cuscuta sandwichiana prefers a tropical or temperate environment with abundant sunlight and rainfall, and soil with good moisture content. It is not frost tolerant, and even a thin dusting of snow will destroy it. Its native habitat is North America, but it has become a widespread invasive species and a noxious weed in many other parts of the world. In Hawai’i, it is considered an introduced invasive species, but some local botanists consider it to be naturalized.
PROPAGATION & GROWTH
The numerous tiny seeds produced by Dodder are easily propagated by wind, water, birds, and humans. They germinate at relatively high soil temperatures (in nine days at about 14°C; in three days at 20-30°C). Seed productivity can exceed 100,000 seeds per plant. Germination occurs at or near the surface of the soil. The new seedling must reach a host plant within 5 to 10 days of germination, otherwise it will die. Some species of Dodder are able to “smell” their next victims. Research has shown that at least one species (Cuscuta pentagona) uses airborne (that is, volatile) chemical cues to locate host plants in its vicinity. Dodder seedlings displayed directional growth responses to volatiles released by species of host plants. When given a choice between volatiles exuded by a preferred host species and those of a non-host plant, the Dodder seedling actually “grew” toward the preferred host by extending its thread-like vine in the right direction. Once the seedling has attached itself to a host, it extends spiral-shaped vines that completely wrap themselves around the stem, branches, and leaves of the host. It then inserts syringe-like appendages (known as hyphae) into the tissue of the host. At this point, the parasitic appendage increases the surface area in contact with cell walls of the host’s tissues by producing haustoria (see photo) that exude enzymes to break down the cell-wall material, enabling the movement of organic nutrients (carbon, etc.) from host to parasite. The original root of the Dodder still anchored in the soil then dies; from then on it obtains all of its nutrients from the host.
Microscopic view of hyphae (long structures) & haustoria (dark spheres). Photo courtesy of E. Boutet.
Throughout its lifecycle, a Dodder continues to grow and attach itself to multiple host plants. In tropical areas (like our Hawaiian rainforests) it lives its life as a perennial vine that grows continuously, reaching high into the forest canopy. In drier temperate regions (such as the leeward shores of individual islands) it exists as an annual plant restricted to relatively low vegetation that can be reached by its offspring seedlings each spring.
HARMFUL EFFECTS OF DODDER
Dodder parasitizes a wide variety of native, weed, and crop plant species. The severity of an infestation depends on the species of the host plant, the climate and time of year of the infestation, and whether there are viruses and other pathogens present in the host. Dodder causes stunted growth or death in its host by disrupting its metabolic processes, literally sucking the nutrients out of it. It also decreases the host plant’s ability to resist diseases; it may spread diseases from one host to another by attaching itself to multiple host plants.
A spreading mat of Dodder (Cuscuta sandwichiana) parasitizes the vines of Beach Morning Glory (Ipomoea pes-caprae) at Kanaha Beach, north Maui.
METHODS OF CONTROL
Physical control of Dodder in agricultural crops is difficult because it requires the destruction of the host plant: this typically involves mowing and burning the cut material where it lies, spraying it with oil or another herbicide, and burning the slash. Effective control of any kind must begin prior to flowering of this aggressive parasite. When controlling an infestation of Dodder in crop species, it is important to keep seed stock physically isolated from the infesting plants, otherwise the Dodder’s numerous tiny seeds can easily intermix with those of the crop species. This means completely eradicating Dodder infestations from nearby roads and access trails, boundary strips, and adjacent wastelands. Fields that were previously infested by Dodder must be sown with crops or other plant species that are not susceptible. Biological controls have yet to be developed, but agricultural researchers in Russia have described a few species of weevils and fungi that prey on Dodder.
Tags: biochemistry, botany of Hawaii, botany of Maui, C. sandwichiana, Cuscuta, Cuscuta sandwichiana, Dodder, genus Cuscuta, haustoria, haustorium, Hawaii, Hawaiian plant life, Hawaiian weeds, hyphae, introduced plant species, invasive plants of Hawaii, invasive species, Invasive species in Hawaii, invasive weeds, invasive weeds in Hawaii, Kaunaʻoa, Maui's Ecosystems, Maui's weeds, Natural history of the Hawaiian Islands, naturalized plants of Hawaii, non-native Hawaiian plants, parasite, parasitic, parasitic plants, parasitism, plant biochemistry, plant ecology, plant growth, plant life of Hawaii, plants of maui, propagation, Stranglevine, Strangleweed, vine, weed control, weeds, Witches’ Shoelaces
