Dodder, Cuscuta sp

Thursday, July 23, 2009

Nature Notes
by Bob Thomas

Next time you travel to Baton Rouge during the summer, keep your eyes peeled for what appears to be spaghetti growing on plants in the marsh. This tannish-yellow, pasta-looking stuff is dodder, Cuscuta sp. At least nine species occur in Louisiana, and about 150 worldwide. It is a member of the morning glory family (Convolvulaceae, though some specialists place it in the Cuscutaceae), and is a true obligate parasite (meaning that it must parasitize to live) on other plants. Dodder expert Dr. Colin Purrington, Swarthmore College, says, its "host is rarely killed, but always inconvenienced."

Among species of dodder in the United States, coloration ranges from yellow to bright orange depending on, among other things, the concentration of carotenoids. Being a near perfect parasite, dodder is virtually devoid of chlorophyll except in its fruit, stems and buds (dodder has no leaves).

Dodder is a flowering plant, with flowers white or tinged with pink or yellow.

Seeds drop to the ground and germinate the following growing season if there are host plants in the vicinity. If there are none, seeds may go five years without germinating.

All curl (circumnutate) counterclockwise around stems of their host plants. The next logical question for the curious naturalist is if species in the southern hemisphere have clockwise circumnutation. Dr. Purrington says no, they twist counterclockwise in Australia, too.

Like vines on the rainforest floor, newly-germinated dodder vines have the ability to find even a distant host. Research shows that they find host plants by perceiving differences in light frequencies surrounding the potential hosts and sensing chemical cues resulting from volatile compounds emanating from nearby plants.

Climbing dodder vines are smooth surfaced. When they curl around a plant that is a good candidate for food, modified adventitious roots called haustoria, resembling prolegs of caterpillars (the leg-like appendages on the caterpillar's abdomen), penetrate the stem by a combination of brute force and the secretion of specialized enzymes that dissolve the cuticle of the plant.

Other than directly affecting the health of its host, dodder may also be a vector of phytoplasma (specialized bacteria that were once called mycoplasma-like organisms). Phytoplasma causes a plant disease called yellows, or yellowing, that has decimated the coconut population in the Caribbean and beyond. Sugarcane is also host to yellows (this is in addition to the sugarcane yellow leaf virus).

Dodder can be very intrusive, not only parasitizing plants that it consumes, but overwhelming other plants due to the expansiveness and density of its growth, strangling most along the way. Dodder can be controlled by a variety of herbicides, but one of the best control methods is to cut and destroy plants infested with dodder.

Enjoy viewing dodder during the summer, but know that it is basically up to no good and, in the grand scheme of things, we have yet to discover its virtues. Hmmm. Maybe if we start with a roux ...

For a wealth of information and many photographs, visit Dr. Purrington's website.

                                        

Patrick Thomas poses with dodder overwhelming                               Dodder growing on vegetation in the
beach vegetation in Navarre, Florida.                                                LaBranche Wetlands beside I-10 just
Photo by Bob Thomas.                                                                 north of Kenner.
                                                                                                        Photo by Bob Thomas.

                                       

Another view of dodder in the same                                                Dodder tendrils climbing Arabidopsis.
vicinity.                                                                                        Photo by Colin Purrington.
Photo by Bob Thomas.

                                     

Close-up of dodder entwined around                                             Dodder penerates the parasitized stem using
a Chenopodium stem.                                                                    its haustorial invaginations (the yellow
Photo by Colin Purrington.                                                        
projections entering the plant cross-section
                                                                                                  above). It then extracts water and other
                                                                                                  nourishing elements from the host.
                                                                                                     Photo by Colin Purrington.