On a walk in the Smoky Mountains last summer, we came across a patch of plants that were covered with what looked like yellow Silly String. It was crazy looking! Strands and strands of fine golden strings were draped over and twining around these poor plants.
Eric knew what it was right away – dodder; a parasitic vining plant also known as scaldweed, strangleweed, devil’s hair, devil’s guts, love vine, and many other equally descriptive common names. Though this is not a disease, I decided to place this page in with "Disease Problems" because I wasn't sure where else to put it!
Dodder (Cuscuta spp.) is an annual plant that is effectively leafless and has contact with the soil only as a young seedling. These plants do not manufacture chlorophyll so they are unable to produce their own food through photosynthesis like green plants can. With the exception of the first few days after germination, dodder is entirely dependent on a host plant for the water and nutrients needed to complete its life cycle – an obligate parasite!
Despite its lack of chlorophyll, dodder IS a flowering plant and produces many clusters of tiny flowers from June through the fall. If pollinated, the flowers will form seeds just like other like flowering plants. In fact over the course of a season, a single dodder plant is capable of producing thousands of seeds!
Dodder seeds typically germinate in the spring and the seedlings, which have thin, vine-like stems, grow upwards and twine around the first solid object they encounter. Since the dodder seeds generally drop to the ground under the mother plant, the seedlings normally germinate among suitable host plants. For the first few days of growth, the dodder seedlings survive on nutrients stored in the seed. If they do not contact a host plant within 5-10 days, they will run out of food and die.
Once dodder contacts a host plant, it quickly twines around the stem of the plant and small structures called haustoria are produced along the dodder stem. The haustoria penetrate the vascular system of the host and begin to extract carbohydrates and water from its stem. At this point, the dodder plant is completely supported nutritionally by the host. The original (seedling) stem of the dodder eventually withers and this parasitic plant loses contact with the soil.
Once attached to a host plant, dodder continues to grow and reattach in multiple places along the host stem. Individual plants will often spread to nearby host plants creating a mass of stringy orange stems which can cover large areas. This is what we came across on our hike in the Smokies.
Parasitic organisms rarely kill their hosts since they rely on them for sustenance. Dodder is no exception and, though they may weaken and stunt the growth of the host plants, they generally do not kill established plants. However, if they attach to seedlings, they can seriously weaken them and may end up killing these young plants.
If it becomes established in agricultural fields, dodder can cause a significant reduction in crop yield. Alfalfa and sugarbeets are common host plants for some species of this parasitic plant. It can also infest ornamental plants including many perennials and annuals. Japanese dodder (Cuscuta japonica) often parasitizes trees and shrubs, including many types of fruit trees.
It can become a serious garden pest and unfortunately it is usually difficult to control. If you pull the stems off the host plant, any haustoria that remain embedded in the host stem will resprout and continue to parasitize the plant. If it is allowed to set seed, thousands of seeds can drop to the ground!
A pre-emergence herbicide can prevent the germination of dodder seeds but these seeds have a hard seed coat and have been found to remain viable in the ground for more that 20 years! Yikes!
One recommendation for control of dodder in the garden is to completely remove all the host plants and replace them with a non-host species. Thus any dodder seedlings that germinate will not find a suitable host and will die off.