While it is a general annoyance in regards to the ornamental plants, it is a serious threat to the agricultural crops. Because dodder spreads across many host plants, it can quickly infect an entire field. Each stem can grow up to 3 inches a day, and a single dodder can cover a circle of about 30 feet in diameter (Ristau 1998). Image one is a photograph taken by Jack Kelly Clark of dodder infesting a tomato crop.
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Image 1.
(http://www.ipm.ucdavis.edu/PMG/WEEDS/dodder.html 2000).
Infestation can cause more problems than just depleting the plant of nutrients; it can serve to transfer diseases to the many plants that one dodder can infect simultaneously. The basis of drawing nutrients from a host plant is tapping into its conductive tissues; this suggests that molecules that are solvated can travel from the dodder into the host plant, as well as into the dodder from the host (Hibberd and Jeschke 2001). This intimate joining of the two plants makes the dodder a great carrier for "yellow diseases". These diseases were first thought to be viral in nature, but were discovered to be caused by Phytoplasma. Dodders can cause many of these diseases, including pear decline, aster yellows, tomato big bud, vinca virescence and elm phloem necrosis (Ristau 1998). Dodders can also transfer bacteria that can be harmful to the host plant. A good example of this is the rickettsia-like bacteria that inhabit in the phloem tissue. .
While there are many species of Dodder plants, each is specific to one or more plant species that it attacks. Dodders select their hosts based upon what types of nutrients are available and the general suitability of the plant. Kelly and Horning (1999) have shown that Cuscuta attenuata grows more vigorously in patches containing multiple species of host plants, as opposed to patches with multiple hosts of the same species. This is illustrated in figure 1.
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Figure 1- Dodder growth on varying hosts.