Cuscuta

Cuscuta can be identified by its thin stems of 1-3 mm in diameter appearing leafless, with the leaves reduced to minute scales.^[10] In these respects it closely resembles the similarly parasitic, but unrelated genus, Cassytha. From mid-summer to early autumn, the vines can produce small fruit that take the same color as the vine, and are approximately the size of a common pea. It has very low levels of chlorophyll; some species such as Cuscuta reflexa can photosynthesize slightly, while others such as C. europaea are entirely dependent on the host plants for nutrition.^[11]

Cuscuta has a cosmopolitan distribution, distributed throughout both tropical and temperate regions. Approximately 75% of the species are native to the Americas (the New World). The genus ranges from deserts and saline environments to riparian and littoral zones. It is distributed from lowland grasslands and forests to disturbed sites and mountain regions.^[14] The genus becomes less common in cool temperate climates; for instance, only four species are native to northern Europe.^[3]

After a dodder attaches itself to a plant, it wraps itself around it. If the host contains food beneficial to dodder, the dodder produces haustoria that insert themselves into the vascular system of the host.^[1] Far-red light signal and physical contact with its host plant are required to initiate haustoria formation.^[15][16] The vestigial root of the dodder in the soil then dies. Through the developmental process of the haustoria, the dodder's searching hyphae reach the vascular tissue of the host and form plasmodesmata connections.^[17] In addition to water and nutrients, the exchange of mRNA, small RNA, and small peptides occurs between the host and the dodder through this connection.^[18][19][20]

Host range

Cuscuta species exhibit diverse host preferences; while some are generalists, others are specialists restricted to specific host groups. The genus is known to infect a wide range of plants, including a number of agricultural and horticultural crop species, such as alfalfa, sugar beet, lespedeza, flax, clover, potatoes, chrysanthemum, dahlia, helenium, trumpet vine, ivy and petunias. As an ectoparasite and holoparasitic plant, or a plant that is non-photosynthetic and is completely dependent on a host.^{[21][22][23][24]} The dodder can grow and attach itself to multiple plants. In tropical areas, it can grow more or less continuously and may reach high into the canopy of shrubs and trees; in cold temperate regions, it is an annual plant and is restricted to relatively low vegetation that can be reached by new seedlings each spring.

Less is known about host defenses against dodder and other parasitic plants than is known about plant defenses against herbivores and pathogens. In one study, tomato plants were found to employ complex mechanisms to defend against dodder. Two pathways, using jasmonic acid and salicylic acid, were activated in response to attack by Cuscuta pentagona. Dodder attack was also found to induce production of volatiles, including 2-carene, α-pinene, limonene, and β-phellandrene. It is not known if or how these volatiles defend the host, but they could potentially interfere with the dodder's ability to locate and select hosts. Also, the presence of trichomes on the tomato stem effectively blocks the dodder from attaching to the stem.^[26]

Dodder ranges in severity based on its species and the species of the host, the time of attack, and whether any viruses are also present in the host plant. By debilitating the host plant, dodder decreases the ability of plants to resist viral diseases, and dodder can also spread plant diseases from one host to another if it is attached to more than one plant. This is of economic concern in agricultural systems, where an annual drop of 10% yield can be devastating. There has been an emphasis on dodder vine control in order to manage plant diseases in the field.^[5][27][23]

Many countries have laws prohibiting import of dodder seed, requiring crop seeds to be free of dodder seed contamination. Before planting, all clothes should be inspected for dodder seed when moving from an infested area to a non-infested crop. When dealing with an infested area, swift action is necessary. Recommendations include planting a non-host crop for several years after the infestation, pulling up host crops immediately, particularly before the dodder produces seed, and use of preemergent herbicides such as Dacthal in the spring. Examples of non-host crops include grasses and many other monocotyledons. If dodder is found before it chokes a host plant, it may be simply removed from the soil. If choking has begun, the host plant must be pruned significantly below the dodder infestation, as dodder is versatile and able to grow back from its haustoria.^[23]

C. chinensis seeds (simplified Chinese: 菟丝子; traditional Chinese: 菟絲子; pinyin: túsīzǐ) have long been used for osteoporosis in China and some other Asian countries.^[6] C. chinensis is a commonly used traditional Chinese medicine which is believed to strengthen the liver and kidneys.^[28] Cuscuta species are also used as medicine in Himalayan regional medical traditions.^[29]

• List of Cuscuta species

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  Cuscuta sp. with a gall
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  Cuscuta sp. flowers
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  Flowers and pollen grains of Cuscuta from Mumbai, India
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  Cuscuta sp. form
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  Cuscuta sp. form
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  Cuscuta sp. form
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  Cuscuta sp. form
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  Dodder Forming a Net on its Host
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  Cuscuta in Flower, Iran