I recently had a conversation with a local farmer about his alfalfa fields and the presents of Dodder. Dodder, is a parasitic annual plant that infests many crops, ornamentals, native plants, and weeds. More than 150 species occur worldwide, although dodder is most prevalent in the Americas.
Dodder is a parasitic weed that causes serious problems in forage and seed alfalfa fields. Parasitized alfalfa plants grow less vigorously, often becoming so weakened that they die. Because dodder is succulent, heavily infested alfalfa may require at least an extra day to cure. A severe dodder infestation can thus reduce stand, cause improper curing of hay, and lower forage and seed yields. As a result, hay prices may be discounted, often by more than $10 a ton
Dodder has slender, twining or threadlike stems that vary from pale green to yellow or bright orange; the bright stems can be readily seen against the foliage of the host plants.
Three species of dodder have been found in alfalfa: largeseed dodder (Cuscuta indecom), field dodder (C. carnpestris), and smallseed dodder (C. planzflora). Largeseed is the only dodder species in the high desert region.
Most dodder seeds are hard, with only a small percentage of the seed present in an alfalfa field germinating in any one year. As a result, once an alfalfa field is infested with dodder, it can be expected to be a problem for several years.
Field observations have shown that dodder first emerges in late winter to early spring, depending on the location. Initial emergence occurs in late May in the northern intermountain alfalfa production region and March in the high desert.
Dodder emerges as a rootless, leafless stem, dependent on the food reserve stored in the seed for its immediate survival. A suitable host, such as alfalfa or certain weeds, must be found within a few days or the dodder seedling will die.
Once the dodder twines around the host, it embeds suckerlike structures (haustoria) into the stem. Its contact with the soil is then severed and it lives at the expense of the host plant. New shoots are initiated at the point of attachment and become attached to other stems on the same plant or adjacent plants.
Dodder can form dense mats greater than 10 to 15 feet in diameter, which often coalesce, turning large areas of the field an orange color. During the summer, numerous clusters of small white flowers are produced near the alfalfa stem. Seed production is prolific, with each flower capable of producing up to four seeds.
Dodder has been an extremely difficult weed to control. Previously available pre-emergence herbicides provided only erratic, short-lived control. Growers have relied on nonselective contact herbicides and on flaming with propane-fueled burners to combat this tenacious pest after it has become attached to the alfalfa. These methods of dodder control are time-consuming, costly, injurious to the alfalfa, and largely ineffective.
The most successful control involves a systematic approach that combines several methods; you usually cannot eliminate dodder with a single treatment or in a single year. If you see native dodders infesting plants, take immediate action to eliminate or reduce the infestation.
Clean and inspect clothing and equipment before moving from infested to “clean” areas. Once you know an area is infested, you must manage it to prevent the further production of dodder seed. Isolate small infestations, and remove them before the plant produces seed. Monitor larger infestations, and mow, burn, or spray herbicides to prevent seed production.
In agricultural settings, cultivate dodder before it attaches to a host plant, since cultivation done after dodder has attached itself to the host is of no benefit. Hand pulling, cutting, or mowing also can reduce dodder infestations. Be sure to break off, cut, or mow the host plant just below the point of dodder attachment (about 1/8 to 1/4 inch) for these methods to be effective. Close mowing is an effective management tool for dodder in alfalfa.
Burning reduces a dodder infestation as long as you destroy the invaded tissue of host plants along with the dodder to prevent regeneration of the dodder from embedded haustoria. Burning kills only some of the dodder seed; the amount of seed destroyed depends on the duration and intensity of the fire.
Several disease organisms are known to infect dodder including A. alternata and Geotrichum candidum, which attack field dodder (C. pentagona). Researchers in China have found that a suspension of Colletotrichum gloeosporioides can selectively control the dodder species C. chinensis and C. australis in soybeans. Difficulty in culturing and applying these organisms has limited their commercialized use.
Where dodder has been a persistent problem in certain commercial agricultural fields, apply pre-emergent herbicides (e.g., trifluralin) before dodder seed germinates; where practical, follow up with close mowing, burning, or spot removal of parasitized host plants to control dodder plants that escaped the herbicide application.
Usually post-emergent herbicides, which you apply directly to the dodder plant to control it, do not selectively control dodder without injuring the host plant and are not a good choice for controlling established infestations.
Treflan® and Prowl® have also been reported to suppress dodder germination (Mueller, 2006). However, in most cases, pre-emergent applications often do not retain enough residual activity to provide control for the rest of the season.
Results from post-emergent applications to control dodder after it has attached to host plants can be more variable. Post-emergent application recommendations for dodder control are few and inconsistent.
Dawson and Saghir (1983) reported Dactathol® (DCPA) achieved 100 percent dodder control three weeks after application. However, Dactathol® is not labeled for alfalfa or clover, crops that often experience dodder problems.
Glyphosate has been reported to control dodder post-emergent and can be applied as a spot treatment of a 1-2 percent solution to alfalfa. However, be aware that the alfalfa will be damaged where glyphosate is applied.
The use of Roundup Ready® alfalfa would be a good option in fields that have dodder problems.
Raptor® can suppress dodder at 5 fl oz/A when applied after dodder emergence and applied before it is three inches tall. Pursuit DG® also can suppress dodder after emergence, but as soon as dodder attaches to the host plant, suppression drops.
The Pursuit® label recommends using it with Crop Oil Concentrate (COC) or methylated seed oil to suppress dodder.
Dodder: A Parasite in the World of Plants, Glenn Nice Bill Johnson Tom Bauman Purdue Extension Weed Science
Dodder Control in Alfalfa, Steve B. Orloff, Ronald N. Vargas, David W. Cudney, W. Michael Canevari, Jerry Schmierer
Dodder, Pest in the Garden and Landscape, W. T. Lanini, Weed Science/Plant Sciences, UC Davis; D. W. Cudney, Botany/Plant Sciences emeritus, UC Riverside; G. Miyao, UC Cooperative Extension, Yolo Co.; and K. J. Hembree, UC Cooperative Extension, Fresno Co.