By JOY AUFDERHAAR and ROGER BENDER
(Editor’s note: The following is reprinted from the OSU Sheep Team Newsletter Oct.-Dec. 2009.)
COLUMBUS — As you looked across your pasture and hay fields this past September you may have noticed not only were the surrounding trees turning fall colors but your red clover and alfalfa were also showing colors of fall? But this is not a color of fall we like to see especially in our red clover and alfalfa.
Yellow or orange threadlike stems were reported in red clover and alfalfa fields in several western Ohio counties in September. The stems are stringlike, twining, smooth and branching to form dense masses in some fields.
Purdue’s Glen Nice said that dodder is a parasitic plant without any leaves or any chlorophyll to produce its own food. It lives by attaching to a host with small appendages (called “haustoria”) and extracting the host plant’s carbohydrates.
Weaken host plants
Dodder can weaken host plants enough to reduce yield, quality and stand. If infestations are severe enough, dodder may kill host plants. When looking at broadleaf plants, single dodder plants may be missed if you don’t look close enough. They appear as yellow strings winding up the stems or over the leaves of other plants. Dodders are annuals that spread by seed.
Having a hard seed coat, it is suspected that gas and water levels control seed dormancy. Although neither toxic nor unpalatable to sheep, dodder is not readily eaten but to reduce the spread, contaminated hay and feeds should not be used and grazing infested pastures should be avoided.
The seed is spread readily in water by natural streams or by irrigation. Sheep can also spread the seed in feces and in mud on their hooves. Dodder seed can remain viable after ingestion and it is known to grow in the feces of sheep.
This information was taken from the State of New South Wales in Australia which deals with 14 different species of dodder and as you know they deal with 100 million head of sheep potentially spreading dodder seed.
Controlling and eradicating
Dodder seed may be able to survive in the soil more than 20 years. Some have speculated this summer’s cooler conditions have enhanced the growth of dodder.
Short-term control of dodder with herbicides depends on the crop in which you wish to control it. Some herbicides may affect dodder, but also may affect the crop, or not be labeled for use in that crop. Always read and follow herbicide labels.
In many cases, dodder control may be more effective if herbicide applications are made before the plant attaches to the host. Pre- applications of Kerb have provided good control of dodder in ornamentals and turf (Anonymous). Treflan and Prowl have also been reported to suppress dodder germination (Mueller, 2006).
However, in most cases that Nice has experienced, pre-applications often do not retain enough residual activity to provide control for the rest of the season. Glyphosate has been reported to control dodder post 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. Others suggest little or no control with glyphosate. 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 COC or methylated seed oil to suppress dodder.
For long-term control if you have heavily infested pasture and/or hay fields NSW Department of Primary Industries recommends the following:
– Prevention is best. Dodder is not readily eaten by livestock but to reduce the spread, contaminated hay and feeds should not be used and grazing infested pastures should be avoided.
– Sow clean seed. Pasture seed infested with golden dodder is an important source of new infestations. Because of their similar size, dodder seed is very difficult to remove from lucerne and small clover seeds. It is essential, therefore, to use certified seed.
– Buy clean fodder. Contaminated hay and grain can introduce dodder to a clean property. Hay containing dodder should be destroyed as mature seed is often present.
– Mark and quarantine areas of infestation. Stock grazing on areas known to be infested with golden dodder should be kept in ‘quarantine’ for at least two days before they are moved to dodder-free pastures. Stock can transport seed and even pieces of plant.
Wherever infestations are found they should be marked and the area quarantined from activities such as hay-cutting and grazing. Care should also be taken to avoid spreading seed in soil, on machinery or boots.
– Overall weed control. The control of weeds that are the preferred hosts for golden dodder will reduce its chance of spreading and prevent them becoming sources of infestation. Since dodder can be spread by seed and by its extending tendrils, it is important to always treat outside the obvious areas of infestation.
A buffer zone of at least one meter diameter outside the initial infestation should be treated or burnt to prevent this occurring.
– Cutting or burning. Dodder can be controlled by cutting the host plant as close as possible to ground level and burning it. Burning can be carried out after cutting and drying. Use flame throwers or other combustible material such as old hay or distillate.
Burning is more effective following application of a desiccant herbicide, or diesel. The hotter the burn the better the kill of dodder, but the greater the risk of killing the host plant.
– Crop rotation. Clover stands which are severely infested should be cultivated and replaced with less susceptible crops or pastures. Cereal crops such as wheat, barley, oats, triticale and cereal rye are poor hosts to dodder. Summer grain crops such as maize and sorghum are resistant to dodder.
Deep ploughing can help reduce the seed burden by burying dodder seed. Most golden dodder seed will not germinate from a depth of greater than 7.5 cm. When re-sowing susceptible clovers, late summer to autumn sowings are less likely to be infected by dodder during the establishment period.
– Follow-up control. Under ideal conditions, the seed of dodder can survive in the soil for up to 20 years and some plants could still re-infest paddocks several years after a successful control program. It is essential that a follow-up control program be implemented.
For more on dodder, refer to the following Web sites: www.btny.purdue.edu/weedscience/2005/Dodder05.pdf, www.ppdl.purdue.edu/ppdl/dodder.html, and http://www.dpi.nsw.gov.au/agriculture/pests-weeds/weeds/profiles/golden-dodder/agfact.
(Joy Aufderhaar is an agriculture program assistant, OSU Extension Shelby County and Roger Bender is a Shelby County Extension educator.)
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