Dodder

 

YELLOW THREAD STUFF

I know we have not seen this for a while but it happens when it rains.  That is why we haven’t seen it.  It is coming up everywhere this yellow or reddish thread weed, alfalfa farmers know it well it is dodder, Cuscuta species, 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. The genus Cuscuta is in the Cuscutaceae family, but sometimes it is included in the family Convolvulaceae (morning glories).

This weed germinates when on warm moist nights usually within a few days after it rains or heavy irrigation. It emerges and the stem starts spiraling until it touches a plant; it severs the root immediately as sends it hyphae in to the green plant cells and lives off the photolysis of the green plant.  An interesting study a few years ago indicated that dodder actually when multiple species of host plants were available had a preference for some.  As dodder plants grow, they continually reattach to the host. When other suitable hosts are nearby, dodder shoots spread from host plant to host plant, often forming a dense mat of intertwined stems, like spaghetti.  Shaded areas greatly reduce twining and attachment. It has no root system, but it does set seed which can remain dormant in the soil for long periods of time, greater than 10 years. 

Impact varies from moderate to severe reductions of plant growth and, in some cases, complete loss of vigor and death. The severity of an infestation depends on the growth stage of the host plant at the time of initial dodder attachment. With native dodder, the greatest growth reduction occurs when the parasite attaches to seedlings; the infestation usually doesn’t kill established host plants, but when multiple attachments are made to the same host plant, death can occur. The weakened state of infected plants also predisposes them to diseases and insect and nematode invasions.

Effective management requires control of the current population, prevention of dodder seed production, and suppression of new seedlings in subsequent years. Where extensive infestations exist, remove the infested host plants if possible.

Prevention

The use of dodder-free planting seed has long been a primary way of preventing the spread of dodder infestations in alfalfa. Many countries and states have seed laws that prohibit the presence of dodder seed in planting seed.

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 by hand before the plant produces seed. Monitor larger infestations, and mow, prune, burn, or spray herbicides to prevent seed production. 

Dodder seed can survive soil solarization—a method for killing weeds using a clear, plastic tarp and the sun’s heat—probably because of its hard seed coat. Although studies haven’t been conducted, composting might kill most dodder seed, because higher temperatures are reached in the composting process are higher than in solarization.

One of the bad things of doing research is about 13 years ago I did a blind study on dodder with a product I only know by a number, it worked great.  I don’t know if it will ever be labeled but one application killed it in alfalfa and on turf.  Other that this  there is not a lot of option except pre-emergent products in alfalfa.

Generally chemical control isn’t necessary in the home garden and landscape, since you can control dodder for the most part by cultivating seedlings or through hand removal or pruning. Although pelargonic acid (Scythe) is effective, it also kills any plant tissue it contacts; consequently, good coverage and careful spraying are important, so desirable plants aren’t damaged.  Usually post-emergent herbicides, which you apply directly to the dodder plant to control it, don’t selectively control dodder without injuring the host plant and aren’t a good choice for controlling established infestations unless it is on weed you want to control anyway.  Dodder is not toxic to wild life, but horses which consume large amounts are prone to colic.  Other ruminant livestock over a prolong period of consumption show signs of depression and become unthrifty.