Giant Asian Dodder

An invasive parasitic plant known as giant Asian dodder, Cuscuta japonica Choisy, was first detected in the greater Houston area in 2001. Since then, it has been found in half a dozen neighborhoods, primarily in southern residential districts of the city.   

The invasive species threatens a variety of trees and landscape plants. Giant Asian dodder is often cultivated in Asia for medicinal products and may have been intentionally introduced into Houston for similar purposes.  

The problem is that it has escaped and invaded street trees and landscape plants in several Houston neighborhoods.  

Identification

Giant Asian dodder is a yellow-green vine that resembles spaghetti. It is able to parasitize a wide range of hosts including many agricultural crops like alfalfa, tomatoes, and onions, woody ornamentals like live oak, crape myrtle, and Ligustrum, and a variety of other herbaceous plants.  

Dodder is usually considered an annual plant, but in Texas, especially in southern regions, dodder is believed to survive for more than one year. It is hardy to a temperature of nine degrees. Temperatures below nine degrees will effectively kill the dodder plant. Temperatures in southern Texas rarely reach nine degrees, though frosts do occur. When frosts take place, the exposed foliage of the dodder plants is affected, but the entire plant is not killed. As a result, it will be able to continue to grow and parasitize its host when warm weather returns. 

Impact 

Giant Asian dodder does not have chlorophyll. This means it requires a living host in order to obtain nutrients needed to survive. It gets nutrients from the host plant by producing specialized structures called haustoria, which attach themselves to the host plant. It takes sugars, amino acids, and certain nutrients from the host at the site of contact. This parasitic action drains the host of vital resources needed for healthy growth and may weaken and eventually kill it. 

The plant grows at a rate of six inches per day and can rapidly spread from one host to another nearby. It can spread to a new area by two methods. 

One method of spread is seed production. In spring, it produces flowers and, if fertilized (usually by self-fertilization or, rarely, by insects), the flowers yield an abundant number of seed. 

Seeds are the size of coffee grains and have thick seed coats that are impermeable to oxygen and water. This allows the seed to remain viable until suitable conditions are present for germination. Seed can remain viable in the soil for a period of 10-20 years. Usually, most seed produced will germinate the following year. 

Seedlings have been reported to emerge from soil four inches deep. Though most seedlings originate from seeds located ½ inch or less.  

Once the dodder seeds germinate, a rootless and leafless seedling is produced. The seedling absorbs water initially from the soil, in a manner similar to a root. It uses the soil as an anchoring point for finding a nearby host to parasitize. Giant Asian dodder seedlings can survive for several weeks without a host plant. During this time, the seedling lengthens and rotates counterclockwise, seeking a host for a source of nutrients and water. If the seedling does not find a host within a few weeks, it dies. 

The second method of spread is by fragmentation. This happens when sections of the dodder vine itself are removed from the main plant and establish on another host. Haustoria are produced from these fragments, establish a connection to the host to obtain necessary nutrients, and continue to grow. 

Management

Methods used to control or eradicate dodder include post-emergence herbicides, pre-emergence herbicides, injected herbicides, rogueing, and eradicative pruning. 

• Pre-emergent herbicides are used to prevent dodder seeds from germinating and seedlings from parasitizing available host plants. 

• Post-emergent herbicides may be used in conjunction with rogueing infected plants, removing host material, and eradicative pruning. The post-emergent herbicides effectively kill the dodder in place, so the potential for spread of the parasite during host plant removal and transport is minimized. When using eradicative pruning, such as in managed landscapes, a barrier treatment should be implemented as well. When possible, remove plants that are within 10 feet of the dodder-infected plants in order to create a host free barrier. 

• Herbicides injected into infected host plants are used in some locations when large trees are being attacked by the dodder or other unique difficulties are posed in safely dealing with the disease. A large tree will increase the likelihood of fragmentation and dispersal of the parasite while the host is being felled and cut for transport. Therefore, trees injected with systemic herbicides will rapidly kill the tree and the attached parasite. 

Using the control methods described above, efforts are currently underway to eradicate giant Asian dodder from known sites, before this invasive plant can become established over a wider area.