Botany: Weed Of The Month October 2009 - Cuscuta Spp., Dodder

Botany

October 2009: Cuscuta spp., dodder

A group of leafless, parasitic vines that can be a serious problem in field and nursery crops, as well as in the home garden.

Family: Convolvulaceae, the Morning-glory Family

Distinguishing characteristics: Rapidly growing, twining, parasitic vines, without chlorophyll; roots present only on young plants; leaves inconspicuous or absent; stems yellowish or orange, threadlike, fleshy, with tooth-like projections called “haustoria” that penetrate the phloem tissue of the host plant; flowers tiny, about 1/8 inch in diameter, often borne in dense clusters; corolla white, somewhat bell-shaped, with 5 often reflexed lobes (as seen with magnification); pistil with two separate styles, or the styles united; fruit a whitish berry, opening by a cap-like lid, or more commonly disintegrating on the vine or the soil and releasing the seeds gradually.

Distribution: The genus contains about 150 species with a worldwide distribution. In this country, one or more species is found in every state except Alaska. Many species are restricted to a single host, but others have a wide host range. All of the species native to Florida belong in the latter category.

Occurrence in Florida: Eight species are native in Florida, and of these, Cuscuta pentagona is the most widely distributed. But one species or another is found in virtually every county. Of the exotic species, only C. japonica has been collected in Florida, and that only in Gadsden County.

Similar species in Florida: When it is not flowering or fruiting, the love vine or woe vine, Cassytha filiformis of the Laurel Family (Lauraceae), is virtually indistinguishable from dodders. That plant however is more tropical or subtropical in its distribution and is not found north of Brevard, Polk, and Hillsborough counties. The love vine contains some chlorophyll, and the stems are green when young, but quickly become orange; they emit a pungent odor when crushed. The stems of dodders are orange from the beginning, and are not aromatic when crushed. The fruits of the love vine are ¼ inch in diameter, green with black seeds, and are borne singly along the stem. Those of dodders are about 1/8 inch in diameter, whitish, and appear in dense clusters. In general, the love vine attacks woody plants while dodders infest herbaceous plants, but there are many exceptions.

Means of dispersal: Dodders produce seeds profusely, and these must be scarified, by natural weathering or fungal attacks, before they will germinate. The seeds can remain viable in the soil for a period of 20 years or more.

Comments: Dodders are obligate parasites, completely dependent on their host plant for nutrition. They generally do not kill their host, but can substantially weaken it. A few plants, such as grasses and other monocots, appear to be immune from infestation. Dodders are difficult to control once established and move rapidly from plant to plant, especially in a nursery situation. Herbicides such as glyphosate generally kill the host as well as the parasite. Contact herbicides such as acetic acid or pelargonic acid applied carefully only to the dodder might work for a homeowner. Manual removal can also be successful on a small scale, but vigilance is required, since overlooked pieces can regenerate the parasite. If an infestation has been a problem in the past, a pre-emergent herbicide will prevent dormant seeds from germinating in the first place. Plants of all species are frost-susceptible and perish during the winter, at least in northern Florida.

Florida's noxious weed list (5B-57.007, F.A.C.) includes all Cuscuta species worldwide, except for the eight that are native to the state. The USDA list specifically excludes 52 native American species. However, all dodders are listed as mandatory quarantine items when detected in nursery stock by FDACS/DPI under the authority of Florida Statute 581.031.

Further information: Haynes, Coile, and Schubert, Botany Circular # 30; www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7496.html