Submitted to: Madrono
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2003
Publication Date: 8/1/2003
Citation: JEWETT, D.K., GAFFRI, M.S., SPENCER, N.R. A COLLECTION OF CARDARIA DRABA (BRASSICACEAE) AND RELATED TAXA FROM THE WESTERN UNITED STATES AND ITS IMPLICATIONS FOR THEIR MANAGEMENT. MADRONO. 2003. v. 50(3). p. 203-208. Interpretive Summary: Hoary cress is a perennial mustard introduced to the United States approximately 150 yrs ago as an ornamental and as a contaminant of both ship ballast and alfalfa seed from Turkistan. Presently, it occupies one-half million acres throughout the western United States, contributing to outbreaks of disease on vegetable crops because it is a reservoir of pathogens transmitted my numerous insect species. Having spent much of autumn and winter dormant as indistinct rosettes, plants develop during early spring and have bloomed and set seed by midsummer. Despite differences in morphology of their seed heads later in the year, other plants often are mistaken for hoary cress. Among them are lens-podded and globe-podded hoary cresses. Hybridization between two of the species further confounds identification. Often, only rosettes are present and distinguishing among specimens is inconvenient. Difference in their response to herbicides has been quantified, but anecdotal reports of their ineffectiveness against hoary cress may simply reflect the frequency with which plants in the field are misidentified. Furthermore, inability to identify specimens is an obstacle to developing a biological control program because potential agents may not be reconciled with identity of plants from which they were collected. During 1999 and 2000, 97 specimens of these three different plants were collected. Many could not be identified because their development was incomplete, illustrating the obstacles to developing an integrated management for hoary cress. Before developing a management program, identification of more convenient ways for distinguishing among hoary cress and related taxa is advised.
Technical Abstract: To illustrate the frequency with which systematic resolution is not adequate in the field for successful management, specimens of Cardaria draba (L.) Desv., C. chalepensis (L.) Hand.-Mazz., and C. pubescens (C.A. Mey.) Jarm. from throughout the western United States were collected. After their taxonomic assignment by Dr. Ihsan Al-Shehbaz (MO), they were deposited either at the Missouri Botanical Garden or at the University of Georgia, Department of Botany. Of the 97 vouchered specimens, Al-Shehbaz identified 47 as C. draba, 22 as C. chalepensis, and 11 as C. pubescens. Seventeen of the 97 specimens were not readily identifiable; 8 are C. chalepensis with distinct features of C. draba, and they may be of hybrid origin. These intermediates possess either distinctly carinate fruits, reticulate valves, or only slightly cordate fruits. Nine specimens are either C. draba or C. chalepensis, but further resolution is not possible because only young fruits are present. Difficulty identifying immature material of Cardaria has significant implications for management. Any integrated management program directed against the three Cardaria spp. in North America will be of limited value without more convenient and reliable means of identifying specimens in the field. Differential response of each species to a variety of herbicides is dose dependent, and anecdotal reports that herbicides are ineffective against hoary cress may simply reflect frequency of misidentification in the field. Attempts to develop a biological control program for Cardaria spp. may be encumbered by inability to reconcile survey for potential agents with identity of plants from which they have been collected.