Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/17/2010
Publication Date: 9/1/2010
Citation: Caesar, A.J., Caesar, T., Lartey, R.T. 2010. First report of anthracnose stem Canker of the invasive perennial weed Lepidium draba caused by Colletotrichum higginsianum in Europe. Plant Disease. 94:1166-1166. Interpretive Summary: Searches for weed pathogens are carried out in the native range of invasive weeds to better focus searches for domestic versions of the same weed pathogens. A stand of white top (Lepidium draba) was found in Switzerland with stunting, chlorosis and reddish stem cankers associated with wounds suggestive of insect feeding. A fungus was isolated from lesioned stem tissue with spore shape, size and presence of additional appendages called setae that were characteristic of Colletotrichum higginsianum, which has been reported to cause leaf diseases of crucifers. When spores of the fungus were produced in culture and sprayed on leaves, complete collapse of leaves resulted. Apparently leaf death was so rapid that infections did not spread into petioles or further host tissue and leaves formed from regrowth after inoculation were disease free. The fungus is apparently highly virulent on tissue it infects. Furthermore, in the field stem lesions, often extensive, can occur when there is injury, apparently by insects. This fungus, if found in the U. S. and if restricted in host range to white top and furthermore if it could be applied in conjunction with insects that damage stem tissue. Thus, this fungus has great potential as a component of a multi-pathogen and/or multitrophic strategy for the biological control of the invasive perennial weed white top or hoary cress.
Technical Abstract: The exotic perennial Lepidium draba occurs as an invasive weed in dense stands in rangelands and disturbed areas in several states of the western U. S., and as an agricultural weed in the prairie provinces of Canada. To help determine strategies for biological control of the weed such as a potential multi-pathogen strategy, surveys that included the native range were conducted to detect diseases that occur on this weed. Several stunted and chlorotic plants were found scattered throughout a stand of L. draba growing in a vacant lot near Riddes, Switzerland. Affected plants had reddish brown cankers on the lower stems, usually elongated and irregular in shape and slightly sunken. Insect injury was associated with the cankers. The symptoms often occurred on plants that were also infected with Rhizoctonia solani. After surface sterilization, tissue adjacent to and including lesions were plated on acidified PDA and incubated at 20-25º C for one week. Zonate, dark grey colonies with sparse mycelia resulted, which exhibited abundant, faintly pink spore masses with numerous dense clusters of black setae. Spores were single-celled, hyaline and cylindrical to oval shaped, measuring 13.5 to 19.5 by 4 to 5.5 µm, and setae were 1- to 3-septate and measured 20 to 42 by 3 to 5 µm. These morphological traits correspond to Colletotrichum higginsianum. For pathogenicity tests, three 4-month-old L. draba plants were sprayed with a 106 conidia per ml suspension of the fungus and incubated for 72 h in plastic bags at 20-25º C in a quarantine greenhouse. Within 4 days, water-soaked lesions appeared which coalesced, resulting in chlorosis and collapse of inoculated leaves. Such symptoms are typical of infection by C. higginsianum and similar necrotrophic species. Fungi isolated from inoculated leaves were identified as C. higginsianum. To assess the host range of C. higginsianum, 3 plants each of turnip, radish, mustard greens, kale, broccoli raab and Chinese cabbage were inoculated under the same conditions for the pathogenicity tests. Control plants in pathogenicity and host range tests were sprayed with sterile distilled water, and all experiments were repeated at least once. All control plants were symptomless. Leaf necrosis occurred on radish and turnip, and to a lesser extent on the lower leaves of Chinese cabbage and broccoli; dark necrotic flecks and small grayish leaf spots occurred on kale and mustard greens, respectively. These results are similar to previous studies involving a cultivated species as the host in the field. The ITS1, 5.8S, and ITS2 sequences of this fungus (GenBank HM044877) were 99% similar to sequences of multiple isolates of C. higginsianum (GenBank #AB042302, AB042303, AB455253, AJ558109, AJ558110). This is the first report of C. higginsianum on a wild species of the Brassicaceae, although a Colletotrichum sp. was reported on wild radish in Australia.