Exotic perennial Lepidium draba, native to Eurasia, is an invasive weed in dense stands in rangelands and disturbed areas in several states of the western United States and an agricultural weed in the prairie provinces of Canada. To determine strategies, such as a potential multipathogen strategy (1), for biological control of the weed, surveys that included the native range were conducted in spring 2009 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 (46°08′22.99″N, 7°9′19.02″E): (http://maps.google.com/maps?source=earth&ll=46.13983490,7.15503250&layer= c&cbll=46.13983490,7.15503250&cbp=1,360,,0,5). 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. Symptoms often occurred on plants that were also infected with Rhizoctonia solani. After surface disinfestation with 0.1% sodium hypochlorite, tissue adjacent to and including lesions were plated on acidified potato dextrose agar and incubated at 20 to 25°C for 1 week. Zonate, dark gray colonies with sparse mycelia resulted that exhibited abundant, faintly pink spore masses with numerous dense clusters of black setae. Spores were single celled, hyaline, cylindrical to oval shaped, and 13.5 to 19.5 × 4 to 5.5 μm. Setae were 1- to 3-septate and 20 to 42 × 3 to 5 μm. These morphological traits correspond to Colletotrichum higginsianum. For pathogenicity tests, three 4-month-old L. draba plants were sprayed until runoff with a 106 conidia/ml suspension of the fungus and incubated for 72 h in plastic bags at 20 to 25°C in a quarantine greenhouse. Within 4 days, water-soaked lesions appeared that coalesced, resulting in chlorosis and collapse of inoculated leaves. Such symptoms are typical of infection by C. higginsianum and similar necrotrophic species (4). Fungi isolated from inoculated leaves were identified as C. higginsianum. To assess the host range of C. higginsianum, three plants each of turnip, radish, mustard greens, kale, broccoli raab, and Chinese cabbage, all in the Brassicaceae to which L. draba belongs, were inoculated with the same conditions used 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; numerous scattered dark necrotic flecks and small grayish leaf spots occurred on kale and mustard greens, respectively. These results are similar to previous studies (2,3) involving a cultivated species as the host in the field. The ITS1, 5.8S, and ITS2 sequences of this fungus (GenBank No. HM044877) were 99% similar to sequences of multiple isolates of C. higginsianum (GenBank Nos. AB042302, AB042303, AB455253, AJ558109, and AJ558110). To our knowledge, 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 (1).
References: (1) A. Maxwell and J. K. Scott. Australas. Plant Pathol. 37:523, 2008. (2) R. O'Connell et al. Mol. Plant-Microbe Interact. 17:272, 2004. (3) R. P. Scheffer. N. C. Agric. Exp. Stn. Tech. Bull. 1950. (4) H. Sun and J. Z. Zhang. Eur. J. Plant Pathol. 125:459, 2009.
