The exotic, rangeland weed Lepidium draba L., a brassicaceous perennial, is widely distributed in the United States. For example, Oregon contains 100,000 ha of land infested with L. draba (2). Because it is capable of aggressive spread and has the potential to reduce the value of wheat-growing land (4), it is the target of biological control research. The application of multiple pathogens has been advocated for control of other brassicaceous weeds, including the simultaneous application of biotrophic and necrotrophic pathogens (3). In pursuit of this approach, in 2007, we discovered the occurrence of leaf spots on approximately 90% of L. draba plants near Shepherd, MT, which were distinct from leaf lesions caused by Cercospora bizzozeriana (1). The lesions were initially tiny, black spots enlarging over time to become circular to irregular and cream-colored around the initial black spots and sometimes with dark brown borders or chlorotic halos. Conidia from the lesions were light brown, elongate and obclavate, produced singly from short conidia, with 8 to 12 transverse septa, and 2 to 6 longitudinal septa. The spore body measured 25 to 35 × 200 to 250 μm with a beak cell 42 to 100 μm long. On the basis of conidial and cultural characteristics, the fungus was identified as Alternaria brassicae (Berk.) Sacc. Leaf tissues bordering lesions were plated on acidified potato dextrose agar. Colonies on V8 and alfalfa seed agar were black with concentric rings, eventually appearing uniformly black after 10 to 14 days. The internal transcribed spacer region of rDNA was amplified using primers ITS1 and ITS4 and sequenced. BLAST analysis of the 575-bp fragment showed a 100% homology with a sequence of A. brassicae Strain B from mustard (GenBank Accession No. DQ156344). The nucleotide sequence has been assigned GenBank Accession No. FJ869872. For pathogenicity tests, aqueous spore suspensions approximately 105/ml were prepared from cultures grown at 20 to 25°C for 10 to 14 days on V8 agar and sprayed on leaves of three L. draba plants. Inoculated plants were enclosed in plastic bags and incubated at 20 to 22°C for 72 to 80 h. In addition, three plants of the following reported hosts of A. brassicae were inoculated: broccoli, canola, Chinese cabbage, collards, broccoli raab, kale, mustard greens, radish, rape kale, and turnip. Within 10 days, leaf spots similar to those described above developed on plants of radish, canola, Chinese cabbage, and turnip and A. brassicae was reisolated and identified. Control plants sprayed with distilled water remained symptomless. These inoculations were repeated and results were the same. To our knowledge, this is the first report of a leaf spot disease caused by A. brassicae on L. draba in North America. A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI No. 878750A).
References: (1) A. J. Caesar et al. Plant Dis. 93:108, 2009. (2) G. L. Kiemnec and M. L. McInnis. Weed Technol. 16:231, 2002. (3) A. Maxwell and J. K. Scott. Adv. Bot. Res. 43:143, 2005. (4) G. A. Mulligan and J. N. Findlay. Can. J. Plant Sci. 54:149, 1974.