Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2009
Publication Date: 8/1/2009
Citation: Caesar, A.J., Lartey, R.T. 2009. First Report of a Leaf Spot Caused by Alternaria brassicae on the Invasive Weed Lepidium draba in North America. Plant Disease. 93(8):846-846.
Interpretive Summary: The exotic rangeland weed Lepidium draba L., a brassicaceous perennial, is widely distributed in the U. S., e.g., Oregon contains 100,000 ha of land infested with L. draba (2). It is capable of aggressive spread and patch size can expand more than 3 M in diameter per year (5). It further has the potential to reduce the value of wheat growing land (4). Thus, it is the target of biological control. 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 near Shepherd MT of leaf spots on L. draba plants, distinct from leaf lesions of Cercospora bizzozeriana reported recently (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 25-35 × 200-250 µm with a beak cell 42 to 100 µm long. Based on conidial and cultural characteristics, the fungus was identified as Alternaria brassicae (Berk.) Sacc. Leaf tissues bordering lesions were plated on acidified PDA and resulting dark grey colonies were further purified. Colonies on V-8 and alfalfa seed agar were black with concentric rings, eventually appearing uniformly black after 10-14 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 ca 105 ml-1 were prepared from cultures grown at 20-25° C for 10-14 days on V-8 agar and sprayed on leaves. Inoculated plants were enclosed in plastic bags and incubated at 20-22° C for 72-80 hr. 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. 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).
Technical Abstract: A second leaf spot disease on the perennial invasive weed white top, aka hoary cress, was found in a stand of white top in south central Montana. The plant pathogen causing the lesions was identified as the fungus Alternaria brassicae. It was isolated, purified grown on a V-8 agar growth medium and sprayed on leaves of white top and produced leaf spots identical to those seen in the field. The fungus was also sprayed on 10 close relatives of white top and caused visible disease on 3 species. This is the first report of the occurrence disease caused by this fungus on white top in North America. It may have utility for biological control of white top if applied with other pathogens of white top.