|Souissi, T - TUNISIAN NAT'L INST AGRON|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 5, 2008
Publication Date: January 1, 2009
Repository URL: http://hdl.handle.net/10113/43462
Citation: Caesar, A.J., Lartey, R.T., Berner, D.K., Souissi, T. 2009. First report of leaf spot caused by Cercospora bizzozeriana on Lepidium draba in the United States. Plant Disease. 93(1):108-108. Interpretive Summary: Lepidium draba, also known as white top or hoary cress is an invasive weed species of Eurasian origin. In field surveys to assess the occurrence, distribution and apparent impact of plant pathogens of this deep-rooted perennial weed invading rangelands in the west of the U.S., a leaf spotting disease was first found in the U.S. in 1997 at Shepherd, MT. The circular, lesions usually on lower leaves contained fungal spores the long thin shape and microscopical measurements of which were characteristic of the fungus Cercospora bizzozeriana. The fungus isolated in pure culture and grown on carrot leaf extract agar to produce spores. The spores in an aqueous suspension were sprayed on young plants of white top, sugarbeet, and three week old seedlings of collard, mustard, radish, cabbage and kale in the greenhouse, and plants were covered with plastic bags for four days. Lesions identical to those seen in the field occurred on the lower leaves of on white top. No other species exhibited lesions. Searches for the same disease between 1998 and 2003 in neighboring states failed to find it. The disease was last reported in Manitoba in 1938. This is the first report of the occurrence and distribution of the leaf spot of white top caused by Cercospora bizzozeriana in the U. S. The isolate we found is genetically identical for critical DNA sequences to an isolate of the same species found in North Africa. This fungus is reported as common in Europe. It causes little damage to white top in the U.S.
Technical Abstract: The herbaceous perennial Lepidium draba L. is an invasive weed of rangelands and riparian areas in North America and Australia, infesting large areas in Oregon, Wyoming and Utah. L. draba is the target of biological control research being conducted in Europe and the U.S. Little is known of plant pathogens occurring on L. draba, especially in the U. S., some of which might be useful for biological control of the weed. Leaf spots, mostly circular, sometimes irregularly shaped, whitish to pale yellow and dappled with minute dark spots within darker raised borders, were first noted on a stand of L. draba near Shepherd, MT in 1997. Diseased leaf samples of L. draba were collected that year and in 2007. Conidia, borne singly on dark grey unbranched conidiophores (from dark stromata late in the season), were elongate, hyaline, multiseptate, 38-120 ×2-6 µm (mostly 38-50 × 2-5 µm) and had bluntly rounded tips and wider, truncate bases. These characteristics were consistent with the description of Cercospora bizzozeriana Saccardo & Berlese. The ITS sequences of this fungus were identical to those of C. bizzozeriana from Tunisia (GenBank # DQ370428). Spores were picked from leaf lesions, suspended in sterile water and spread on plates of water agar. Germinated spores were transferred to PDA. For pathogenicity tests, aqueous suspensions of 104 spores per ml from cultures grown on carrot leaf decoction agar were sprayed on 6 wk-old L. draba plants and on seedlings of sugarbeet, collard, mustard, radish, cabbage and kale. Plants were covered with plastic bags and placed on the greenhouse bench at 20-25 for 96 hr. No symptoms occurred on plants other than L. draba. Koch’s postulates were completed by reisolating the fungus from the circular leaf spots of L. draba that appeared within 10 days, usually on lower leaves. Searches for the disease in other states were made from 1998-2003 and none were found. This, to our knowledge, is the first report of Cercospora bizzozeriana in the U. S. The initial report of the fungus in North America was from Manitoba in 1938. It has recently been reported as occurring on L. draba in Tunisia, Russia, and in Europe.