First Report of Downy Mildew Caused by Peronospora trigonellae on Fenugreek in the United States

Fenugreek (Trigonella foenum-graecum) is a member of the Fabaceae family and is grown worldwide for culinary and medicinal purposes. The leaves are used as an herb while the seeds are used whole, ground as a spice, or germinated and used as sprouts. In November 2008, a fenugreek plant exhibiting leaf spotting and severe stunting was submitted to the CDFA Plant Pest Diagnostics Laboratory from the Los Angeles County Plant Diagnostic Laboratory. The county had received the sample from a homeowner who reported severe dieback of the fenugreek in his backyard planting. The fenugreek is grown by the resident as an annual and is propagated each year from the previous crop's seed. The seed was originally obtained from a local ethnic grocery store in Lakewood, CA. The homeowner stated that he had noticed symptoms for a number of years and that they seemed especially severe during the winter months. The adaxial surfaces of the leaves exhibited small chlorotic spots often at the leaf margins, while the abaxial surfaces exhibited a grayish violet, felty growth. Conidiophores found on the underside of the leaves branched dichotomously 6 to 10 times and were terminally forked. Conidiophores measured 280 to 525 μm (average 420 μm) with slightly swollen bases (7.5 to 10 μm broad). Conidia were slightly pigmented, oblong to ellipsoid, and measured 23 to 33 × 18 to 23 μm (average 27.8 × 20.3 μm). Globose oospores with verruculose walls measured 30 to 40 μm in diameter (average 36.1 μm) and were found embedded in the leaf tissue of older lesions. The pathogen was identified morphologically as Peronospora trigonellae Gaum. (3). Sequences of a portion of the rDNA, including the internal transcribed spacer regions, were obtained using primers DC6 and ITS6 (1). Sequence data for P. trigonellae had not previously been entered into GenBank and no identity was obtained. Pathogenicity experiments attempted by spraying healthy fenugreek seedlings with conidial suspensions were unsuccessful, presumably because of the age of the inoculum. Since fenugreek is not commercially grown in California, the economic importance of this disease is limited. Although P. trigonellae has been reported on fenugreek in Algeria, India, Pakistan, and the United Kingdom (2-4), to our knowledge, this is the first report of its occurrence in California and the United States. A specimen of P. trigonellae has been deposited in the U.S. National Fungus Collection (BPI 879153). References: (1) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (2) D. F. Farr et al. Fungal Databases. Systematic Mycology and Microbiology Laboratory. Online publication. ARS, USDA, 2009, (3) E. A. Gaumann. Beitr. Kryptogamenflora Schweiz 5:216, 1923. (4) D. R. Jones et al. Plant Pathol. 56:891, 2007.