First Report of Tan Spot of Wheat Caused by Pyrenophora tritici-repentis in the Pacific Northwest.

In late May 2001, lesions resembling tan spot were observed on lower leaves of winter wheat (Triticum aestivum L.) in early boot stage in Nez Perce County, ID. Abundant sporulation was observed from tan lesions with chlorotic haloes after 2 days incubation in a moist chamber at room temperature. Conidia were multicelled, straw colored, approximately 100 × 15 µm, rounded at the apex, and borne singly on dark brown conidiophores. The fungus fit the morphological description of Drechslera tritici-repentis (Died.) Shoemaker, the anamorphic state of Pyrenophora tritici-repentis (Died.) Drechs. (2). Three single-conidial isolates were sampled from infected plants in a 5 × 1 m area of the affected field and induced to sporulate. Two of the isolates were used to spray-inoculate 3-week-old susceptible wheat (cv. Madsen) in the greenhouse (one plant per isolate, 1 × 10 conidia/ml), and tan spot lesions were apparent 3 to 5 days after inoculation with both isolates. DNA was extracted from all three isolates, and the entire nuclear ribosomal internal transcribed spacer (ITS) was amplified with ITS1 and ITS4 primers (4). Similarly, 610 bp of the 5' end of the glyceraldehyde-3-phosphate-dehydrogenase gene (gpd) was amplified with gpd-1 and gpd-2 primers (1). ITS and gpd amplicons were direct-sequenced on both strands, and alignment revealed that all three isolates were identical for both regions. A BLAST search of the NCBI database with the ITS sequence revealed P. tritici-repentis accessions AY004808 and AF071348 and D. tritici-repentis accession AF163060 as the closest matches with 100, 99.8, and 98.8% sequence similarity, respectively. A similar search with the gpd sequence revealed P. tritici-repentis accessions AY004838 and AF081370 and P. bromi accession AY004839 as the closest matches with 100, 100, and 99.0% sequence similarity, respectively. These results, coupled with the morphological identification and inoculation results, confirm the identity of the fungus as P. tritici-repentis. Although reported on other grass hosts in the region (3), to our knowledge, this is the first report of tan spot of wheat in the Pacific Northwest. This disease has been of little concern to wheat producers in the Pacific Northwest due to low rainfall and relative humidity during the growing season. References: (1) M. L. Berbee et al. Mycologia 91:964, 1999. (2) M. B. Ellis, Dematiaceous Hyphomycetes. CMI, Kew, Surrey, UK. 1971. (3) R. Sprague. Diseases of Cereals and Grasses in North America (Fungi, Except Smuts and Rusts). Ronald Press Co. New York, 1950. (4) T. J. White et al. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. Academic Press Inc., New York, 1990.