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First Report of Leaf Blight Caused by Phoma exigua on Acroptilon repens in Turkey

Acroptilon repens (L.) DC. (Russian knapweed, synonym Centaurea repens L., family Asteraceae) is becoming a noxious weed in wheat fields in Turkey. Because it is also an invasive weed in the northwestern United States, A. repens is a target of biological control efforts. In the summer of 2002, approximately 20 dying A. repens plants were found on a roadside near Cankiri, Turkey (40°21′41″N, 33°31′8″E, elevation 699 m). No healthy plants were found in the immediate area. Dying plants had irregular, charcoal-colored, necrotic lesions at the leaf tips and margins, and frequently, whole leaves and plants were necrotic. Symptomatic leaves were air-dried and sent to the Foreign Disease-Weed Science Research Unit, USDA/ARS, Fort Detrick, MD. There, diseased leaves were surface-disinfested and placed on moist, filter paper in petri dishes. Pycnidia producing one-celled hyaline conidia were observed after 4 to 5 days. Internal transcribed spacer regions 1 and 2, including the 5.8S ribosomal DNA, were sequenced for isolate 02-059 (GenBank Accession No. AY367351). This sequence was identical to sequences in GenBank from six well-characterized strains of Phoma exigua Desmaz (1). Morphology was also consistent with P. exigua (2) with the exception that material grown on alfalfa twigs produced pycnidia with 1 to 4 ostioles with necks as much as 80 μm long. Typically, pycnidia of P. exigua produced on agar have 1 to 2 ostioles that lack necks. Conidial dimensions on alfalfa were 4.1 to 7.6 × 1.7 to 3.2 μm (average 5.5 × 2.4 μm). Images of the fungus are located at http://nt.ars-grin.gov under the section ‘Fungi Online’. Stems and leaves of 20 3-week-old plants were spray inoculated with an aqueous suspension (1 × 10⁷ conidia per ml) of conidia harvested from 25-day-old cultures grown on acidified potato dextrose agar, and placed in an environmental chamber at 25°C with constant light and continuous dew for 3 days. Plants were then moved to a greenhouse bench and watered twice daily. After 6 days, symptoms were observed on all plants. Once symptoms had progressed to the midveins of the leaves, the disease progressed rapidly on the plants, indicating the possibility of systemic infection or systemic movement of toxins. Phoma exigua was reisolated from the stems, petioles, and leaves of all inoculated plants. In a separate test, 12 plants were inoculated as described above, and 8 additional plants were sprayed with water only. After inoculation, plants were handled as described above. The first lesions developed after 3 days on all except the youngest leaves of inoculated plants. After 10 days, three inoculated plants were dead, and all other inoculated plants had large necrotic lesions. No symptoms developed on control plants. This isolate of Phoma exigua is a destructive pathogen on A. repens, and severe disease can be produced by inoculation of foliage with an aqueous suspension of conidia. These characteristics make this isolate of P. exigua a potential candidate for biological control of this weed in Turkey and the United States. To our knowledge, this is the first report of P. exigua on A. repens in Turkey. A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI 843350).

References: (1) E. C. A. Abeln et al. Mycol. Res. 106:419, 2002. (2) H. A. Van der Aa et al. Persoonia 17:435, 2000.