First report of tan spot caused by Pyrenophora tritici-repentis (anamorph Drechslera tritici-repentis) in Bulgaria

In June 2004, an unknown leaf spot was observed on wheat, rye and triticale collected from commercial fields in different geographic regions of Bulgaria. The visible symptoms were oval to elongated brown spots surrounded by a chlorotic halo (Fig. 1).

Conidiophores in the necrotic tissues were long, mainly single, hyaline or dark brown (Fig. 2). Conidia were cylindrical, 90-220 x 10-16 μm, with a conical terminal cell at the base and 3-9 septa (Fig. 3). This identified the fungus as Drechslera tritici-repentis as described by Hunger & Brown (1987), Ellis (1976) and Shoemaker (1962). In follow-up collections made in March-April 2005, the teleomorph Pyrenophora tritici-repentis was identified in straw debris collected in North Bulgaria. Pseudithecia were setate, dark brown and spherical. Asci with 8 ascospores - with 3 transverse and 1 longitudinal septa - were observed (Shoemaker, 1962) (Fig. 4).

Conidia and ascospores plated onto PDA formed white to grey cottony colonies secreting a brown pigment. Young wheat plants (second leaf stage) were inoculated with a mycelial suspension by spraying, placed in a moist chamber for 24 h and then incubated under greenhouse conditions. After 7-8 days the typical brown spots with chlorotic halo were observed (Fig. 5).

Pyrenophora tritici-repentis is one of the most common and economically important diseases in many Western European countries. During the last decade it has been reported in the Ukraine, Latvia, Russia, Czech Republic and Hungary (Sarova et al., 2003). It is likely that wet weather, reduced tillage and lack of crop rotation has favoured its spread in Bulgaria.