First report of Thielaviopsis populi on hybrid poplar in Hungary

In summer 2002, an unusual root disease was observed in Hungary in two stands of 3 to 4 year old Populus x euramericana. The symptoms began with bark necrosis at the collar, which was frequently swollen, cracked and wounded by the xylophagous insects Aegeria apiformis Clerck and Anaerea carcharias Linnaeus (Fig. 1). Later, necrosis of the inner bark extended underground throughout most of the root system, before the attacked trees died.

From infected trees, Fusarium solani (Mart.) Sacc. and Cylindrocarpon destructans (Zins.) Sholten fide Booth were frequently isolated. In addition, Thielaviopsis populi (Veldeman ex Kiffer et Delon) Paulin, Harrington et McNew (° Chalara populi Veldeman ex Kiffer et Delon) was less frequently isolated from necrotic bark and wood taken from the root collar. The isolate of T. populi was identified based on cultural characteristics on potato dextrose agar (PDA), the endoconidia (Fig. 2) and aleurioconidia (Fig. 3). The ITS sequence of the nuclear rDNA (GenBank accession AY423551) was identical to that of two other isolates (CBS 484.71 and 486.71) of T. populi from diseased poplar in Belgium (Veldeman 1971). Perithecia of a Ceratocystis species with hat-shaped ascospores were observed on symptomatic wood after 7-10 days incubation in a moist chamber, but the formation of the sexual state could not be reproduced in culture, even in pairings with the CBS cultures. Previously T. populi was thought to be asexual (Paulin-Mahady et al., 2002). A pathogenicity test was conducted under laboratory conditions by inoculating wounds of 2 year old sprouts of P. x euramericana with mycelium. Lesions averaging 25 x 13 mm were produced in the inner bark after 5 weeks (compared to 45-cm lesions produced by Fusarium in similar inoculations). Control inoculations resulted in no necrotic lesions.

Thielaviopsis populi was first described in Belgium, where it caused bark lesions in stems of P. x euramericana cultivars robusta and gelrica (Veldeman, 1971). The fungus shows some similarity to the conidial state of C. fimbriata Ellis and Halsted, which causes cankers on P. tremuloides and P. balsamifera in North America and hybrid poplars in Poland (Gremmen and de Kam, 1977), but T. populi differs from C. fimbriata in its conidial states and ITS sequence (Paulin-Mahady et al., 2002). This is the first report of T. populi outside of Belgium. The fungus appears to be a weak pathogen and plays a perhaps minor role in a complex disease syndrome in Hungary.