First Report of Phoma herbarum and Phoma exigua as Pathogens of Dandelion in Southern Ontario.

Abstract

Diseases of dandelion (Taraxacum officinale) were surveyed in southern Ontario from 1993 to 1997 to identify promising pathogens for biological weed control. Two new pathogens of dandelion, Phoma herbarum Westend. and Phoma exigua Desm., were recovered and characterized from small necrotic lesions on dandelion foliage. Both Phoma spp. were frequently isolated from multiple locations and during various years of the survey, indicating that they were endemic and widespread on T. officinale in southern Ontario. Pathogenicity was assessed by inoculating detached leaves or seedlings with colonized agar disks (6 mm in diameter) or spore suspensions (1 × 106 conidia per ml). Inoculated leaves and seedlings were incubated at 22°C and 48 h of continuous leaf wetness. Lesion diameters were measured 3 days post-inoculation. Isolates that gave rise to necrotic lesions were reisolated from leaves and grown in pure culture. Their growth characteristics were compared with those of the initial isolate. Spores were ellipsoid, hyaline, and 5 × 2 μm for both species. Identification of representative isolates of both species was confirmed by the Centraalbureau voor Schimmelcultures, Oosterstraat 1, 3742 SK Baarn, The Netherlands. This is the first report of these two species as pathogens of dandelion in North America (1,2).