EVALUATION OF THE PATHOGENICITY OF PHOMA EXIGUA VAR. EXIGUA ON CALIFORNIAN THISTLE

Pathogenicity of isolates of Phoma exigua var. exigua Desm. on detached, wounded Californian thistle (Cirsium arvense (L.) Scop.) leaves was determined by measuring lesion development after inoculation with conidial solutions (1 x 10 5 spores/ml). Results showed significant lesion development by all six test isolates compared to control treatments, but inconsistent lesion development was observed between isolates and between assays. The six isolates also gave greater (P<0.05) lesion development than the control treatment 18 days after inoculation of wounded Californian thistle leaves on whole plants, with no significant differences in lesion development between isolates. For whole plants and detached leaves, lesion development correlated with the recovery of P. exigua var. exigua from lesion sites.