A Thesis Presented to The Faculty of Graduate Studies of The University of Guelph by NAEL THAHER In partial fulfillment of requirements for the degree of Master of Science January, 2011 Extensive use of strobilurin fungicides on chickpea (Cicer arietinum L.) in western Canada to manage ascochyta blight caused by Ascochyta rabiei (Pass.) Labr. (teleomorph Didymella rabiei (Kovachevski) Arx) resulted in a rapid and widespread insensitivity to strobilurin fungicides. Assessment of 10 isolates of A. rabiei (3 sensitive, 4 intermediate, and 3 insensitive to pyraclostrobin, a strobilurin fungicide) showed that their pattern of response did not differ substantially, (1) in radial growth or conidial germination assessments, (2) with addition of salicylhydroxamic acid (SHAM), (3) as single-spore or mass-transfer isolates, or (4) against formulated product or technical grade active ingredient. In a greenhouse study, the pathogenic fitness (estimated as ascochyta blight severity and lesion numbers on inoculated chickpea seedlings) of four pyraclostrobin-insensitive isolates and four sensitive isolates did not differ. Baseline sensitivity to a new fungicidal active ingredient (penthiopyrad) was evaluated for 50 isolates presumed to have no previous exposure and used to assess the response of isolates exposed to a related fungicide (boscalid): 12 of 79 exposed isolates exhibited slightly reduced sensitivity to penthiopyrad. There was no correlation in response between penthiopyrad and boscalid. No insensitivity to the protectant fungicides chlorothalonil or mancozeb was detected in an assessment of 50 isolates. Agriculture and Agri-Food Canada; the Development Fund of Saskatchewan; University of Guelph. BASF and DuPont.