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Twig and blossom blight, and stem dieback, has become an impediment to highbush blueberry (Vaccinium corymbosum L.) production in British Columbia. Periodic assessment of disease symptoms and pathogenicity test confirmed Phomopsis vaccinii to be the major causal agent of the disease. Symptomatic tissues sampled at 7- to 10-day intervals in the 2014 and 2015 growing seasons were examined for presence of pycnidia, their developmental stages, and conidial presence in relation to phenological status of blueberry. Based on histological analysis, pycnidia were grouped into a series of developmental stages, referred to as pre-conidial, early-conidial, conidial, conidial-release and post-conidial stages. These different stages of development coincided with the phenological stages of blueberry plants during the growing season. Pycnidia at the conidial and conidial-release stages were predominantly found on symptomatic tissues sampled from bud-break to bloom; these pycnidia had high conidial contents of 1.0–2.0 × 104 conidia pycnidium−1. Pycnidia at the post-conidial stage were mostly found on symptomatic tissues sampled from green-fruit to post-harvest, under warm and dry weather conditions. Pycnidia at the pre-conidial stage were predominantly observed on symptomatic tissues sampled during latter parts of the growing season, from bud-set to leaf-fall. Mycelial growth of P. vaccinii isolates on potato dextrose agar was measured in response to temperatures from 4 to 32°C; the optimum growth occurred between 20 and 28°C. The number of pycnidia produced on quarter-strength potato dextrose agar at different incubation conditions varied amongst P. vaccinii isolates, and the optimum production was recorded at 16°C with a photoperiod of 14 h light/10 h dark or 14 h UV-A/10 h. Percentage germination of conidia of P. vaccinii isolate 11 BB 14 on water agar was higher at 16, 20 or 24°C than at 8 or 12°C. Conidia at 16°C reached 50% germination at ≥56 h incubation, whereas conidia at 20 and 24°C reached 50% germination at ≥72 h. Percentage germination of conidia at 8 and 12°C remained <50% during 104 h incubation. These findings provide an improved understanding of the role of pycnidia in the epidemiology of twig and blossom blight and stem dieback caused by P. vaccinii on highbush blueberry. [ABSTRACT FROM AUTHOR]