While other factors can cause these general symptoms, two fungal diseases are frequently responsible for the dieback. Arborvitae is also susceptible. This disease begins as a foliar infection which then spreads to and kills stem tissues. Newly developing needles in the yellowish-green stage are especially susceptible, while more mature, darker green needles are not. Phomopsis blight can occur during the growing season anytime there is new shoot growth.
Symptoms are evident a week or two after infection.
Small branches up to 1 cm in diameter are generally girdled by the disease. When a side shoot is infected, Phomopsis may progress to a main branch. Lesions on larger branches, however, develop into cankers but they seldom girdle the branch. Well- developed pycnidia can be found on needles and twigs that have dried up and turned grey. The pycnidia appear as tiny black specks at the base of the infected portion.