Forestry Commission scientists identified the pathogen and are testing samples from the other 80 Lawson's cypress trees in the park showing similar symptoms. They are also investigating 27 dead and dying yew trees.
Forestry Commission Scotland director Dr Bob McIntosh said: "This is a very worrying development. P. lateralis is a particularly virulent pathogen and very few trees survive an attack.
"Although its main victim is Lawson's cypress, it can kill other species, particularly the Pacific yew, which is a close relative of our native English yew. It could also be very serious for the ornamental plant industry because Lawson's cypress are some of the most important conifers in our ornamental trade."
He added: "We are working closely with West Dunbartonshire Council to fell and destroy the dead and dying trees and to implement bio-security measures at the park to minimise the risk of spreading the disease."
Councillor Jim McElhill said: "The council's park staff will place disinfectant mats at exit points from the park and public notices are also being displayed to inform visitors of the infection and encourage them to observe sensible bio-security measures."
P. lateralis is predominantly found in the western states of Canada and the USA, but outbreaks were recently recorded in France and the Netherlands. Symptoms include foliage appearing lighter in colour before withering and turning red/brown and tongues of killed inner bark becoming visibly darker.
Any sightings should be reported to the Forestry Commission's disease diagnostic and advisory service on 01314 452176.