They put out the blaze and stopped it spreading to a nearby thatched property.
Mr Johnston added: “The police are looking into it and seem to think it might have been deliberate.
“We will just have to wait and see.
“But I would like to thank the firemen.
“Their quick actions stopped it spreading to the cow shed, which is thatched and next to the barn.
“It could have been even worse.”
The barn was being used to store a number of vehicles and a van, a caravan, a car and a tractor were destroyed.
A spokeswoman for Dorset Police said: “We were called by the fire service at 11.23am on Sunday regarding a barn fire on a farm at Pymore, Bridport.
“It is believed to have been started deliberately and we are working with Dorset Fire and Rescue to establish exactly what happened.”
No arrests have been made, the spokeswoman added.
Speaking after the incident on Sunday, Dorset Fire and Rescue Service incident commander Steve Underhill said: “At 11.18am we received a call that a vehicle was on fire, believed to be a caravan.
“Whilst en route crews discovered there was a thatched property close to the incident so additional resources were requested.
“On arrival it was established a fire was taking place and crews got to work extinguishing the fire using hose reel jets and main lines, wearing breathing apparatus.”
Mr Underhill said that the fire was under control by 1pm and crews remained on scene during the afternoon to begin the process of dampening down and scaling down the incident.
He added: “Crews worked hard to prevent the fire spreading further.”
Police confirm they are treating West Dorset barn blaze as arson
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