The branch from an oak tree along the oak avenue access road came down onto the passenger side of a car delivering goods to the castle for the weekend’s events.
The branch also clipped a passing double decker bus run by South Devon Railway which was due to be used as a shuttle service by the castle to ferry in spectators to watch the celebrations on a big screen.
The car was a write-off after the incident which happened just after 10am on Saturday.
A spokesman for South Devon Railway said the bus had been parked up having arrived and waiting to take the load of passengers.
The driver and conductor were having a quick cup of tea before starting the trips when they heard a crack and saw a branch of the oak tree come down on the roof of a passing car which was coming the other way.
The spokesman said: ‘It clipped the front wing of the bus and the damage was not too serious.
‘I understand the car has been written off.
‘Our bus staff helped the driver get out of the car.
‘They checked the driver was OK and then checked the bus.’
Damage to the bus was able to be repaired.
A spokesman for Powderham Castle said the branch fall happened along an access only route which was not used by the public.
He said the tree limb had fallen on the passenger side of the car and caught the edge of the bus.
A few weeks previously, the tree had been part of a wider tree survey on the estate which had concluded no action needed to be taken.
The spokesman said: ‘Remarkably it was an unlikely event.
‘The car belongs to a local who lives in the next village.