Under Ontario law, both drivers are 50% responsible for the accident:
If the incident occurs when automobile “A” is turning left at a private road or a driveway and automobile “B” is overtaking automobile “A” to pass it, the driver of each automobile is 50 per cent at fault for the incident.
Now, if the tractor was turning onto a cross road instead of a private drive, then fault would be 25% tractor, 50% truck:
If the incident occurs when automobile “A” is turning left at an intersection and automobile “B” is overtaking automobile “A” to pass it, the driver of automobile “A” is 25 per cent at fault and the driver of automobile “B” is 75 per cent at fault for the incident.
The only way the truck driver would be judged 100% at fault is if he also overtook traffic that stopped behind the tractor prior to the tractor making the turn:
If the incident occurs when automobile “A” is turning left at a private road or a driveway and automobile “B” is passing one or more automobiles stopped behind automobile “A”, the driver of automobile “A” is not at fault and the driver of automobile “B” is 100 per cent at fault for the incident.
(Revised Regulations of Ontario 1990, Reg. 668: FAULT DETERMINATION RULES)
The double yellow line does not affect the above, but truck disobeying a "No passing" or tractor disobeying a "No Turns" sign would change things.
Also, the rules don't apply if the tractor driver was charged with impaired driving, or exceeding the speed limit by more than 16kph but oddly, the rules still apply if the truck driver was impaired or speeding.
Other jurisdictions will have their own rules, of course. But fault is not always assigned the way we think it should be.