"I think the Myrtle Rust thing is a bit of a non-issue as these...

"I think the Myrtle Rust thing is a bit of a non-issue as these two companies are selling carbon credits, not timber"

Whether you sell timber or carbon is irrelevant. Any statement from the carbon plays along these lines is a red herring and mute point. At the end of the day if you are growing trees you are selling growth and this disease attacks new growth which cannot then go on to form new photosynthetic surface area. It is the expansion of photosynthetic capacity which allows growth to be fuelled for expansion of wood biomass (which is both timber and carbon).