First of all, don't panic. Second, don't let a home inspector, mold inspector, realtor or buyer panic either. This is a VERY common problem with a simple solution. First, what you are probably seeing is decay of the plywood sheathing caused by a lack of adequate attic ventilation. I haven't seen your house but I've seen plenty like this in the 20 years I spent as a home inspector. If I had to guess, I'd say your home is around 30 years old with a minimal amount of attic ventilation, most likely a gable vent, tiny soffit vents and maybe a roof vent or two. This isn't nearly enough to flush the moisture which naturally accumulates, causing the attic mold.If I inspected your home for a potential buyer, the only concern I would have would be the structural integrity of the plywood. If the decay is severe, the plywood may be weak and not sound enough to do its job supporting the roof and the shingles. If all you are seeing is some black fungus (notice I did not say "black mold" because mold is not always present in the decay), I'd improve the attic ventilation by adding continuous ridge and soffit vents. If this problem is handled for what it is, as opposed to what it isn't, it should not be a deal breaker. However, if you go down the path of talking to mold remediation companies, they may take you for a ride -- driving up your remediation costs and driving down the profit on selling your house.For more tips on black mold and mold proofing your home, see our Mold Resource Guide.