any advise or ideas… thanks julie
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Why is the plywood in my attic turning black?
No air sealing at the attic floor to stop warm, conditioned air from escaping into the attic (warm air/ winter issue) coupled with a lack of adequate attic ventilation creating conditions that are right for condensation that adheres to the plywood and eventually forms mold.
Hope that helps.
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If you could upload pictures that can be enlarged, that would be helpful.
I’m not sure how to help you to do that, though. 
Maybe someone else can direct you.
Oh, I see one can be enlarged.
Aside from Marcel’s observation. the plywood may have had the discoloration on it before installation because I don’t see much, if anything, on the adjacent rafters.
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And the nails look rusted so I would lean toward lack of ventilation in the attic.
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Good point. Looking closer, they do.
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Looking closely, I think they have seen their fair share of moisture as well. Just doesn’t show up as well I guess.
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Looks like mold from here, and I agree with others that it would most likely be caused by moisture due to a lack of ventilation.
It looks like a hip roof and those normally have good soffit venting installed at the time of construction, and I do see a few rafter vents or baffles installed in one of the pictures. I always try to look for daylight shining up through the soffits which is a good indication that they are open and not blocked by insulation.
Also be sure to check for proper vent at the ridge, I have found quite a few ridge vents that didn’t have an adequate gap cut to allow for good flow, and some hips aren’t long enough for just a ridge vent and may require additional vents be installed.
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As others have said, this is lack of ventilation. I see it often up here in NE Ohio. Sometimes in attics when additional insulation is added and blocks the soffit vents.
Un-insulated attic hatches/accesses also contribute to this.
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And more importantly… NO seal/weather stripping!
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thank you everyone
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It looks like some of the plywood has lost some of its strength, dry rot?
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Fire would blacken everything, not just the roof sheathing.
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I know this is late for info - However:
In the NW - I see this a lot. Even with good venting sometimes. Another major contributing factor is the roof condition. If there is a lot of tree debris or moss on the roof, it can trap moisture in the shingles that will raise the moisture level as well back into the sheathing.
None of your posted shots are of the roof vents - but you will traditionally see no staining within a foot or so of the roof vents as they get air flow and the sheathing stays dry. Similar to the PVC venting penetration, there is a small ring (Less air leaking) of clear wood around it. Rafters generally will not stain up, but will. OSB will stain up and blacken much worse than real plywood. North facing roof planes are substantially more affected with a lack of sun exposure to cook off moisture and slow moss growth.
I usually will take a few moisture readings to confirm if active - There is not really any way to know if it is from 10 years ago and moss is no longer present, or if it is actively getting worse. Compare the moisture readings from the traditionally cleaner southern exposure side. Companies exist to spray the attic and easily kill the growth. It will be white when done - and can easily tell if it becomes a problem after treatment is done. Sorry for the long story - but I see it frequently in the Seattle Area.
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Appears to be, or resemble, “an unknown, unidentified microbial growth, for the moment,” that many refer to as mold. Referring for an Indoor Air Quality assessment would be of good value.
Why?
In certain species of ‘microbial growth,’ repeat exposure can increase a person’s sensitivity, causing more severe allergic reactions, if this is the case.
As well, the fasteners in darker areas of the sheathing are rusted/corroded, which tends for me to believe that there is poor attic curculation allowing high humidity to remain between the rafters on the sheathing and that possibly there is no barrier or the barrier is degraded between roof covering and sheathing, in others words, the roof covering is nearing its life expectancy and moisture is wicking through to the sheathing.
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Yep, textbook mold… see it all the time in Oregon. It’s not on the rafters because they are slightly warmer and not such a target for the condensation and subsequent mold. You’ll usually see it worst on the north side and/or over bathrooms. The moisture will cause the plywood roof deck to de-laminate and lose a lot of its strength since the plies of wood can now slip past one another and allow deflection. I see it worst on 90s houses after the code change (in Oregon anyway) that increased attic insulation to R38. More heat staying in the house = colder attic = condensation on cold surfaces = mold. Porting out bath fans completely rather than just terminating near a passive roof vent helped somewhat but mold is still often a problem on fairly new houses.
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