pretty sure that lack of fresh air influx plays a big part.
absolutely. no tunnel should be closed ever really. temperature shoouldnt take priority over air flow. much better to have a cooler tunnel and no mould.
also the effect of plant respiration soon leads to a lack of oxygen that stunts growth and leads to the 'greenhouse effect'
my small tunnel has never been closed and i have all the vents open permanantly. ill only start to close them down as the nights average 5 ish degrees.
if you have viable plants left to crop, remove all affected growth, hose the thing down and open all the vents.