By the way, the dehydrate sheets must be dry ... or the water moisture could also causing molding.
I dry my tomatoes on no higher than 41C (about 115F). 115F is the highest produce can be dehydrated at and retain its natural enzymes. Higher temperatures destroy enzymes and temperatures lower than 105F can actually become breeding grounds for bacteria. These tomatoes dried for 3 days ... and then I put left them in for another day because I could detect a little moisture in a few that had been cut thicker than the others.