It is not the landlord's responsibility to provide a dehumidifier, but there are other issues here due to the mold.In every lease there is an implied warranty of habitability which requires the landlord to maintain the premises in a habitable condition by complying with state and local housing codes.  The mold is a health issue which constitutes a breach of the implied warranty of habitability.  When there is a breach of the implied warranty of habitability, the tenant notifies the landlord as you have done and the landlord is required to respond within a reasonable time by making the necessary repairs.  When the landlord fails to respond within a reasonable time by making the necessary repairs, the tenant has the following options:  The tenant can make the repairs (contact someone to remove the mold) and deduct the cost from the rent or the tenant can move out and terminate the obligation to pay rent for the balance of the term of the lease or if the tenant stays on the premises, the tenant can withhold rent and defend against eviction.  Another alternative is to sue the landlord for breach of the implied warranty of habitability.  You can also contact your local housing code inspector, who can bring an enforcement action against the landlord for housing code violations.