*How can fungus be avoided?
Reduce the relative humidity to less than 60% (never under 30% as it is dangerous for the instrument) by storing:
in climate-control cabinets in which hygrometers maintain environmental conditions
next to driers (e.g. silicagel orange packs) in the containers
in a special cabinet whose interior is heated to 40°C (max. 50°C) using a fan heater/ incandescent lamps, thereby reducing the relative humidity
in hermetically sealed cabinets with fungicides with high vapor pressure (fungicide depot must be replaced at regular intervals)
in an dehydrator above driers
After the work is done, Immediately clean the instruments. If possible, you can use a fan to facilitate evaporation of surface moisture. Do not use containers made of leather, textiles or wood for storage. Short solar radiation or irradiation with UV light may also help avoiding fungus.
http://www.zeiss.com/camera-lenses/en_de/website/service/fungus_on_lenses.html