The spores of these microfungi are present in the atmosphere and when they settle on suitable substrates they can quickly grow under favourable conditions of temperature and humidity. The best protection against mildew is to make certain that the fabrics are dry when stored and that they do not become damp during storage. Fabrics which are to be used out of doors for awnings, beach umbrellas, military uniforms, sails, tarpaulins, tents, truck and boat covers, shoes and shoe linings, are treated with a fungicidal finish to protect them from mildew damage and rotting. Algal greening may also occur on fabrics which remain wet for long periods and can cause particular problems in the tropics. Animal fibres are more resistant to mildew growth than plant fibres.
Pure silk is less susceptible if completely degummed.
Wool decays only slowly but chemical and mechanical damage during processing can increase its susceptibility to biodeterioration. The insoluble polymer is converted into soluble sugars which can then be metabolized inside the bacterial or fungal cells. The hydrophobic nature of these polymers is probably an important factor determining their resistance. Also, these synthetic polymers contain chemical bonds which do not occur or are uncommon in nature, and so perhaps they have not been around long enough for micro-organisms to evolve the appropriate enzymes necessary to initiate their breakdown. In most cases this will not affect the strength of the fabric but can give rise to staining and discolouration which are often difficult or impossible to remove.