Mildew is an airborne fungus. As a fungus, it thrives in damp, dark environments. Mildew grows in as diverse areas as Casco Bay, Maine; Tampa Bay, Florida; and all points in-between. It is not restricted to North America by any means.
There does not seem to be any consistent geographic or environmental factors that cause or restrict the growth of mildew, but what we DO KNOW is that when the following are present in various combinations, mildew is very likely to grow and flourish:
If we look at the way sails are constructed and stored we find most of the ingredients necessary for mildew growth. Storing or rolling sails away that are still wet is a very common beginning to the mildew growing process. This is exacerbated in salt water areas as salt left on sails seems to attract and prolong moisture presence. This, coupled with lack of air flow, lends itself perfectly to mildew growth.
Lack of airflow probably rears its ugly head most prominently in the case of Roller furling headsails (especially sails made utilizing laminated constructions with the use of Mylar film), and the newer crop of in-mast and in-boom furling main systems. Since the sails are rolled up in a bundle, layer on layer, air flow is severely reduced, increasing the likelihood of mildew growth. Roll away a wet sail and the likelihood increases (we think). Now you have 2 of the 3 components in the chain, so add a little food (dirt) or glue, and you have an excellent platform for mildew to begin. Not that it is guaranteed, but it is ready to thrive.
The good news is that as unsightly as it is, mildew is not nearly as detrimental and harmful to modern sail fabrics as it once was to materials such as cotton, linen, and other natural fibers; however, if left unwashed or untreated, the mildew will continue to grow and flourish.
Although there is no evidence to suggest that this mildew will harm your sails, there is no evidence that mildew will not harm your sails even if only moderately, and the visual detriment to your sails certainly can not be ignored. In today’s sailing environment, even though we do not have to be worried about mildew deteriorating sail fabrics, it is very hard to ignore the visual and aromatic traces of mildew.