Several Aspergillus spp. produce mycotoxins that may be toxic, mutagenic, or carcinogenic. Aspergillus species are often soil fungi or saprophytes, however some also cause decay of stored foodstuffs and disease in plants, or can be human and animal pathogens. These fungi are difficult to control, spreading efficiently through the production of asexual spores called conidia. Some Aspergillus spp. can survive periods of adverse conditions in crop fields or during storage by forming protective structures called sclerotia.