Mildews are not members of the Plant Kingdom but they, like other fungi, are usually included in general botany texts and in introductory plant courses. Traditionally, they have been aligned with plants although recent research show the fungi to be more closely related to animals than to plants.
Very little information is available on the causative organism or organisms of biblical mildew. The most helpful treatment is in Moldenke and Moldenke (pages 289-290) where several fungi are noted. These include: Microsporium gyseum [sic] (should be gypseum), Aspergillus niger, Actinomyces albus, and Cladosporium herbarum.
The most complete description of mildew is found in Leviticus 13: 47-59.