There are naturally occurring fungi (molds) found in soil. One common fungus is called "white mold" (Sclerotium rolfsii). This fungus usually damages roots, stems, and pods. Good land management practices (crop rotation, irrigation, etc.) or fungicides can be used to control this problem.
Another mold, Aspergillus, has 2 species (A. flavus, A. parasiticus) which can be a problem. During the latter part of the growing season, peanuts are especially susceptible to drought and high temperatures. Under those conditions these fungi, can infect peanuts and produce a chemical (aflatoxin) which can potentially pose health and economic risks (see Grading and Testing page).