Peanut (Arachis hypogaea L.) is susceptible to diseases caused by numerous soilborne pathogens. In the southwestern United States pathogens including Botrytis cinerea Pers.: Fr., Pythium spp., Rhizoctonia solani Kühn AG-4, Sclerotinia minor Jagger and Sclerotinia sclerotiorum (Lib.) de Bary, Sclerotium rolfsii Sacc., and Verticillium dahliae Kleb. routinely affect peanut yield. This region has an arid climate and peanut development is generally later than in other peanut production areas, hence the time plants are exposed to pathogens is increased. These pathogens cause similar symptoms in the field; therefore, proper diagnosis is needed so that the appropriate management strategies can be implemented.