For the purpose of assessment, the following districts are established: District I, Imperial and Riverside Counties; District II, Kern, Kings, Tulare, Fresno, Madera, Merced, Stanislaus, and San Joaquin Counties, and that portion of Los Angeles County lying north of the San Gabriel Mountains; District III, Sacramento, Solano, Yolo, Placer, Sutter, Yuba, Colusa, Butte, and Glenn Counties; District IV, Alameda, Contra Costa, Monterey, San Benito, San Luis Obispo, Santa Clara, Santa Cruz, and Santa Barbara Counties; additional areas of the state may be established in districts by regulation of the secretary if the secretary finds crops in any such district are subject to damage from curly top virus and that any such district is necessary to accomplish the purposes of this article.