Plant diseases causes serious losses to crop production. They may kill plants completely, but more often they simply reduce the yield. Most plant diseases are caused by microorganisms which infect the tissues, the most important being fungi, including mildew, rusts, and smuts. Control methods are based on fungicides. Viruses are the next most damaging group of plant pathogens. Most of them are carried by aphids and other sap-sucking insects and control is largely a matter of controlling these insect-carriers. Bacteria are less important, their main role being in secondary infection, causing the tissues to rot. Deficiency diseases are caused by a lack of available minerals in the soil. Insect pests, such as the boll weevil on cotton, can also cause serious crop damage.