Wheat, the most important food crop in the Western world, is subject to attack by a number of pests, which can seriously diminish yields.
Of the diseases, the rusts can cause very serious losses. The three that attack wheat are stem or black stem rust, leaf or brown rust and yellow or stripe rust. Together they produce average annual losses of 10%, though losses may be as high as 50% in epidemic years. Black stem rust, which is distributed world wide, is the most destructive of all the wheat rusts, doing more harm than all the other diseases put together. Yellow rust is common in the more temperate areas, particularly Europe. Brown rust is very widespread but it only causes losses of economic importance in areas which have high humidity and high temperatures during the summer, such as parts of the USA and India. Another group of diseases of wheat are the smuts. The important species are bunt or stinking smut and loose smut. Bunt is the best known, having been responsible for serious crop losses and for contamination of flour, but now the damage done by this and other smut diseases is relatively small. The principal insect pest is the wireworm which is most important in Northern Europe, USSR, and Canada, though less so in southern regions such as Turkey, Spain, Argentina, South Africa and Australia. Local pests are to be found (such as the hessian fly which causes serious damage) in the eastern half of the USA. Of the weeds, those most difficult to control include the species of Nabricaria, Polyganum and Chrysanthemum segatum, and arable weeds such as Agropyron repens (couch), Avena sativa (wild oat) and Alopecuris pratense (black-grass).