||Barley yellow dwarf (BYD) is a cereal disease caused by an aphid-transmitted virus belonging to the luteovirus group, members of which cause the yellows diseases. The virus, known as barley yellow dwarf virus (BYDV), is actually a group of closely related viruses. BYDV is persistently transmitted, meaning that once an aphid acquires the virus, it will transmit it for life. The virus is not known to multiply in the insects; newborn nymphs (young aphids) are virus-free and acquire BYDV by feeding on infected plants. BYD has been recorded in most areas of the world and its distribution is documented in these proceedings. Losses due to the disease vary, ranging from 1% to 3% annually, though in some years and some locations, losses may be as high as 20-30%. In some cases, crops have been totally destroyed. Because its presence is often masked by other diseases, such as the rusts or other foliar diseases, the effects of BYD often become much more apparent once resistance to these diseases has been achieved. The virus partially plugs the phloem, interfering with translocation. Infection can severely stunt plants, inhibit root formation, delay heading, and reduce yield. Disease symptoms vary depending on the crop species or cultivar affected. On oats (Avena sativa L.) and some wheats (Triticum spp.), the leaves of diseased plants show a yellowing or reddening, but often the symptoms of BYD in bread wheat (T. aestivum), durum wheat (T. turgidum var. durum), and triticale (X Triticosecale) are not particularly apparent, and even experienced cereal workers may have difficulty recognizing them. Formerly, the only practical method for diagnosing BYD was by aphid transmissions to indicator plants with the resulting development of typical disease symptoms. With the development of the enzyme-linked immunosorbent assay (ELISA), an alternative method of diagnosing the disease became available. In many parts of the world, however, researchers do not have the facilities to carry out either aphid transfers or ELISA, and BYDV must still be diagnosed on the basis of symptoms. One of the disquieting facts emerging from recent reports is the number of asymptomatic BYDV-infected cereal plants that are being discovered. However, in most places where BYD-resistant materials are selected. selection is being done to some extent on symptoms. This area of research requires a great deal of additional study. The United Nations Development Programme (UNDP) sponsored a BYD workshop at CIMMYT December 6-8. 1983. The publication of the proceedings of this workshop in 1984 provided a benchmark of the disease's status. In 1985 the Dipartimento Cooperazione Allo Sviluppo (DCAS) of the Republic of Italy awarded a technical assistance grant to CIMMYT to develop and implement an integrated pest management program for the control of BYDV. This project aims to reduce losses caused by this virus disease by supporting the transfer of technology from developed country institutions (including Italian institutions directly funded by the grant) to developing countries, via CIMMYT. Since BYD is both significant and Widespread in its distribution, the development of resistant germplasm could increase cereal production in developing and developed countries by decreasing the losses it currently causes.