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Cassava ( Manihot esculenta , Euphorbiaceae) is one of the main tuber crops and the staple or subsistence food for about one fifth of the world’s population ( Edison, 2000 ). Demand for cassava is expected to rise in the next 20 years due to human population growth ( Scott et al. 2000 ).

More than fifty viruses are known to be able to establish experimental infections in barley ( Hordeum vulgare ), but only about 25 viruses naturally infect this cereal ( Mathre, 1997 ). Among these, only the Hordeivirus, Barley stripe mosaic virus (BSMV), members of the Barley yellow dwarf virus (BYDV) (Genus Luteovirus ) family complex, and two Bymoviruses, Barley mild mosaic virus (BaMMV) and Barley yellow mosaic virus (BaYMV), cause widespread diseases or substantial yield losses. For this chapter, we will focus on only these viruses, since information about resistance to viruses of lesser economic importance is limited.

Tomato yellow leaf curl virus (TYLCV) is one of the most devastating viruses of cultivated tomatoes in tropical and subtropical regions. TYLCV is a monopartite begomovirus, first described in Israel ( Cohen and Nitzany, 1966 ). Although originally found only in the eastern Mediterranean, it is now a problem in the western Mediterranean, the Caribbean, Japan, and the southern U.S. ( Polston and Anderson, 1997 ; Polston et al. 1999 ). Infection of susceptible tomato plants results in cupping of leaves, chlorosis, prominent stunting of the growing point, and flower abscission. Depending on the timing of infection, yield losses can reach 100%. In many tomato-growing areas, TYLCV has become the limiting factor for production of tomatoes in both open field and protected cultivation systems ( Lapidot and Friedmann, 2002 ).