The Global Dimension of Tomato Yellow Leaf Curl Disease: Current Status and Breeding Perspectives

Tomato yellow leaf curl disease (TYLCD) caused by tomato yellow leaf curl virus (TYLCV) and a group of related begomoviruses is an important disease which in recent years has caused serious economic problems in tomato (<i>Solanum lycopersicum</i>) production worldwide. Spreading of the vectors, whiteflies of the <i>Bemisia tabaci</i> complex, has been responsible for many TYLCD outbreaks. In this review, we summarize the current knowledge of TYLCV and TYLV-like begomoviruses and the driving forces of the increasing global significance through rapid evolution of begomovirus variants, mixed infection in the field, association with betasatellites and host range expansion. Breeding for host plant resistance is considered as one of the most promising and sustainable methods in controlling TYLCD. Resistance to TYLCD was found in several wild relatives of tomato from which six TYLCV resistance genes (<i>Ty-1</i> to <i>Ty-6</i>) have been identified. Currently, <i>Ty-1</i> and <i>Ty-3</i> are the primary resistance genes widely used in tomato breeding programs. <i>Ty-2</i> is also exploited commercially either alone or in combination with other <i>Ty</i>-genes (i.e., <i>Ty-1</i>, <i>Ty-3</i> or <i>ty-5</i>). Additionally, screening of a large collection of wild tomato species has resulted in the identification of novel TYLCD resistance sources. In this review, we focus on genetic resources used to date in breeding for TYLCVD resistance. For future breeding strategies, we discuss several leads in order to make full use of the naturally occurring and engineered resistance to mount a broad-spectrum and sustainable begomovirus resistance.