ABSTRACT

Viruses are important groups of plant pathogens. Several viruses are known to cause crop diseases, and 977 plant virus species have been officially or provisionally recognized (Fauquet and Mayo, 1999). Rice tungro virus disease causes heavy losses in rice grown in different countries in Asia (Azzam and Chancellor, 2002). Citrus tristeza virus disease has caused the decline and death of close to 40 million trees grafted on sour orange rootstocks in Spain and several million trees in other growing areas in the Americas, Australia, and South Africa (Terrada et al., 2000). Tomato spotted wilt virus causes significant yield losses in many food and ornamental crops (Garcia et al., 2000). It was difficult to classify plant viruses into class, order, family, genus, and species due to lack of knowledge of genetic relationships between viruses. Recently viruses have been grouped into genera, based on molecular biological studies. A few families and an order have been created. The virus infection process is now studied at the molecular level, and the viral genes involved in the pathogenicity have been characterized. Insects, mites, fungi, protozoa, and nematodes serve as vectors of viruses, and studies of their interaction help to develop integrated virus disease management technology. All these aspects are described in this chapter.