Biotechnology and the control of viral diseases of crops

Abstract

The global impact of plant viruses on crop productivity is difficult to estimate because losses caused by viruses frequently go unnoticed. Viral infection may be inconspicuous – causing little or no obvious symptoms other than yield reduction (Waterworth and Hadidi 1998). The monetary impact of viral pathogens on food production systems is therefore chronically underreported and the calculations of impact underestimated. Yet that impact may be striking in magnitude. For instance, global annual reductions in yield due to virus are estimated at 7 percent in sugar beet and 8 percent in potato (Oerke and Dehne 2004). Of course, in specific cases of viral pathogens, the impact can be much more destructive than global averages. Tomato spotted wilt virus has been estimated to cause more than $1 billion in damage to cultivated plant species annually (Goldbach and Peters 1994). Another striking example is Cocoa swollen shoot virus, which has destroyed 200 million cocoa trees in Ghana alone (Lockhart and Sachey 2001). Management options for viruses are relatively limited. They consist mainly of regulatory and cultural control methods such as exclusion of the viral pathogen from a geographic area and using disease-free seed (Agrios 2005). Unlike fungal pathogens, which may be controlled through the use of fungicides, there is no effective way to control viral pathogens directly. Chemical control of virus vectors has been widely adopted for controlling viral diseases despite limited success (Broadbent 1957). A literature search reveals that recommended rates of pesticide application can be very high, pesticides must be applied frequently, and often they do not effectively control virus spread (Table 5.1). Not only does this disease management strategy have potential negative environmental and health consequences but also viruses continue to be problematic in sprayed fields (Satapathy 1998). Indirect chemical control of nonpersistent viruses, which can be transmitted by a feeding insect in a matter of seconds, is particularly ineffective (Broadbent 1957).