systemic fungicides. It is especially difficult to protect root tips, which are the site of infection for many soil-borne pathogens.
Many fumigants such as dibromochloropropane and ethylene dibromide, two of the most effective nematicides, have been removed from the market; and the cost of using those fumigants that remain on the market often is too great to justify their use on field crops such as cereal and oil crops (Cook et al., 1995).
Plant viruses annually cause crop-yield reductions valued at more than $100 million. In most cases the reductions represent 1 percent to 10 percent of the potential crop yield, a figure generally accepted in cases where disease resistance is not available. In selected crops the impact can be severe, causing losses of up to 80 percent. As with human medicine, there are no chemicals capable of controlling plant viruses—chemical controls must be directed at the arthropod, fungal, or nematode vectors of plant viruses. Control of plant viruses is effected by
management of areas surrounding production areas,
control of the insects and other vectors of the disease agent, and
use of disease resistant varieties.
Agriculturalists have long known that the incidence and severity of virus diseases can be reduced by
eliminating weeds that harbor viruses and serve as alternate hosts from which insect vectors spread the virus;
using insecticides to control the vectors, for example, aphids and white-flies;
crop rotation and crop sanitation practices to reduce the sources of inoculum; and
shifting planting dates to avoid infestations of insect vectors.
Even with recommended agricultural practices, growers face the possibility of substantial losses caused by disease in years in which climactic changes augment heavy infestations of arthropod vectors. For example, outbreaks of wheat streak mosaic virus can occur in the Midwest in the spring following a warm, wet fall season—conditions optimal for the mite vector.
The situation is not as bleak with all crops, however, largely because of the success of plant breeders in identifying and deploying natural genes for resistance. Genes for resistance to tomato mosaic virus; potato viruses X, Y, and M; cucumber mosaic virus in pepper and in some varieties of cucumber, tomato, and cantaloupe are all examples of successful deployment of genetic material. Some but not all virus resistance genes are single genetic loci, easily transferred be-