NEW ROADS TO CROP PROTECTION AGAINST VIRUSES

Farmers and growers, already under pressure to produce food under tigh ter constraints, face the omnipresent threat of new pests and diseases caused by new strains of fungal, bacterial or viral pathogens or thei r vectors, such as insects. The whitefly Bemisia tabaci is an example of such a threat in Europe (Bedford and Markham, 1993). It is feared t hat over-use of pesticides, fertilizers and growth regulators could cr eate major environmental problems, but to survive without them will re quire other strategies, broadly referred to as 'sustainable agricultur e. Traditionally, durable disease resistance has required ingenious br eeding programmes to introduce natural resistance genes from crop-rela ted wild species into crops. However, we can now exploit the power, fl exibility and specificity of recombinant DNA techniques (genetic manip ulation) either to identify existing natural resistance genes and tran sfer them between unrelated crop species, or to create new and more ef fective resistance genes against one or more pathogens or pests. The r ationale behind many of these 'designer genes' has arisen from empiric al field observations or from our deeper understanding of molecular an d genetical aspects of the natural life-cycle of the pathogen.