Plants have evolved various ways to resist infection or attack by pests, and pests often evolve ways to get around a plant’s resistance. This back and forth evolutionary battle generally means that a given crop species includes a range of resistant and susceptible types. In the 1960s, plant breeders found wheat plants with resistance to a particularly damaging disease called wheat stem rust, and that advancement was a major contributor to the success of the Green Revolution. In 1999, a strain of the fungus in Uganda overcame the plant’s resistance and began to spread throughout the world’s wheat crops. Because some institutions maintain extensive seed banks of diverse genetic types, it was possible to find a new source of resistance. The global network of wheat breeders are now cooperating internationally to move that resistance trait into the myriad types of wheat grown around the world.
Genetically-based pest resistance is highly desirable for farmers, but it is best to integrate that strategy with other methods of pest control, otherwise the pest is likely to evolve around the host defense. IPM systems are critical to preserving genetic resistance for as long as possible.
Another genetic pest control strategy involves joining two different plant types. From ancient times, people learned that if they found a particularly desirable example of a fruit or nut, it was possible to grow more of the same trees or vines by grafting a branch or bud of the desirable species on to the roots/trunk of a different kind of the same species or of a related species. For instance, when an insect that attacks grapes was introduced to Europe from North America in the 1870s, the only way to save the crop was to graft the venerated European grape varieties on to rootstocks of North American grape species which were resistant to the pest. This sort of two-variety or two-species grafted plant is a pest resistance strategy that has been widely used in perennial crops.
For high value crops such as tomatoes, peppers and cucumbers, growers are increasingly dividing the task of genetic improvement between the roots and top of the crop. Growers use rootstocks to develop resistance to soil-borne pests and overall plant vigor. Using the above ground part of the plant, growers focus on breeding for yield, quality and resistance to the pests that attack the leaves and fruit. The two plant types are then grafted together for use in commercial production. Sometimes, you can find such grafted vegetables for sale in garden centers.
Genetic engineering provides a means of using genetic pest resistance in situations where ordinary breeding for such a trait is either impossible or far too slow. For instance, a gene for resistance to a bacterial disease of peppers has been moved to tomatoes, making them resistant to that same bacterium. Potatoes are quite difficult to breed, but by transferring a gene from wild potatoes from the Andes, scientists have moved disease resistance into modern, commercial-type potatoes. Potatoes with this improvement have recently been approved for planting in North America. Crops can also be engineered to be genetically resistant to viruses─a strategy that saved the Hawaiian papaya industry.
Genetic resistance through biotechnology could provide solutions to important pest issues in many crops, but opponents of the technology have largely blocked the implementation of this approach. In many cases, the final result of the gene transfer would be the same as what would have happened through a much longer traditional breeding effort. In other cases, biotechnology offers a solution to a threat that may not be successfully addressed by any other approach.
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