This is the VOA Special English Agriculture Report, fromhttp://voaspecialenglish.com

Plant breeders and genetic engineers keep working to give crops the strength to resist threats like insects, diseases, droughts or floods. But before you can resist a threat, you need to understand it.

We told you last week about a newly completed genetic map of the organism that causes late blight. That disease led to starvation in Ireland from potato shortages in the middle of the eighteen hundreds.

The new genome could lead to better ways to protect potatoes, tomatoes and other crops. Science may supply a stronger crop. Yet that does not always guarantee demand.

Nik Grunwald from the United States Agriculture Department worked on the international team that completed the genome. He says it is possible to grow potatoes that resist late blight. But these may not look like Russet potatoes. And most American farmers grow Russets because, as Nik Grunwald puts it, "that is where the demand is."

Another example of scientific progress involves a natural bacterium known as Bt. Bt is used as a pesticide to fight cotton bollworms, corn borers and other pests. Scientists have found a way to grow cotton plants that contain a Bt gene, reducing the need for pesticides.

But sometimes, when one problem gets solved, another one appears.

In China, some farmers and researchers blame a decrease in pesticide use for an increase in pests unaffected by Bt. Also, there are concerns that some organisms could begin to resist the plants designed to resist them.

And in October, scientists reported on what they call the "indirect costs" of a virus-resistance gene in Cucurbita. This is the species of squash that includes pumpkins and gourds.

The scientists say virus-resistant transgenic squash are grown throughout the United States and much of Mexico. The genetically engineered squash are usually larger and healthier than wild squash.

But a three-year study showed that beetles like to feed more on the transgenic plants, increasing cases of wilt disease. The report by a team from the United States and China appeared in the Proceedings of the National Academy of Sciences.

The researchers point out that gene flow between crops and their wild relatives is common and difficult to contain. They note concerns that wild plants could, as a result, gain genetically engineered resistances. And these could affect the natural balance in their environment.

And that's the VOA Special English Agriculture Report.