Passage: It is hard to predict how changes in the climate and the atmosphere’s chemistry will affect the prevalence and virulence of agricultural diseases. But there is a risk that such changes will make some plant infections more common in all climatic zones, perhaps catastrophically so. Part of the problem is that centuries of selective breeding have refined the genomes of most high-value crops. They are spectacular at growing in today’s conditions but genetic variation that can make them resilient to new threats has often been lost. This is good for yields but creates vulnerability: an entire disease or pest may spread rapidly through a genetically homogeneous crop. Which one of the following statements best reflects the central idea conveyed by the passage?