In November, after the harvest, Borlaug would take his four surviving varieties to Sonora, where he would breed them with each other and many other cultivars in an effort to produce new cultivars that both resisted stem rust (as the four survivors did) and produced a lot of grain (as the other strains would if they didn't succumb to rust). In April he would harvest the seed from the best plants and take it to the Bajio, where he would perform a second round of crossbreeding. Because summer in the Bajio was wet, the area was like an incubator for plant diseases. Borlaug could use the second generation as a screen to check susceptibility to diseases other than P. graminis: viruses, bacteria, different types of fungi.