The main pesticide employed against nematodes around Livingston, Sweet Potato Capital of California, is methyl bromide. The Montreal Protocal to ban the use of methyl bromide because it is burning a hole in the ozone layer has been signed by 196 nations. Exemptions are allowed in the US for various crops, sweet potatoes and strawberries among them, because we wouldn't want to damage the income stream of any blameless agriculturalist fumigating his soil with a gaseous compound that is burning a hole in the earth's ozone layer.
Before the discovery of this miraculous soil fumigant, more "organic" means were employed in the years when Livingston was just becoming the Sweet Potato Capital of California. Then, according to an interview with a Livingston farmer buried in the Merced Sun-Star archives, farmers would plant a field with a crop of barley and let it die unharvested,  attracting and starving the nematodes, and clearing the field for a crop of sweet potatoes. The system worked until the growing number of absentee landowners in the region began raising rents so high that the good but starving farmers could not afford to fallow the field by feeding (and killing) nematodes that natural way.
The pesticide dealers descended on the sweet potato deal like another infestation, and the expensive, dangerous, and globally destructive practice of fumigating fields with methyl bromide became the norm.