SciDev.Net recently published a news story about researchers who had genetically modified cassava to boost the size of its starch-rich roots (see GM cassava has 'super size' roots). The researchers produced roots that were double the normal size by inserting a bacterial gene into the cassava DNA.
My colleagues and I have produced cassava with roots that are ten times the normal size without resorting to genetic modification.
The cost of our research was extremely low. We simply hybridised cassava (Manihot esculenta) with the related wild species Manihot caerulescens. No foreign genes were inserted.
An image of the hybrid cassava roots is available online here.