Complex sentence: Potatoes do not keep very well in storage and are vulnerable to molds that feed on the stored tubers and quickly turn them rotten; however, crops such as grain can be stored for several years with a low risk of rot.
Split into simpler sentences: Potatoes do not keep very well in storage and are vulnerable to molds that feed on the stored tubers which quickly turn them rotten. By contrast, grain can be stored for several years with a low risk of rot.