Most likely Black Root Rot, a soil fungi that attacks plant roots. The base of the stems turn brown/black and the leaves become discolored. This usually occurs when same crops are repeatedly grown in the same spot.
It can also be from plants not having regular waterings.
The fungi can be controled, if they are not affected severely. Cut back diseased tissue damaged shoots to healthy plant life. Plant in another location or build up soil with healthy material. Some plants may have to be destroyed if badly affected.