Most fungi love wet conditions, and overwatering can produce such conditions. When the soil your bush is planted in is overwatered and it has poor drainage and water stands too long at the base and around the roots, then your bush is at risk of developing water mold root rot, a condition that is caused by a variety of fungi.
For heavily infected soils, try solarization.
The fungi start a rot that invades the roots and then it works its way up the plant. After that the rot is established you will see dark discolorations in the plant's roots and stems. The leaves look dull and some are even yellowing and wilting.
There is no cure for this disease, so once the bush is infected there is little you can do but remove it. Before you plant another bush in its place, make sure the area drains properly. Good drainage will prevent the wet conditions the fungi love.
The first potatoes were cultivated in Peru about 7,000 years ago.