Even a seed potato purchased from a reputable nursery or garden store may be infected with fungus spores. When the seed potato is planted, the warm soil and moisture can cause these spores to develop into any one of several potato diseases, including black rot. Stop fungus on seed potatoes by sterilizing your seed potatoes before you plant them. Proper sterilization doesn't just help your current potatoes, but also future potato generations since fungus on the potato can linger in the soil long after the infected potato is uprooted.
Borax (Sodium Borate)
Pour 1/3 cup borax into a gallon-sized plastic bucket. Borax is available from most general grocery stores.
Add 1/2 gallon of fresh water into the bucket.
Use a spoon to mix the bucket's contents thoroughly until the borax is dissolved.
Put your seed potatoes into the solution and let them soak for 10 to 15 minutes. After this time is up, remove the potatoes and plant them immediately. Borax effectively sterilizes seed potatoes and kills fungi, according to Clemson University.