My Potatoes Turned Black & Soft After Digging Them Up

Cause

• Pythium leak, or shell rot, is a disease that affects potatoes after they are harvested and while they are in storage. It is caused by a fungal pathogen (Pythium ultimum) that lives in the soil and gains access to the tuber through wounds experienced during harvest. According to Cornell University's Thomas Zitter, some potato varieties are particularly susceptible to shell rot. These include Dark Red Norland Russet Norkotah, Red LaSoda, NorDonna and FL 1533.

Symptoms

• When the potatoes develop pythium leak you may notice a dark line around the outside, with normal-colored shell on one side and a dark brown to black shell on the other side of the line. When cut open, the flesh turns pink. Within 20 minutes, the flesh turns black. As the decay advances, the potato begins oozing and feels soft and mushy. If you squeeze the potato it appears even more watery. The areas of healthy flesh and infected flesh remain clearly defined.

Management

• Don't plant potatoes in the same soil from which the infected potatoes were harvested. Wait at least 4 years to allow the pathogen to die out. Plant the potato seed when the soil warms to over 45 degrees Fahrenheit and remains below 70 degrees Fahrenheit. At planting, apply a fungicide powder with mefenoxam as the active ingredient. Spread it in a 6-inch wide band over the potato seed pieces before covering the row with soil.

Additional Considerations

• Grow potatoes in well drained soil to avoid excess moisture around the tuber. According to agriculturists with the University of Minnesota, excess moisture is a key factor in pythium leak disease. Don't harvest the potatoes while the field is wet or when the tubers are warm. Avoid injury to the tubers during harvest. Don't allow the tubers to sit on warm, moist soil after harvest. According to specialists at Ohio State University, the disease develops rapidly under these conditions.