Your potatoes are affected by early blight. Early blight is a fungal disease caused by the fungus Alternaria linariae. This is a common foliar disease of potato plants that can result in defoliation and reduced yields. The fungus overwinters in soil and on plant debris. Early blight is typically splashed by rainfall onto lower leaves early in the season. Early blight may reduce your yield, however the crop will not be affected.

To help control early blight the following points may help

• Provide adequate spacing to increase air circulation.
• Keep plants well mulched to minimize soil splashing
• Use drip irrigation instead of overhead watering if possible. If not possible, water at the base of the plant
• Do not work around the plants when they are wet
• Remove infected leaves during the growing season. Then remove and destroy infected plants at the end of the growing season
• Rotate your vegetables so that you are not planting potatoes in the same location year after year.
• Several fungicides are registered for use against Early Blight. Preventative sprays are effective but are only necessary in areas with a history of Early Blight. Make sure that you read and follow the manufacturers’ directions carefully.

The following article contains more information on early blight: Early blight in tomato and potato | UMN Extension

Thank you for your question and have a great gardening experience.