Your tomatoes are affected by early blight. Early blight is a fungal disease caused by the fungus Alternaria linariae. This is a common foliar disease of tomato plants that can result in defoliation and reduced yields. The fungus overwinters in soil and on plant debris. It can also be transmitted on seeds and transplants. Early blight is typically splashed by rainfall onto lower leaves early in the season.

To help control early blight the following points may help

• Provide adequate spacing to increase air circulation and remove all suckers that emerge from the plant base
• 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 members of the squash family in the same location
• 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.