7 Companion Plants You Should Never Grow With Tomatoes 

Potatoes: Both tomatoes and potatoes are susceptible to similar diseases, such as blight, and planting them together can increase the risk of infection.

Fennel: Fennel produces chemicals that can inhibit the growth of tomatoes and other plants, so it's best to keep them separate in the garden.

Corn: Corn and tomatoes are both heavy feeders, meaning they require a lot of nutrients from the soil. Planting them together can result in competition for resources and stunted growth.

Cabbage, Broccoli, Cauliflower: These plants are also heavy feeders and can compete with tomatoes for nutrients. Additionally, cabbage family plants can attract pests that may also damage tomatoes.

Walnut Trees: Walnut trees produce a chemical called juglone, which can be toxic to tomatoes and many other plants. Avoid planting tomatoes near walnut trees or using walnut tree wood chips as mulch.

Kohlrabi: Like other members of the cabbage family, kohlrabi can compete with tomatoes for nutrients and may attract pests that can also harm tomatoes.

Dill: Dill can attract tomato hornworms, a common pest that can damage tomato plants. It's best to plant dill away from tomatoes to avoid this issue.